Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/ftp/documents/foc-handbook/foc-handbook.txt Дата изменения: Fri May 6 00:16:19 1994 Дата индексирования: Sat Dec 22 16:56:43 2007 Кодировка: Поисковые слова: reflection nebula

Hubble Space Telescope



Faint Ob ject Camera



Instrument Handbook



[Post-COSTAR]


Version 5.0



May 1994

Revision History



Handbook Version 1.0 May 1985; edited by F. Paresce
Handbook Version 2.0 April 1990; edited by F. Paresce
Handbook Version 3.0 April 1992; edited by F. Paresce
Handbook Version 4.0 February 1993; edited by A. Nota, R. Jedrzejewski, W. Hack
Handbook Version 5.0 May 1994; edited by A. Nota, R. Jedrzejewski, W. Hack
The Space Telescope Science Institute is operated by the Association of Universities for
Research in Astronomy, Inc., for the National Aeronautics and Space Administration.


FAINT OBJECT CAMERA
INSTRUMENT HANDBOOK
[Post-COSTAR]



A. Nota
R. Jedrzejewski
P. Greenfield
W. Hack


Space Telescope Science Institute
3700 San Martin
Baltimore, MD 21218



Version 5.0

May 1994
 FOC Instrument Handbook Version 5.0 i

CAUTION
The procedures for creating a Phase II proposal are being reviewed and revised
as this is written. We strongly recommend that users check the Phase II docu-
mentation carefully. We also recommend checking on STEIS at that time for a
revised version of this Instrument Handbook.



Major changes from the FOC Instrument Handbook,version (4.0)



1. The F/48 Camera is not available in Cycle 5, due to the high background count rate
and high-voltage turn-on problems which have occurred in the past two years. The F/48
was last successfully switched on December 22, 1993, before the deployment of the COSTAR
corrective optics. The camera remained on for the duration of the observation. While the
acquisition image showed a background level which, although high, was consistent with the
previous dark count images, a preliminary analysis of the following images showed immedi-
ately that the background increased dramatically with time, eventually reaching saturation
levels approximately two hours after HV switch-on. Whether this characteristic of increasing
background is a permanent condition of the F/48 is not clear. As a consequence, the
F/48 will not be made available to GOs during Cycle 5, pending further testing
and analysis. See section 6.4 for further details.
2. The Point Spread Function description has been updated to reflect the results from
SMOV and early Cycle 4 calibrations. COSTAR has restored much of the OTA capability,
in that the COSTAR-corrected PSF contains more than 75% of the light within a radius
of 0.1 arcsecond at visible wavelengths while only losing less than 20% of the light to the
two reflections at the two extra mirror surfaces. The net increase in sensitivity is a factor of
approximately 3_4 at visible wavelengths. Section 6.1 contains all the new information.
3. The Absolute Detector Quantum Efficiency has been updated after execution of the
related Cycle 4 Calibration program. A new OTA + COSTAR + FOC central absolute
quantum efficiency curve is provided as a function of wavelength for the four FOC imaging
and spectrographic configurations. The data represent the product of in-orbit measurements
for the new F/96 relay+OTA absolute quantum efficiency, and ground-based reflectance
calibrations of the COSTAR mirrors for the new F/48. The predicted loss of light from
two reflections of MgF2 coated aluminum COSTAR mirrors amounts to a 20% loss in the
visible and a 35% loss in the ultraviolet. The loss due to the COSTAR mirrors is more than
compensated by the improvement in image quality, since the encircled energy performance is
improved from 18% within a 0.1" radius to '80% within the same area, based on theoretical
PSFs. For a detailed discussion, see Section 6.3. In addition, a format dependence sensitivity
effect has been studied, and the results are provided in Section 6.3.1
4. The Format-Dependent Sensitivity Effect has been measured and has been found to
be substantial (as much as a 45% change). Section 6.3.1 lists the relative sensitivities of the
more common imaging formats.
5. The Geometric Distortion discussion has been updated (Section 6.11) and new values
for the FOC plate scale are provided in Section 6.12).
6. The prescription for Estimating Exposure Times has been updated to take into ac-
count the new PSF and DQE parameters (see Section 7.0).
ii FOC Instrument Handbook Version 5.0

CONTENTS


1.0 INTRODUCTION 1

2.0 COSTAR OVERVIEW 3

3.0 INSTRUMENT OVERVIEW 6

4.0 DETAILED INSTRUMENT DESCRIPTION 12

4.1 Transfer Optics : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : :12
4.2 Focal Plane Apertures : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : :14
4.3 Internal Calibration System : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : :19
4.4 Filter Wheels : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : :@
4.4.1 Bandpass and Neutral Density Filters : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :21
4.4.2 Objective Prisms : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : :29
4.4.3 Polarizers : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : :@
4.5 Long Slit Spectrographic Facility : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : :34
4.6 Detectors : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : :@
4.6.1 Image Intensifier and Coupling Lens : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :37
4.6.2 TV Tube : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : :@
4.7 Video Processing Unit : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : :38
4.8 Science Data Store : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : : :39

5.0 OBSERVING ARRANGEMENTS 41
5.1 Imaging, Occultation and Spectrographic Modes : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :41
5.2 Target Acquisition Modes : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : :44
5.2.1 Mode I Target Acquisition - INTeractive ACQuisition : : : : : : : : : : : : : : : : : : :44
5.2.2 Mode III Target Acquisition - Blind Pointing : : : : : : : : : : : : : : : : : : : : : : : : : : : :45
5.2.3 Early ACQuisition : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : :45
5.3 The FOC Target Acquisition Apertures : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :46

6.0 INSTRUMENT PERFORMANCE 47

6.1 The Point Spread Function (PSF) : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: :47
6.1.1 Image Quality and Field Dependence of the PSF : : : : : : : : : : : : : : : : : : : : : : : : :50
6.2 Dynamic Range : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : :51
6.2.1 Uniform Illumination : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : :51
6.2.2 Non-Uniform Illumination : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : :52
6.3 Absolute Quantum Efficiency : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : :54
6.3.1 Format-dependent Sensitivity : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : :58
6.4 Detector Background : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : :58
6.5 Stray Light : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : :@
6.6 Detector Overload : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : : :62
6.7 Overhead Times and Multiple Exposures : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :63
6.8 Guiding Modes with the FOC : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : :63
6.9 Uniformity of Response (Flat Fielding) : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
*63
6.10 Visible Leaks : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : : : : : @
6.11 Geometric Distortion and Stability : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: :70
6.12 Plate Scale : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : : : : : @
 FOC Instrument Handbook Version 5.0 iii

7.0 OBSERVER'S GUIDE (PRESCRIPTION FOR ESTIMATING
EXPOSURE TIMES) 73

7.1 Point Sources : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : : : : : : @
7.1.1 Imaging : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : : :@
7.1.2 Spectroscopy : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : : :78
7.2 Extended Sources : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : : :79

8.0 THE FOC EXPOSURE TIME SIMULATOR, FOCSIM 83

9.0 LIMITING MAGNITUDES 85

10.0 FOC DATA ANALYSIS AND PRODUCTS 87

10.1 Pipeline Processing : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : : : : :87
10.2 General Procedures : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : : : : : : : : :88
10.2.1 Dark-Count Subtraction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : :88
10.2.2 Format-Dependent Photometric Correction (ITF) : : : : : : : : : : : : : : : : : : : : : : :88
10.2.3 Correct for Zoom Mode : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : :88
10.2.4 Compute Absolute Sensitivity : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :*
* : : :88
10.2.5 Geometric Correction : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : *
*: : : : : : : :88
10.2.6 Relative Calibration or Flat Field Correction (normal images only) : : : : : : :90
10.2.7 Spectrographic Detective Efficiency Correction : : : : : : : : : : : : : : : : : : : : : : : : : : :90

11.0 ACKNOWLEDGEMENTS 91

12.0 APPENDIX 92
iv FOC Instrument Handbook Version 5.0

LIST OF FIGURES


Figure 1. COSTAR correction principle for pre-COSTAR F/96 relay : : : : : : : : : : : 3
Figure 2. COSTAR deployed showing FOC light path. : : : : : : : : : : : : : : : : : : : : : : : : : 5
Figure 3. FOC Operational and Data Flow Block Diagram : : : : : : : : : : : : : : : : : : : : : 7
Figure 4. A Schematic Drawing of the FOC : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 8
Figure 5. The Transfer Optics Block Diagram : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 13
Figure 6. The Schematic Optical Layout for the New Cameras : : : : : : : : : : : : : : : : : 14
Figure 7. Location of the FOC Entrance Apertures on HST Focal Plane : : : : : : : : 16
Figure 8. The New F/96 Camera Entrance Aperture : : : : : : : : : : : : : : : : : : : : : : : : : : : 17
Figure 9. The New F/48 Camera Entrance Aperture : : : : : : : : : : : : : : : : : : : : : : : : : : : 18
Figure 10. Normalized Emission Spectra of the Calibration LEDs : : : : : : : : : : : : : : : 20
Figure 11. Transmittance of the Long Pass and Wide Band Filters for
the New F/96 relay : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :2:4: *
*: : : : :
Figure 12. Transmittance of the Visible, Medium and Narrow Band Filters
for the New F/96 Relay : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :2:5: *
*: : :
Figure 13. Transmittance of the UV Medium Band Filters
for the New F/96 Relay : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :2:6: *
*: : :
Figure 14. Transmittance of the Neutral Density Filters
for the New F/96 Relay : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :2:7: *
*: : :
Figure 15. Transmittance of all the Filters for the New F/48 Relay : : : : : : : : : : : : : : 28
Figure 16. Optical Layout of the Focal Plane of the New F/96 Relay with
the Objective Prism : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :3:2: *
*: : : : :
Figure 17. The Physical Layout of the FOC Polarizers : : : : : : : : : : : : : : : : : : : : : : : : : : 32
Figure 18. Image Configurations on the Focal Plane of the New F/96 Relay
for the Polarizers : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :3:3:*
* : : : : : : :
Figure 19. Transmittance of the Polarizers : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :3:3:
Figure 20. Optical Layout of the Focal Plane for the New F/48 Relay
with the FOPCD : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :3:6:*
* : : : : :
Figure 21. Physical Layout of the Imaging Photon Counting Detectors : : : : : : : : : : 37
Figure 22. Typical Raster Scan Output of the Detectors : : : : : : : : : : : : : : : : : : : : : : : : 39
Figure 23. Radial profiles of pre-COSTAR aberrated PSF and COSTAR-
corrected PSF : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :4:7: *
*: : : : : : : :
Figure 24. The encircled energy fraction and PSF profile for the COSTAR-
corrected F/96 and pre-COSTAR F/96 relays : : : : : : : : : : : : : : : : : : : : : : : : 49
Figure 25. Images of PSFs taken with the COSTAR-corrected F/96 camera
through the F486N, F120M, and F372M filters : : : : : : : : : : : : : : : : : : : : : : : 50
Figure 26. Flat-field Linearity Plots for Both Relays : : : : : : : : : : : : : : : : : : : : : : : : : : : : 53
Figure 27. Point Source Non-linearity Characteristics : : : : : : : : : : : : : : : : : : : : : : : : : : : 53
Figure 28. Baseline Overall (OTA+COSTAR+FOC) Central Absolute
Quantum Efficiency : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :5:6: *
*: : : : :
Figure 29. Increase in the background count rate over time in the new F/48 relay 60
Figure 30. Stray Light Illumination in V Magnitudes : : : : : : : : : : : : : : : : : : : : : : : : : : : : 61
FOC Instrument Handbook Version 5.0 v

Figure 31. The Earth's Average Daylight Nadir Radiance in Rayleighs A-1 : : : : : 62
Figure 32a. Plot of Vignetting Function for the Extended Format for
the New F/48 Relay : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :6:5:*
* : : : :
Figure 32b. Plot of Vignetting Function Along the Spectrographic Slit : : : : : : : : : : : 65
Figure 33. Contour Plots of Flat Field Images for the New Relays : : : : : : : : : : : : : : 65
Figure 34. Plot Across a Row of UV Flat Field Images for the New Relays : : : : : : 67
Figure 35. Ratio of External to Internal Flat Field Images for the New
F/48 and the New F/96 Relays : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :67
Figure 36. The Expected Monochromatic Count Rate for the New F/96 Camera : 69
Figure 37. The Overall (Optical + Detector) Distortion Field for
the New F/48 Relay. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :7:1:*
* : : : :
Figure 38. The Overall (Optical + Detector) Distortion Field for
the New F/96 Relay. : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :7:1:*
* : : : :
Figure 39. Residual 1216 and 1304A Airglow Contribution : : : : : : : : : : : : : : : : : : : : : : 76
Figure 40. Zodiacal Light Contribution to the FOC Background Counting Rate : 77
Figure 41. Exposure Time Required to Reach S/N = 10 (point source) : : : : : : : : : 86
Figure 42. Exposure Time Required to Reach S/N = 10 (extended source) : : : : : : 86
Figure 43. Flow Diagram of the FOC Imaging Data : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 89
Figure A1. Extended Format (512z x 1024) pre-COSTAR F/48 Image : : : : : : : : : : : 92
Figure A2. Extended Format pre-COSTAR F/96 Image : : : : : : : : : : : : : : : : : : : : : : : : : 93
Figure A3. Extended Format pre-COSTAR F/48 Spectrographic Image : : : : : : : : : : 94
Figure A4. 512x512 Format pre-COSTAR F/96 Far-UV Objective Prism Image : 95
Figure A5. Extended Format pre-COSTAR F/48 Image Showing
High Background Features : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :9:6:*
* :


LIST OF TABLES


Table 1. Summary of FOC Performance Characteristics I. Imaging : : : : : 10
Table 2. Summary of FOC Performance Characteristics II. Spectroscopy 11
Table 3. The Optical Element Characteristics for the New F/96 Relay : : 22
Table 4. The Optical Element Characteristics for the New F/48 Relay : : 23
Table 5. FOC Objective Prism Characteristics : : : : : : : : : : : : : : : : : : : : : : : : : 31
Table 6. Standard Imaging, Occultation, and Spectrographic Modes : : : : 42
Table 7. Target Acquisition Formats : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :46
Table 8. Measured Energy Fraction ffl(~) for the New F/96 Relay : : : : : : : 48
Table 9. Calculated Flat-Field Linearity Parameters : : : : : : : : : : : : : : : : : : : 52
Table 10. Overall (OTA+COSTAR+FOC) Absolute Quantum Efficiency 55
Table 11. Format-Dependent Sensitivity Ratios : : : : : : : : : : : : : : : : : : : : : : : : : 58
Table 12. Zodiacal Light Intensities in S10 Units. : : : : : : : : : : : : : : : : : : : : : : : 75
FOC Instrument Handbook Version 5.0 1

1.0 INTRODUCTION_______________



The Faint Object Camera (FOC) is a long-focal-ratio, photon-counting device capable of
taking high-resolution two-dimensional images of the sky up to 14 by 14 arcseconds squared
in size with pixel dimensions as small as 0.014 by 0.014 arcseconds squared in the 1150 to
6500A wavelength range. Its performance approaches that of an ideal imaging system at low
light levels. The FOC is the only instrument on board the Hubble Space Telescope (HST)
to fully use the spatial resolution capabilities of the Optical Telescope Assembly (OTA) and
is one of the European Space Agency's contributions to the HST program.


The HST was placed in orbit on April 24, 1990. A few months later, it was realized that the
2.4m diameter primary mirror suffered serious optical degradation due to a manufacturing
error at Perkin-Elmer. The primary mirror of the HST OTA was incorrectly figured with
the wrong conic constant, which produced severe spherical aberration at the OTA image
plane (Burrows et al. 1991, Ap.J.Letters, 369,L21.). This aberration, which caused the light
from a star to be spread out into a circular Point Spread Function (PSF) of 2.5 arcseconds
radius, could not be removed by changing the secondary mirror focus position or by mov-
ing the primary mirror actuators. In the fall of 1990, a Strategy Panel was convened to
investigate possible methods for removing the spherical aberration, and made the following
recommendations: 1) replace the Wide Field Planetary Camera (WF/PC) with the WFPC2
at the earliest possible opportunity, with the optics of the WFPC2 re-designed to counteract
the OTA spherical aberration, and 2) replace the High-Speed Photometer with an instru-
ment designed to deploy corrective optics in front of the remaining axial instruments (FOC,
GHRS and FOS)_ COSTAR. Both these recommendations were approved by NASA, and
were implemented in the first Servicing Mission executed in December 1993.


COSTAR (Corrective Optics Space Telescope Axial Replacement) has restored the two prime
scientific objectives of the FOC: deep imagery and photometry of very faint celestial objects
and imagery of bright objects at the highest possible resolution available from HST. The
FOC is now capable of detecting a star of U magnitude 27.5 in a 5 hour exposure with a
S/N = 5 and of resolving bright sources in the near UV up to an effective angular resolution
of '0.04 arcseconds.


Other main scientific objectives of the FOC include, but are not restricted to, the study of
the physics of planets, search for planets and proto-planetary condensations around nearby
stars, search for massive black holes in globular clusters, study of the ionization structure of
shock waves in the interstellar medium, high spatial resolution studies of very young stars
and cataclysmic variables and their interaction with the surrounding interstellar medium, the
study of stellar content of globular clusters, observation of optical emission associated with
radio lobes and jets in galaxies, the observations of velocity dispersion and mass densities
in the central regions of normal and compact elliptical galaxies, observation of extended
structure around QSOs at high spatial resolution, and the study of gravitational lenses.


The basic aim of this handbook is to make relevant information about the performance of
the FOC+COSTAR system available to a wide group of astronomers, primarily to aid in
2 FOC Instrument Handbook Version 5.0

applying for HST time, and to aid those who have had FOC proposals accepted in planning
and specifying their exposures. At this point it needs to be emphasized that, due to
the high background count rate and high-voltage turn-on problems which have
occurred in the F/48 relay (see Section 6.4), the F/48 camera will not be made
available in Cycle 5, pending further testing and data analysis.



The information contained in this handbook has been obtained from the combination of the
inflight characterization of the FOC, COSTAR ground and flight calibrations and represents
our best present knowledge of the performance of the FOC+COSTAR system. For Cycle
5, this handbook supersedes all the previous versions. However, if the reader is
interested in preparing an Archival Proposal for observations taken during the
previous cycles (before COSTAR was deployed), the Cycle 3 Handbook (Version
3) must be used as a reference instead.



A brief overview of COSTAR and its effect on the OTA image quality is presented in Sec-
tion 2. The FOC, as presently configured, is briefly described and some basic performance
parameters summarized in Section 3. A more detailed, in-depth perspective on the FOC can
be found in Section 4. In Sections 6 and 7, the readers will find the detailed FOC perfor-
mance parameters and instructions on how to derive approximate FOC exposure times for
the proposed targets and some useful examples. The last section deals with the expected
data products and calibration plans. This plan should allow readers to choose the level of
detail required to match their previous degree of understanding of the instrument with the
degree of complexity of the proposed observing program.
FOC Instrument Handbook Version 5.0 3

2.0 COSTAR_OVERVIEW__________________



COSTAR replaces the High Speed Photometer in the Axial bay of HST, in the -V2, +V3
quadrant (see Figure 7). It is a "passive" instrument, in that it has no detector of its
own, its sole purpose being to deploy a set of mirrors in front of the other Axial Scientific
Instruments (ASI). These mirrors, and their associated mounts and arms, serve only to
block the aberrated OTA beam from entering the ASI entrance apertures and to correct
the spherical aberration of a different part of the OTA field of view before re-directing the
corrected beam into the ASIs. A schematic diagram of the COSTAR optics in front of the
F/96 relay is shown in Figure 1. There is a separate set of mirrors that corrects the F/48
channel, mounted on the same arms as those for the F/96 channel.

Figure 1. Diagram showing the COSTAR correction principle for the F/96 relay. The
aberrated OTA beam (dashed line) is blocked from reaching the FOC aperture
by the M2 mirror.
Before COSTAR, the aberrated F/24 OTA beam at 6.565 arcminutes from the V1 axis formed
an image at the FOC focal plane aperture 110mm from the V1 axis. After the deployment of
COSTAR, the M2 mirror and its mounting arm blocks this light from entering the aperture.
The beam that would have formed an image 4.658 arcminutes from the V1 axis is re-directed
by the spherical M1 mirror to form an image of the OTA exit pupil on the M2 mirror. This
mirror becomes the exit pupil of the OTA+COSTAR optical system, and the anamorphic
aspheroidal figure fulfills three functions:



o it re-directs the corrected beam into the FOC, forming an image at the FOC focal
plane aperture;



o it corrects for the OTA spherical aberration;



o it increases the astigmatism from that present at 4.658 arcminutes to that appropriate
to 6.565 arcminutes because the FOC is designed to correct for exactly the latter
amount of astigmatism.
4 FOC Instrument Handbook Version 5.0

However, there is one major difference between the COSTAR-corrected beam and the un-
aberrated OTA: the F/ratio is increased from F/24 to F/37. This results in a change in
the F/number of the FOC relays from F/48 to F/75.5 and from F/96 to F/151.
Because the names "F/48" and "F/96" are deeply-rooted in the HST ground
system at all levels, from proposal entry to data archiving, we are forced to retain
these names despite the fact that they do not describe the true focal ratios of
the cameras anymore. As a result, the user must take particular care in reading
this handbook, for whenever we mention the new F/48 and the new F/96 relays,
we are really referring to the relays with new focal ratios of F/75.5 and F/151
respectively.



A schematic diagram of COSTAR when deployed is shown in Figure 2. The FOC M1
mirrors are mounted on an arm which can be tilted in each of 2 orthogonal directions. This
adjustment is necessary to accurately center the image of the OTA exit pupil on the M2
mirror, and was done on orbit. Any error in this centering results in the introduction of
coma into the PSF. Since the M1 mirrors for both of the new relays are on the same mount
("ganged"), only one channel (the new F/96) can be optimized in this way. It is anticipated
that the tilt of the new F/48 M1 mirror relative to that of the new F/96 mirror was set
during ground alignment to sufficient accuracy that no appreciable residual coma is left
in the new F/48 PSF. Optimization of the M1 tilt was accomplished during the Servicing
Mission Observatory Verification (SMOV) period shortly after the installation of COSTAR.



The M2 mirrors for both of the new channels are also on a single arm mounting, but there
is no tip/tilt capability. Both arms are connected to the Deployable Optical Bench (DOB),
which can be commanded to move over a 16mm range parallel to the V1 direction. This was
used to focus the new F/96 channel during SMOV. Again, it is anticipated that the focus
position of the new F/48 channel relative to that of the new F/96 channel was set to an
accuracy of 0.1mm or less during ground alignment.



The correction of the spherical aberration of the OTA by COSTAR does introduce a small
number of side-effects that were not present in the original design of HST+FOC. Firstly,
the focal plane tilt of the OTA at the FOC entrance apertures cannot be duplicated by the
2-mirror COSTAR system. The tangential and sagittal image planes produced by COSTAR
are tilted with respect to those in the OTA, and tilted appreciably with respect to each
other. This means that perfect astigmatism correction can only be achieved at one field
point. Similarly, the mean focal surface produced by COSTAR is tilted with respect to the
mean focal surface produced by the OTA, so that there is a field-dependent focus variation.
These effects are described more fully in Section 6.1.



Secondly, the HST+COSTAR exit pupil is now only 530mm from the FOC entrance apertures
(compared to 7m for the uncorrected OTA). This causes unavoidable vignetting in the new
F/48 camera for field positions more than about 12 arcseconds away from the optimally-
corrected field point. The effects of this vignetting are discussed in sections 4.2 and 6.9. The
new F/96 channel does not suffer from vignetting.
FOC Instrument Handbook Version 5.0 5

Figure 2. A Schematic View of COSTAR, after deployment, showing the FOC light paths.
For clarity, the FOS and GHRS M2 arms are omitted.

For this reason, it was decided to make the new F/96 channel the `preferred' channel of the
FOC, and attempt to optimize the image quality of this channel.
6 FOC Instrument Handbook Version 5.0

3.0 INSTRUMENT_OVERVIEW_______________________



The Faint Object Camera (FOC) is one of the four axial scientific instruments sharing
the HST focal plane. It is located in the -V2, -V3 quadrant (see Figure 7), has overall
dimensions of 0.9 x0:9 x 2:2 meters, weighs 320 kg and consumes 130W of power on average
in operation. An overall operational and data flow block diagram of the instrument is
shown in Figure 3 with the FOC itself contained within the dashed line. Radiation from
an astronomical source focused onto the OTA focal plane is reimaged by COSTAR and fed
into either of two separate and independent cameras each with its own entrance aperture,
imaging optics and detector system.


One camera magnifies the image on the OTA focal plane by a factor of two to an effective
focal ratio of F/75.5 while the other magnifies the focal plane by a factor of four to an
effective focal ratio of F/151. This transfer is accomplished mainly in order to match the
OTA resolution performance with the available detector pixel size.


Each optical relay consists of segments of a full optical figure of revolution the axis of which is
perpendicular to the OTA focal plane at the FOC entrance aperture location. Both cameras
have the same overall length and operate at the same distance from the OTA optical axis.
The re-imaging optics transfers the COSTAR corrected OTA image onto the photocathode
of a photon counting detector with negligible spherical aberration or coma and corrects for
the residual OTA off-axis astigmatism. The FOC optical system also provides means for
dispersing, filtering, attenuating, polarizing and focusing the image formed by the OTA and
for in-flight calibration of the relative and absolute response in the visible.


All the optical elements and both detectors are supported on an optical bench which is
rigidly connected to the focal-plane structure of the OTA and is contained within the load-
carrying structure which also provides a light-tight enclosure. To meet the image stability
requirements, the internal surfaces of the load-carrying structure which enclose the optical
bench are actively thermally controlled during all operational modes with a stability of better
than 0.5O C. An exploded schematic view of the FOC is presented in Figure 4.


The two detectors are two dimensional photon counting devices of identical design. Each
consists of a three-stage image intensifier which is optically coupled by a relay lens system
to an Electron Bombarded Silicon Target (EBS) TV tube. The tube detects scintillations
at the output of the intensifier corresponding to the arrival of individual photons at the
first stage photocathode. The central x-y position of each burst of visible light is measured
by a dedicated video processing unit (VPU), and the contents of a memory location in the
scientific data store (SDS) unit associated with that position are incremented by one. At
the end of the exposure, the accumulated image in the SDS is sent directly out of the FOC
to a dedicated unit in the ST Scientific Instrument Control and Data Handling (SI C&DH)
subsystem which consists of a computer with a reprogrammable non-volatile memory. The
two detectors in orbit produce a dark noise typically of ' 7 x 10-4 counts sec-1 pixel-1 for
the new F/96 relay and ' 2 x 10-3 counts sec-1 pixel-1 for the new F/48 relay (see also
Section 6.4).
FOC Instrument Handbook Version 5.0 7



Figure 3. FOC Operational and Data Flow Block Diagram

The SDS storage capacity is adapted to an image area of 512 x 512 resolution elements
and provides a 16 bit data word for each pixel. This results in a memory capacity of 256K
words of 16 bits each or 0.5 Mbyte in total. The word length can also be commanded to
8 bits to store data in a 512 x 1024 pixel format with reduced dynamic range. Because of
operational constraints, a time interval of at least 3.9 minutes must elapse between the end
of an exposure and the start of the next.



The detectors are sensitive to radiation between 1150 and 6500A, the lower limit being set
by the MgF2 input window and the upper limit by the bialkali photocathode material. The
useful photocathode area is 40 millimeters in diameter while the size of an independent
resolution element (pixel) is on average normally ' 24 x 24 microns squared, but one
dimension can be stretched (zoomed) to ' 48 microns. The longer pixel dimension is in the
8 FOC Instrument Handbook Version 5.0




Figure 4. Schematic Drawing of the FOC

TV frame scan direction and perpendicular to the dispersion direction of all but one (the
FOPCD) of the dispersing elements. The plate scales for the new F/48 and the new F/96
relays are 1.131 and 0.569 arcseconds mm-1 respectively.



These parameters, coupled to the quoted maximum SDS capacity, imply that the new F/48
camera has a maximum achievable field of view of ' 28 x 28 arcseconds squared imaged
at a pixel size of 0.056 x 0.028 arcseconds squared (512 zoomed x 1024, 48 x 24 micron
squared pixels with 8- bit words). The corresponding values for the new F/96 relay are ' 14
x 14 and 0.028 x 0.014 arcseconds squared. Smaller fields can be imaged at higher spatial
resolution and extended dynamic range (see Table 6 for a partial list, and Section 5.1 for
more details).



In summary, the key operating features of the FOC are its low noise, high angular resolution,
high sensitivity in the UV range and extreme versatility due to its occultation, polarization,
and objective prism capabilities. Its most significant limitations, on the other hand, are
its relatively small field of view, further restricted by COSTAR, and the non-linearity of
response at high count rates which limits its useful application to objects yielding less than
' 2 counts sec-1 pixel-1 , corresponding to a U ' 18:5 A0V star observed through the
F342W filter with the new F/96 relay, for example. A summary of the most important
FOC Instrument Handbook Version 5.0 9

performance characteristics of the FOC as presently known is given in Tables 1 and 2 for
the imaging and spectrographic modes, respectively.
10 FOC Instrument Handbook Version 5.0

FOC Instrument Handbook Version 5.0 11

12 FOC Instrument Handbook Version 5.0

4.0 DETAILED_INSTRUMENT_DESCRIPTION__________________________________
4.1 TRANSFER OPTICS



A component block diagram of the FOC transfer optics is shown in Figure 5. A conceptual
schematic optical layout in a plane containing the V1 axis and the chief ray is shown in
Figure 6. Radiation from the COSTAR corrected OTA enters the FOC through a baffled
tube that leads to a field-defining entrance aperture located in a plane tangential to the OTA
focal surface and centered on or near the best focus point at the position of each relay. Just
beyond the entrance aperture, the radiation encounters a light tight shutter mechanism that,
in its closed position, introduces a calibration mirror into the beam to intercept light emitted
by an internal source of visible radiation and to uniformly illuminate the FOC object plane.


Once past the shutter, radiation impinges on a two element aplanatic optical system consist-
ing of a spherical concave primary and an elliptical convex secondary mirror. This optical
system magnifies the OTA focal plane by a factor of two for the new F/48 relay and four for
the new F/96 relay with negligible spherical aberration or coma. The mirrors are all made
of Zerodur and overcoated with Al + MgF2 for a reflection efficiency exceeding 0.7 above
1200A.


Near the exit pupil, in the new F/96 relay are located four independently commandable
rotating filter wheels. Two such wheels are located at or near the exit pupil in the new F/48
relay. The filter wheels for the new F/96 relay each have 12 equidistant working positions
while for the new F/48 relay each wheel has 8 equidistant positions. Each wheel has one
clear position. These devices carry a full complement of wide, medium and narrow bandpass
and neutral density filters, polarizing and objective prisms.


In order to fold the light beam back onto the detector and to focus the FOC, a cylindrical
concave mirror is placed into the slowly converging beam past the filter wheels. This mirror
also corrects for the residual off-axis OTA astigmatism and is made of the same materials
as the primary and secondary mirrors. This mirror is mounted on a commandable focusing
mechanism that allows it to internally compensate for variations in optical path length
introduced by the OTA focus variations, FOC internal stability and by the differing optical
thicknesses of the various optical elements on the filter wheels. The focusing mechanism
changes the length of the optical path by 16 millimeters maintaining the position of the
image on the detector typically within 0.05 millimeters whatever the location of the mirror
along the stroke. The FOC focal plane is designed to coincide with the detector photocathode
plane. The detector samples an area of 24.6 x 24.6 millimeter squared corresponding to 1024
x 1024 pixels, each ' 24 micron squared in size, averaged over the field of view.


Absolute image position on the FOC focal plane can be referred to a grid of 17 x 17 reseau
marks, each 75 x 75 microns squared in size evaporated on the inner surface of the pho-
tocathode MgF2 window. The overall wavefront distortion of the FOC+COSTAR optical
system is less than ~/10 for the new F/48 relay and the new F/96 relay at ~6328A.
FOC Instrument Handbook Version 5.0 13


Figure 5. The Transfer Optics Block Diagram. The removable components are shown in
the dashed frames.
In the new F/48 relay, the beam from the folding mirror may be relayed by a removable
toroidal convex mirror to a fixed spherical concave reflection grating which re-images a
spectrum of a portion of the field of view onto the detector photocathode. This portion
contains a fixed width rectangular slit that is located on the entrance aperture (see Figure
9). The grating works with a divergent beam in the Rowland condition at fixed wavelength
ranges in the first (3600-5400A), second (1800-2700A), third (1200-1800A) and fourth (900-
1350A) order at a resolution ~/ ~ ' 1000. Only the 1150-1350A portion of the fourth order
spectrum can be measured in practice, of course, due to the MgF2 cut-off of the detector.
14 FOC Instrument Handbook Version 5.0



Figure 6. The schematic optical layout of the two cameras in the planes containing the V1
axis and the chief rays.

Wavelength range selection is accomplished by introducing suitable bandpass filters into
the optical path of the new F/48 relay or by using the objective prism (FOPCD) whose
dispersion axis is oriented at ' 90O to the grating dispersion direction as a cross disperser.
4.2 FOCAL PLANE APERTURES



The two FOC field defining entrance apertures are each located in a plane tangent to an
OTA focal surface at the center point of the aperture. The center of each aperture is located
at a linear distance of 110 millimeters from the V1 axis. Each FOC channel of COSTAR
redirects part of the OTA field of view into an FOC entrance aperture. For the new F/96,
FOC Instrument Handbook Version 5.0 15

the field is centered on a point 4.658 arcminutes from the V1 axis, while for the new F/48
the field is centered on a point 4.312 arcminutes from V1. The projections of these apertures
onto the plane of the sky are shown in Figure 7; the dotted circles show the projection
through the OTA only (i.e. the pre-COSTAR positions), while the solid circles are the new
positions through the COSTAR+OTA optics. In this figure, the V1 axis runs into the paper
at the center of the WFPC2 field of view and V1, V2, V3, U2, and U3 are the HST axes
defined in the Call for Proposals and Proposal Instructions. The observant reader will notice
that in Figures 8 and 9 the V2-V3 directions are reversed from previous versions of the FOC
Handbook: this is because COSTAR forms an intermediate image between the M1 and M2
mirrors.



At the locations of the FOC entrance apertures, the OTA focal plane makes an angle of 10.05O
with the normal to the ST axis. This plane is the object plane for the FOC optical relays.
The COSTAR-corrected focal plane is inclined to this plane, which induces a field-dependent
focus variation that is described more fully in Section 6.1.2. The axes of symmetry of the
two FOC cameras D96 and D48 that run through the center of the apertures, perpendicular
to and intersecting on the V1 axis form an angle of 30O . The D96 axis forms an angle of 30O
with the +V2 axis and D48 an angle of 30O with the +V3 axis.



An expanded view of the two apertures in exactly the same perspective is shown in Figures
8 and 9. The camera aperture for the new F/96 relay is a circular diaphragm of 10.5
millimeters in diameter corresponding to 24 arcseconds on the sky in the COSTAR-corrected
field, centered at point O with two 2 millimeter-long protruding opaque metal fingers oriented
' 30O to the D96 line and parallel to the V2 axis. The finger on the right is 0.112 millimeters
thick (0.25 arcsecond in the sky) while the other is 0.223 millimeters thick (0.5 arcsecond
in the sky). The directions of increasing sample (S) and line (L) numbers for the extended
SDS format define the image coordinate system with its center at point C96 . This system is
aligned with the X, Y reference system used to designate the orientation of the apertures on
the sky in the Proposal Instructions. The corners ABCD of the 512 x 1024 zoomed format
are marked on Figure 8. The large 14 x 14 arcsecond square marks the limit of the extended
format. The opaque coronographic fingers are indicated by the hatched regions. The V1
axis is 4.658 arcminutes from O in the direction indicated to V1. The fully corrected field
point for the new F/96 relay coincides with C96 .



The new F/48 entrance aperture is shown in Figure 9. The center O of the main aperture
coincides with the center of the extended SDS format and lies on the sagittal focus while the
center J of the slit lies on the tangential focus of the OTA.



Because of vignetting induced in the FOC by moving the OTA exit pupil from 7 meters to 530
millimeters from the new F/48 entrance aperture, it is not possible to have the center of the
extended field unvignetted as well as all of the slit. The best compromise, which maintains
the utility of the long slit as well as providing an imaging field useful for spectrograph
acquisitions, was to move the optimally corrected field point from the center of the extended
format to the point P, which is where the hypothetical extension of the slit would meet the
edge of the extended imaging format. The default 512 x 512 imaging format has therefore
16 FOC Instrument Handbook Version 5.0



Figure 7. The location of the FOC entrance apertures on the HST focal plane projected
onto the plane of the sky. In this perspective V1 is directed into the paper at
the center of the WFPC2 pattern. V1, V2 and V3 form the HST right handed
coordinate system defined in the Call for Proposals.

been moved to that corner of the extended format, and has been outlined in Figure 9. In
this way, over half of the slit suffers less than 20% vignetting while the newly moved 512 x
512 imaging format is unvignetted over more than 80% of its area. Full vignetting contours
are shown in Section 6.9.



The main aperture is essentially a circular diaphragm with a diameter of 21 millimeters
corresponding to 48 arcseconds on the sky in the COSTAR-corrected field, except for an
oblique truncation at points E and H. A thin, 0.15 arcsec wide opaque finger points to O
from point G. A 5.689 millimeters (12.8 arcsecond) long, 0.028 millimeters (0.064 arcsecond)
wide slit centered at J is located between points I and K. This slit forms the defining aperture
of the spectrograph for the new F/48 relay. The corners of the 512zx 1024, 28 x 28 arcsecond
FOC Instrument Handbook Version 5.0 17




Figure 8. The camera entrance aperture for the new F/96 relay projected onto the sky.
 18 FOC Instrument Handbook Version 5.0




Figure 9. The camera entrance aperture for the new F/48 relay projected onto the sky. KI along
the spectrograph slit coincides with the reference axis x that is used to designate the
orientation of the apertures on the sky in the Proposal Instructions. The point P is the
optimally corrected field point for the new F/48 relay and the new 512 x 512 imaging
format is shown outlined with a bold line.
FOC Instrument Handbook Version 5.0 19

squared extended imaging format are given on Figure 9 as points A, B, C and D. When the
spectrograph mirror is in place, the aperture is imaged onto the extended SDS format as
shown in Figure 9 with the dashed lines representing the inner and outer edge of the spectrally
dispersed image of the slit and the edge of the main aperture drawn for the specific case of
the Hg line at 4358A. The opaque target acquisition finger is indicated by the dark region.
V1 is 4.312 arcminutes from P in the direction indicated to V1. A part of the dispersed
main aperture falls in the right hand quarter of the extended format and may be eliminated
by tailoring the observing format to the region inside the slit area. Wavelength increases in
the direction indicated by ~ from 3600A to 5400A in first order. Slitless spectroscopy can
be performed in the clear region to the right of the dashed lines (see Section 3.6 and Figure
A3 in the Appendix).


In order to predict with reasonable accuracy the location and orientation of an extended
source in the FOC fields of view and to determine whether or when the required instrument
orientations are compatible with the HST roll angle restrictions, it may be useful to locate
with respect to the S, L axes on Figures 8 and 9 the celestial reference axes for that particular
target and viewing configuration. To accomplish this, simply follow the procedures described
in the Call for Proposals.


To specify a particular orientation of the apertures with the ORIENT special requirement of
the exposure logsheets (see Proposal Instructions, section 7.2 for a more detailed description),
place the object to be observed in the proper configuration on the entrance aperture shown
in Figures 8 and 9. This will determine the desired positions of the N and E directions on
the same apertures. The angle between these directions and the U3 axis drawn on these
figures (measured E from N) is the angle to specify in this special requirement.


The aperture configurations described in this section for the F/96 correspond to the position
determined during the first inflight calibrations of the FOC + COSTAR system. More similar
calibrations will be performed in Cycle 4 to assess the stability of these positions. No inflight
calibration has been performed for the F/48 yet. Actual images of the extended 512zx 1024
pixels squared pre-COSTAR F/48, pre-COSTAR F/96, and pre-COSTAR F/48 spectrograph
fields obtained in orbit with external flat field illumination of the FOC entrance apertures
are shown in Figures A1-A3 in the Appendix. The extended format 512zx 1024 pixel images
have been dezoomed and displayed in a 1024 x 1024 pixel format. The occulting fingers,
spectrograph slit, reseau marks appear clearly together with some blemishes and large scale
response inhomogeneities. The latter are discussed in more detail in Section 6.9.
4.3 INTERNAL CALIBRATION SYSTEM



When the shutter is closed, an Al + MgF2 mirror (see Figure 6) reflects the light beam
from a light emitting diode (LED) calibration source into the optical path of the relay. The
position of the source and the curvature of the mirror insure a quasi flat field illumination of
the object plane. The unit consists of seven LEDs (two red, two yellow, two green and one
blue) illuminating an integrating sphere. Their normalized emission spectra are shown in
20 FOC Instrument Handbook Version 5.0




Figure 10. Normalized Emission Spectra of the Calibration LEDs.

Figure 10. The unit is capable of illuminating both calibration mirrors simultaneously. Each
LED output can be set to 256 separately commandable intensity levels. The calibration
system will be used to determine the detector's intensity transfer function, the uniformity
of response, the FOC response to visible light and the geometric distortion. A ground-
based comparison between external and LED flat field illumination of the detectors at the
same wavelengths shows that the spatial variations of LED illumination are less than 3%
peak to peak over most of the field of view. The only exception is due to one edge of the
circular mirror on the back of the shutter for the new F/48 relay preventing LED light from
illuminating the upper left hand corner of the new F/48 frame. The calibration unit does not
experience the same vignetting of an external source. This must be taken into account when
FOC Instrument Handbook Version 5.0 21

comparing internal and external flat fields. As mentioned previously, the F/96 detector is
not expected to have any vignetting, nor is there any evidence of it. UV flat fields must be
obtained by observing an external UV source since there is no onboard UV lamp. UV flat
fields are currently obtained using observations of the inner region of the Orion Nebula.
4.4 FILTER WHEELS



The FOC has six commandable rotating filter wheels holding 58 optical elements and six
clear apertures. Four wheels are on the the new F/96 relay and two on the new F/48
relay. The filter wheels of the new F/96 camera have 3 long pass, 9 wide band, 20 medium
band, 2 narrow band and 5 neutral density filters. They also contain 3 polarizers and 2
objective prisms. The filter wheels of the new F/48 relay contain 3 long pass, 8 wide band,
and 3 objective prisms. A complete list of the optical elements ordered by increasing peak
wavelengths ~o is given in Table 3 for the filters of the new F/96 relay and in Table 4 for the
filters of the new F/48 relay. In these tables, FW indicates the filter wheel number (1-4 for
the new F/96, 1-2 for the new F/48), ID the filter identification code, ~0 the wavelength at
the peak of the curve representing the product of the filter transmission (T) with the OTA
+ FOC response function, (Q) described in Section 6.3, ~ the full width at half maximum
of this curve, and T(~0 ) is the filter transmission at ~0 , and QT(~0 ) is T(~0 ) multiplied by
Q. m gives the magnitudes of attenuation of the neutral density filters at 3000A. These
filters are placed on the wheels in such a way as to allow beam attenuation in increments of
1 magnitude from 1 to 9 magnitudes.


The filter wheel system of the new F/96 camera allows, in principle, up to 124 or 20,736 and
up to 82 or 64 different combinations of optical elements for the new F/48 relay. Clearly,
only a fraction of these will find a useful astronomical application. Observing configurations
requiring more than one filter on the same wheel are not possible, of course. Filter positions
on the wheels were carefully selected in order to minimize this possibility. The time required
to change some filter combinations may reach 3 minutes. This implies a considerable expense
in overhead time for programs requiring extensive cycling between filters.
4.4.1 Bandpass_and_Neutral_Density_Filters__________________



In general, the long pass filters are Schott colored glass combined with a low pass filter, the
wide band filters are metallic UV filters, the medium band filters are multi-dielectric multi-
element with ZnS-Th F4 layers, and the interference filters are multi-dielectric multi-element
with ZnS chiolithe layers. The measured transmission versus wavelength curves for all filters
and attenuators for the new F/96 relay and the new F/48 relay are shown in Figures 11-15.


In order to suppress ghost images, the external faces of all mono-element filters are parallel
to within 5 arcseconds or better. For multi-element filters the tolerance is 1 arcminute. The
cemented elements have a wedge angle of 1O or less. In order to minimize losses in the
modulation transfer function, the external faces are flat to ~/5 peak to peak at 6300A and
22 FOC Instrument Handbook Version 5.0

FOC Instrument Handbook Version 5.0 23

24 FOC Instrument Handbook Version 5.0


Figure 11. Transmittance of the long pass and wide band filters on the filter wheels of the
new F/96 relay as a function of wavelength.
FOC Instrument Handbook Version 5.0 25


Figure 12. Transmittance of the visible medium and narrow band filters in the filter wheels
of the new F/96 relay as a function of wavelength.
26 FOC Instrument Handbook Version 5.0




Figure 13. Transmittance of the UV medium band filters on the filter wheels of the new F/96
relay as a function of wavelength. The F120M and F130M filter transmission
curves remain essentially flat at 10-4 beyond ' 2500A.
FOC Instrument Handbook Version 5.0 27


Figure 14. Transmittance of the neutral density filters on the filter wheels of the new F/96
relay as a function of wavelength.
28 FOC Instrument Handbook Version 5.0


Figure 15. Transmittance of all the filters on the filter wheels of the new f/48 relay as a
function of wavelength.
FOC Instrument Handbook Version 5.0 29

the internal faces in the multi-element filters are flat to ~/2 peak to peak. The refractive
index is homogeneous to a level of n< 2 . 10-5 to be consistent with the ~/5 flatness
constraint. These conditions have been complemented by the introduction of appropriate
tilt angles of the different filter wheels themselves. Transmission non-uniformities are held to
within 5% over the whole surface. Ground tests of the FOC with a point source projector
have been performed with all filters. A few have been found to exhibit faint ghost images
and image shifts. These filters are flagged in the Comments column.
4.4.2 Objective_Prisms_________



The objective prisms consist of either a single 30 millimeter diameter, 3 millimeter thick
wedged crystal of MgF2 (the FUVOP and FOPCD, called PRISM 1 and PRISM 3 in the
Instructions) or a combination of two wedged crystals of MgF2 and SiO2 glued together (the
NUVOP called PRISM 2 in the Instructions). The former operates down to 1150A with a
wavelength resolution ~/ ~ ' 50 at 1500A while the latter has a dispersion ~/ ~ ' 100 at
2500A but transmits only above ' 1600A. All of the prisms disperse in a direction oriented
roughly anti-parallel to the increasing line number (L) direction except FOPCD on FW #
1 of the new F/48 camera that, instead, disperses in a direction roughly perpendicular to
L or about 90O to the others. This last one is meant as a cross disperser (CD) for the long
slit spectrograph (see Section 4.5). The MgF2 prisms (FUVOP and FOPCD) on the new
F/48 relay (Prisms 1 and 3) are both preceded by a 3 mm. thick CaF2 window in order
to reduce geocoronal Lyman alpha contamination. These prisms, therefore, have negligible
transmissions below ' 1250 A.



The essential features of the FOC objective prism facility are listed in Table 5 and illustrated
schematically in Figure 16 for the FUVOP of the new F/96 relay. The left hand side of this
diagram corresponds to a view of the extended format for the new F/96 relay in the same
orientation as the one shown in Figure 8 and!-approximately to scale. The direction of
dispersion of the prism is represented by the vector I emanating from the center C96 of the
format and making an angle ` with -L with ` increasing clockwise-from!-L. The spectrum
of an object located at C96 will lie along the line defined by I . The position of any specific
wavelength is defined then by giving the linear coordinate x in pixels from C96 on this line
with negative values-for!positions above C96 (towards +L), positive below it (towards -L)
consistent with the I directions shown in Figure 16.



A blow-up of this spectrum extending from 1200 to 6000 A as dispersed by the FUVOP is !-
shown on the right hand side of this figure where the solid curve gives the position x along I
of any wavelength for this case. The reciprocal of the slope of this curve yields the resolution
R in A/pixel given in the figure for several representative wavelengths. Values of the linear
coordinate x(~), R(~), T(~) the transmission of the prism and the value of ` for each prism
is listed in Table 5 as a function of wavelength. Please note that the angle ` for the prisms of
the new F/96 relay increases clockwise from -L while it increases counterclockwise from -L
for the prisms of the new F/48 relay due to the different orientation of the new F/48 format
shown in Figure 9. The position of the entire dispersed FOV with respect to the undispersed
30 FOC Instrument Handbook Version 5.0

FOV is also shown in Figure 16. The former is displaced upwards by 5.88 millimeters at the
red limit at 6000A at the upper edge and 9.96 millimeters at the far UV limit at 1200A at
the lower edge of the field.



A composite image showing the central 256 x 512 pixels of the undispersed image of a star
in the post-COSTAR F/96 256 x 1024 image and its associated Far-UV objective prism
spectrum is shown in Figure A4 in the Appendix. In this 256 x 1024 centered format, the
star was placed well below the center of the format so that in the next image, the dispersed
FUVOP spectrum is roughly centered. The two images were then co-added to produce
Figure A4. It should also be apparent from an inspection of Figure 16 and Table 5 that
careful consideration must be given to the positioning of the format and/or the target object
in the format in order to ensure that the ensuing spectrum falls on the correct part of the
frame. This is especially critical for the FUVOP's that have a large offset and a spectrum
length which is a sizable fraction of a typical field of view. The simplest way to handle
this problem is through judicious use of the POS TARG special requirement described in
the Proposal Instructions. Suppose, for example, that one desires to place a particularly
interesting feature in the spectrum of an object located at ' 1500 A close to the center of
the image for the new F/96 relay listed in Table 6 using the FUVOP. According to the data
given in Table 5 and the situation illustrated in Figure 16, one would specify a POS TARG 0,
-4:4 because 1500A falls 312 pixels or 4.37 arcseconds from the undispersed position of the
object in the positive Y(or L) direction specified on Figure 10.2 of the Proposal Instructions.
4.4.3 Polarizers____



The FOC polarimeter consists of three MgF2 double Rochon prisms located on FW1 in
the new F/96 relay. Each prism consists of an optically contacted double Rochon prism
combination acting as a three element birefringent beam splitter. The pass directions of the
prisms are at 0O , 60O , and 120O , counterclockwise from the image X axis (-S direction) as
projected onto the sky. A schematic drawing of the device is shown in Figure 17.



The optical axes of the outer components A and B are oriented perpendicular to the optical
beam axis while the central component C has its optical axis parallel to the beam axis. The
entrance face is at the base of the central prism. In this configuration, the ordinary ray
is transmitted without deviation while the extraordinary rays are deviated by the interface
between the outer and central prisms. Thus, three exit beams emerge from the polarizer.
The orientation of polarization is parallel to the face of the octagon to within 5 arcminutes
and the external faces are parallel to within 5 arcseconds. This insures that the wavefront
distortion is less than ~/10 at ~ 6328A. The beam deviation ffi depends on the ordinary and
extraordinary indices of refraction and the prism wedge angle. These parameters were chosen
such that the angular separation of the beams will yield a central undeviated 11 arcseconds
squared image without overlap of the two orthogonally polarized beams. Thus, ffi = 1:155O
for ~ =1300A and 1.165O for ~ =6328A. In these conditions, the images on the focal plane
of the new F/96 relay will be located as shown in Figure 18 for the three prisms.
FOC Instrument Handbook Version 5.0 31

32 FOC Instrument Handbook Version 5.0



Figure 16. Optical layout of the focal plane of the new F/96 relay with the FUVOP inserted
in the beam. The star is assumed located at C96 in the entrance aperture of
Figure 7.
Figure 17. The Physical Layout of the FOC Polarizers. Dimensions are in millimeters.
FOC Instrument Handbook Version 5.0 33



Figure 18. Image configurations on the focal plane of the new F/96 relay for the three
polarizers.
Figure 19. The major principal transmittance TMAJ and the minor principal transmittance
TMIN of the three FOC polarizers as a function of wavelength.
34 FOC Instrument Handbook Version 5.0

The major principal transmittance (TMAJ ) of the undeviated beam through the three prisms
and the minor principal transmittance (TMIN ) of the normal nonpolarized light are given
in Figure 19. Notice that one of the polarizers (POL60) does not transmit below ' 1800A.
There are limitations on the accuracy which is attainable with this facility. A major factor
is that the three different polarizers have somewhat different throughputs, even longward
of 3000 Angstroms. While the filter transmissions have been measured, filters do change
with time, and color variations in the source will result in small differences in the actual
throughput. In addition, the light reaches the polarizers only after reflection off the OTA,
COSTAR and the FOC primary and secondary mirrors (but before the fold mirror). Each of
these six reflections is at a non-zero angle of incidence, ranging from a few minutes of arc for
the OTA primary to about 11.5 degrees for the FOC secondary. Such reflections introduce a
phase shift in incident polarized light and a slight instrumental polarization, but these effects
are below the calibration accuracy. The reflections off the COSTAR optics are at angles of
incidence that are comparable to the angles for the FOC optics, so we expect the polarizers
to perform as well as before COSTAR. At the time of the writing, post-COSTAR calibration
of the F/96 polarizers has not yet been performed.
4.5 LONG SLIT SPECTROGRAPHIC FACILITY



Due to the high background count rate and high-voltage turn-on problems which
have occurred in the F/48 relay (see Section 6.4), the F/48 camera will not be
made available in Cycle 5, pending further testing and data analysis.



This facility consists of the following four elements:

1. a rectangular (0.063 x 12.5 arcsecond) slit placed on the new F/48 camera entrance
aperture at the OTA tangential focus as shown in Figure 9,

2. order sorting bandpass filters and/or a cross dispersing objective prism on the filter
wheels of the new F/48 relay,

3. a removable toroidal convex mirror which picks off the new F/48 beam between the
folding mirror and the DHU and reflects it towards

4. a fixed spherical concave reflection grating which reimages the slit spectrum onto the
detector photocathode as shown in Figure 6.
The last two optical elements are slightly tilted and decentered with respect to the
optical relay axis of the new F/48 in order to center the spectrum of the slit onto the
photocathode. Since, as shown in Figure 9, the slit is considerably offset from the extended
28 x 28 arcseconds squared normal imaging field, its image falls outside the scanned area of
the photocathode in the normal imaging mode. Only when the convex mirror is placed into
the new F/48 beam does the dispersed image of the slit become visible on the scanned area
in the position indicated in Figure 9. The effective wavelength range of the device in first
order is 3600-5400A, in second 1800-2700A, in third 1200-1800A and in fourth 900-1350A.
The MgF2 window of the detector limits this last range to 1150-1325A.
The spectrograph mirror and the grating are both made of Zerodur overcoated with
Al + MgF2 with a reflection efficiency exceeding 0.7 beyond 1200A. The grating is ruled
FOC Instrument Handbook Version 5.0 35

with 150 grooves mm-1 and a blaze angle of 1.94O for maximum efficiency at 4500A in first
order. Its radius of curvature is 94cm and the angle of diffraction is 2.6O . This implies
a linear dispersion at the photocathode of 71, 36, 24, and 18 A mm-1 and, with a beam
diameter of ' 20 mm., a theoretical resolving power of ' 3000, 6000, 9000 and 12000 for the
four orders, respectively. The FOC spectrograph resolution, however, is limited, in practice,
by the slit size and the OTA Point Spread Function (PSF) that correspond to ' two to
three 24 micron pixels. Using the Rayleigh resolution criterion, the actual resolving power
of the instrument is ' 1150 in all orders with a spectral resolution of 4, 2, 1.3, and 1A for
first, second, third, and fourth orders respectively. These values have been confirmed by
ground-based calibration using line source stimulation.
Both the spectrograph mirror and the grating work with unit magnification. The con-
vex mirror corrects the astigmatism introduced by the spectrograph's optical elements. The
resulting image is nearly free of astigmatism and image tilt with respect to the photocathode
plane. The fixed grating configuration of the long slit spectrograph implies that light from
all orders falls simultaneously on the same area of the detector. Because of the limitations of
the UV bandpass filters, any order may be contaminated with light from another, resulting in
possible ambiguities in line identification and degradation of achievable signal to noise ratio
(S/N) due to line or continuum overlap. This can be a serious problem in some applications,
especially those involving objects with a bright visible spectrum where the spectrograph's
overall quantum efficiency peaks.
Even in the most complicated situations, however, it is still possible, at least in prin-
ciple, to separate the different orders by executing a number of exposures with judiciously
chosen bandpass filters. Light from the first order, for example, can be unambiguously iden-
tified by means of the F305LP filter that completely blocks radiation below 3000A. Since
the filter transmissions are well known, shorter wavelength information can be recovered
from a confused spectrum by appropriately subtracting the calibrated data. The F220W
for the second order, F150W for the third and F140W for the fourth may be considered
as the standard FOC spectrograph order sorting filters but others may be selected, at the
discretion of the observer, instead of or in addition to these for more specialized applications.
This procedure can always be used at the expense of increased observation time required
by the multiple exposures and of degraded S/N due to the effectively increased background
uncertainty. For extended sources larger than 1.6 arcsecond in size this is the only viable
alternative.
For objects of limited spatial extent (including point sources), the four overlapping
orders can be spatially separated by using the FOPCD objective prism as a cross disperser.
The position of the four orders on the detector field of the new F/48 relay in this case is shown
in Figure 20. The prism dispersion direction PD is orthogonal to the grating dispersion
direction GD and close to antiparallel to the increasing sample number direction S. The
reader is referred to Figure 9 for a broader perspective of this viewing configuration. The
largest achievable physical separation between orders is 7 pixels (0.20 arcseconds) between
the first (I) and second (II), 15 pixels (0.42 arcseconds) between the second and third (III)
and 36 pixels (1.01 arcseconds) between the third and fourth (IV) order. The background is
significant only for wavelengths which are harmonics of bright geocoronal lines like OI, 1304A.
This option is very attractive because of its high efficiency due to the spectral multiplex
gain of a factor of 4 and to the gain of a factor of 3 - 5 resulting from the elimination
36 FOC Instrument Handbook Version 5.0

of the bandpass filters. The CaF2 blocking filter on the FOPCD effectively removes the
contaminating effect of the bright geocoronal line at Lyman ff at 1216A.


Figure 20. Optical layout of the new F/48 focal plane in the spectrograph mode and the
FOPCD in the beam. The coordinates are line numbers (L) as ordinates and
sample numbers (S) as abscissas. Notice the different scales for S and L.


The open area to the right of the dotted line in the extended format of the new F/48
relay in the spectrograph mode shown in Figure 9 and Figure A3 of the Appendix is normally
blanked out by selecting the 256zx 1024 format with initial sample and line positions 160 and
0. It should be kept in mind, however, that dispersed light from this part of the aperture
is still falling on the photocathode and, if the field here is very bright (a bright galactic
nucleus, or the central part of a nebula, for example), some contamination of the right edge
of the slit spectrum due to scattering should be expected. On the other hand, this area can
be exploited for slitless high spatial and spectral resolution observations of compact sources.
For this purpose, the user should specify the standard format 512 x 1024-CD described in
Table 6. Provided the undispersed source is placed with a POS TARG special requirement
FOC Instrument Handbook Version 5.0 37

near the end of the opaque spectrograph finger in the new F/48 image aperture shown in
Figure 9, a full two dimensional series of quasi monochromatic images of the object will
appear in this special format which has the initial sample and line positions set at 512,0.
Because of the 4.6 arcseconds offset of the slit from the finger along the L direction, the
wavelength range covered by the slitless mode is slightly different than that of the standard
slit mode i.e., ' 3300 to 5100A in first, 1650 to 2550A in second, 1150 to 1700A in third
and 1150 to 1275A in fourth.
The slitless mode is, obviously, most advantageous when dealing with compact line
emission sources that minimize the risk of overlapping monochromatic image contamination
and is not recommended for extended continuum sources. One important advantage of the
slitless mode for point sources is the elimination of the uncertain slit function which depends
on the OTA PSF at the time of observation and the accuracy of the target acquisition
procedure that places the object on the narrow spectrograph slit. A disadvantage is the
rapidly decreasing sensitivity with increasing distance of the object from the slit finger due
to vignetting, which can be seen in Figure A3.


4.6 DETECTORS

The transfer optics described in Section 4.1 relay the image produced on the OTA
focal plane to the photocathode of a two dimensional photon counter drawn schematically
in Figure 21. The detector consists of three basic parts: a three stage image intensifier tube,
a coupling lens, and a TV camera tube.

Figure 21. Physical Layout of the Imaging Photon Counting Detectors


4.6.1 Image_Intensifier_and_Coupling_Lens_________________

The intensifier is an EMI 9614 three stage tube magnetically focused by means of a
permanent magnet. The first-stage photocathode (like the following two) is a hot bialkali
for the highest quantum efficiency in the UV-blue region and the lowest dark-count rate at
17O C. It has a useful diameter of 40mm and is deposited on a MgF2 input window.
38 FOC Instrument Handbook Version 5.0

The photoelectrons generated at the first stage are accelerated by a 12 kilovolt poten-
tial and impinge on a P11 phosphor layer coupled by a 4 micron thick mica membrane to
the second photocathode. This amplification process is repeated in the second and third
stages to achieve an overall photon gain of 1.3 x105 . Focusing of the intensifier electrons is
accomplished with a carefully shaped permanent magnet assembly and a trimcoil is added
around the third stage for fine adjustments.
The limiting spatial resolution of the intensifier is 35 line-pairs per millimeter. The
dark current at an ambient temperature of 17O C is less than 10 counts cm-2 s-1 (10-4
counts pixel-1 s-1 in the normal mode). Both of these characteristics are essentially limited
by the first stage of the intensifier tube.
A lens assembly consisting of 9 components in a double Gaussian design is used to
transfer the image from the output phosphor of the intensifier to the fiber-optic faceplate
of the TV camera. It is designed to operate at f/2.7 with a slight magnification (1.15) to
compensate for the demagnification of the image intensifier. The 80% energy width for point
object images varies between 22 and 35 microns over the whole of the useful area and the
light transmission is more than 60%.


4.6.2 TV_Tube_______

The camera tube is a Westinghouse WX32719 low-light TV tube. This is a high-
sensitivity, high resolution EBS tube (Santilli and Conger in Photo-Electronic Devices,
AEEP, ed. L. Marton, 33A, 1972) with a 25 millimeters square diode array target, magnetic
focus and deflection coils and an electrostatically focused image section with a 40 millimeter
diameter useful photocathode area. The S-20 photocathode is evaporated onto the concave
inner surface of the input fiber-optic faceplate. The emitted photoelectrons are accelerated
by a potential of up to 12 kilovolts and focused onto the target which is an N-type silicon
wafer with diffused P-type regions arranged in an hexagonally-packed diode array.
During operation each diode is reverse biased. Incoming photoelectrons generate
electron-hole pairs which discharge the diodes. An amplified charge pattern correspond-
ing to the image is then stored in the diodes. The charge flowing in the target lead, when
the scanning beam recharges the diodes, is the signal current. The target gain is about 2500
and the modulation transfer function is 50% at 8 line-pairs per millimeter. The video signal
coming from the TV tube is amplified by the preamplifier and then transmitted to the Video
Processing Unit (VPU).


4.7 VIDEO PROCESSING UNIT

Each camera has its dedicated VPU which accepts the amplified signals from the
camera preamplifier. The purpose of the VPU is to determine the x-y centroid of each event,
determine if a true photon event has occurred and to increment the SDS memory address
corresponding to the location where the photon event was detected. During any one scan of
a frame of duration of 30 milliseconds for the 512 x 512 format down to 520 microseconds
for the smallest 64 x 64 format there will only be a few scattered photon events. A photon
event is typically a spot with a diameter of 3 or 4 pixels. It is read by the scanning beam on
successive lines of the raster scan. Figure 22 illustrates how such a signal would look using
the z axis to represent the magnitude of the charge.
FOC Instrument Handbook Version 5.0 39

As a line is scanned, a gaussian shaped pulse is produced. As successive lines are
scanned, additional pulses, corresponding to slices of the event, increase in peak amplitude
until a maximum is reached. The pulse amplitude then decreases. This video signal is
amplified and presented to the VPU which takes the incoming video lines and produces two
signals needed to analyze the waveform, Peak Signal and Extent Signal. The Peak Signal
corresponds to the point of maximum amplitude of an event on a single scan line or slice.
The Extent Signal is used to determine the time or extent of the event during a single line.
A true photon event is present on several successive lines. Analysis of these events
characterized by the peak and extent signals on successive lines is the task of the VPU. By
using delay lines and shift registers, each event is examined in a 4 x 9 pixel area by real
time analysis so that the same event on successive lines can be analyzed. The z-dimension
event center is tagged in the x and y direction. The Pattern Recognition Logic analyzes the
event's shape to determine true photon events and reject other noise and ion events.

Figure 22. Schematic Drawing of a Typical Raster Scan Output of the Detectors


4.8 SCIENCE DATA STORE

As an event is detected and classified as valid, the video processing unit causes the
science data store (SDS) to increment by one the memory location corresponding to the
event centroid. The image is gradually built up over the exposure time. After stopping the
exposure, the SDS can be read out without disturbing the stored image. The SDS can accept
a 512 x 512 line image in the 16 bit word mode or a 512 x 1024 line image in the 8 bit
word mode. The cycle time for the SDS is compatible with the camera scan rate of 106
pixels per second. This rate is maintained for all formats and zoom. In this latter mode,
the camera read beam scans the target twice as fast in the line direction as it does in the
normal imaging mode but the pixels are twice as long. The detector generates an increment
command for every pixel in which a photon event has been detected. A scan of 256K pixels
occurs in less than 30 milliseconds while a scan of 4K pixels (i.e., a 64 x 64 pixel squared
format) takes place in approximately 512 microseconds.
There are two different interface circuits in the SDS (SDS-1 and SDS-2), with unit 1
dedicated to the camera of the new F/48 relay and unit 2 to the camera of the new F/96
40 FOC Instrument Handbook Version 5.0

relay. The SDS memory is physically divided into 22 modules of 16K 16-bit data words
each holding 32 words from each of 512 lines, but only 16 modules are active at any given
time. These 16 modules are accessed in sequence to reduce the required memory cycle time.
Each SDS word has 22 bits, with the extra 6 bits being used for "single-bit" error correction
and "two-bit" error detection. Included in the engineering telemetry are error detection and
correction bits set for each logical module. If more than 6 SDS memory modules fail, the
memory can still be operated in a reduced data mode. In this case, zeros will appear in the
downlink for those modules that are not available. Data loss occurs from the "right-hand"
side, so if only 15 modules are up, words 0-479 for each of the PDA lines of 512 pixels would
be obtained.
The SDS can be operated in either the normal imaging mode or the SDS dump mode
so it is necessary to interrupt the pixel increment commands from the detector to read out
the SDS memory to the downlink. Each readout is a dump of the 256K 16-bit words of SDS
memory, and hence contains 4M data bits regardless of the image format. Readout of science
data is normally done under control of the NSSC-1, which controls the gating of signals to
the Remote Interface Unit (RIU) of the SI C&DH including the Science Data Formatter
(SDF). From the SDF, the data is fed to the downlink or the tape recorder.
FOC Instrument Handbook Version 5.0 41

5.0 OBSERVING_ARRANGEMENTS____________________________



5.1 IMAGING, OCCULTATION AND SPECTROGRAPHIC MODES

Operationally, the observation of an astronomical source with the FOC is defined once
the following physical parameters are specified:

1. The configuration or optical relay (the new F/96 or the new F/48 camera)

2. The positions of the filter wheels (4 for the new F/96, 2 for the new F/48 camera)

3. The spectrograph mirror position (in or out of the new F/48 beam)

4. The SxL imaging format (S 512, L 1024 pixels with S the number of SDS pixels in
the line scan direction and L the number of SDS pixels in the increasing line direction)

5. The word length (8 or 16 bits per word)

6. The pixel size (normal 24 x 24 microns squared or zoomed 48 x 24 microns squared)

7. The position (S0 , L0 ) of the starting pixel. This can be specified with a least increment
corresponding to 0.25 pixel both in the S and L directions. Telemetry monitoring,
however, can only verify the starting pixel with an accuracy of 32 pixels.

8. The position of the target in the chosen format.
Due to the high background count rate and high-voltage turn-on problems
which have occurred in the F/48 relay (see Section 6.4), the F/48 camera will
not be made available in Cycle 5, pending further testing and data analysis.
In general, selections 4, 5, and 6 have to be made consistent with the 4 Mb SDS
memory size limitation. This means that the word length is completely defined, once the
format is selected, as all formats larger than 512 x 512 pixels squared will automatically
require an 8-bit word length while any format of that size or less will be imaged with 16-bit
words.
Obviously, only a small fraction of all the possible observational modes allowed in prin-
ciple by the FOC will find practical astronomical application and, therefore, be accurately
and extensively understood and calibrated prior to use. Table 6 lists the main characteristics
of the anticipated standard imaging and spectrographic observing modes for the new F/96
and the new F/48 relays. The first column in this table gives the format size (SxL), the
second the pixel size in microns and arcseconds squared in the sky, the third the starting
pixel (S0 , L0 ), the fourth the word length, the fifth the zoom configuration, the sixth the
overall field of view in arcseconds squared for that format, the seventh the maximum count
rate per pixel NMAX for that format (see Section 6.2.1), and the last column lists the main
scientific justification for the selection, the mode names used in the Proposal Instructions
for HST, if appropriate, and the proposal entry required for that format. S0 is given in the
1-1024 range on a dezoomed extended format.
For the spectrographic modes of the new F/48 relay, the pixel size and the FOV are
given in units of arcseconds x Angstroms in first order. The pixel size (selection 6 in the
list above) is considered an optional parameter in the Phase II Exposure Logsheet of the
ST Proposal Forms. The default value, if none is specified, is 24 x 24 microns squared for
imaging and 48 x 24 microns squared for spectroscopy. The position and orientation of the
target in the aperture (selection 8) should be specified in the Special Requirements column
of the Phase II Exposure Logsheets.
42 FOC Instrument Handbook Version 5.0

FOC Instrument Handbook Version 5.0 43

44 FOC Instrument Handbook Version 5.0

5.2 TARGET ACQUISITION MODES

Two acquisition modes are available for use with the FOC as described in the Proposal
Instructions: INTeractive ACQuisition (Mode I) and blind pointing (Mode III). On-Board
Acquisition (Mode II) is not available any longer due to technical limitations. In addition, an
EARLY ACQuisition can be specified where an examination of the field is necessary prior to
science exposures to help measure a target in a crowded field or to determine a slit alignment
angle, for example.


5.2.1 Mode_I_Target_Acquisition_-_INTeractive_ACQuisition__________________________

Designed to be the most accurate acquisition procedure for use with the corono-
graphic fingers, the slit or small image formats, this procedure requires the involvement of
the observer, in real-time, to identify the field and measure the center of the target on an
FOC image. First, using a blind pointing acquisition, the target is placed in a standard field
of view and an image is taken, with instrument parameters such as filters and exposure time
selected by the user. The resulting image of the field is then read down for immediate display
in the Observation Support System (OSS) area at ST ScI. Once the observer identifies the
target using an interactive image display system and measures the target position from the
display screen, a slew request is generated and up-linked, and the telescope is maneuvered
to place the target in the selected destination. There is no verification of the subsequent
field unless specifically requested by the observer on the exposure logsheets.
INTeractive ACQuisition is expected to be the standard acquisition procedure for
OCC mode. For the long slit spectrograph, which has a slit width of only 0.06 arcseconds,
INTeractive ACQuisition is the only safe way to ensure proper centering of a point source.
The procedure also may be useful in IMAGE mode when an object needs to be placed in a
particular place in one of the imaging apertures or when using very small fields of view.
In IMAGE mode, objects of 9th magnitude or brighter require a more accurate IN-
Teractive ACQuisition strategy to ensure the safety of the detector. The following steps will
be automatically scheduled whenever an acquisition is required of a target brighter than 9th
magnitude:
o Step 1 will be the first acquisition exposure taken with sufficient neutral density and
color filters to ensure the FOC is operating in a safe regime. The spacecraft will be
pointing at a blank field 30" away from the bright target. It is the responsibility of the
observer to include this pointing offset in the target coordinates, and to coordinate this
with the Instrument Scientist. The image is downlinked to OSS where the Instrument
Scientist will check that the proper filter configuration is in place and will agree on the
small angle maneuver to bring the bright target into the FOC field. In an attempt to
reduce the overhead times, this first exposure will be a DARK, and will be executed
during Earth occultation. If and only if the proper filter configuration is in place, the
spacecraft will be commanded to slew to the bright target. The exposure time for
the first image is selected by the user.
o If the filters are properly placed, Step 2 will be the scientific exposure, with the same
filter configuration and format used for the acquisition. If the telemetry indicates that
the correct filters are not in place, no maneuver will be executed, and the observation
will continue on the blank field.
FOC Instrument Handbook Version 5.0 45

In general, the interactive nature of this acquisition, requiring TDRSS links, means
that the procedure is classed as a "limited resource" in the Proposal Instructions. Fur-
thermore, our initial testing of the acquisition procedure suggests that it is extremely time
consuming for the user, and therefore must be used only when strictly required. The current
estimates of the overhead times give 1 orbit for each Bright Object Acquisition. The proce-
dure must be repeated whenever a different filter configuration is selected. We are currently
working on an alternative Bright Object Acquisition procedure which will be fully automatic
and will execute in a much shorter time. Please contact the Instrument Scientist for further
details.


5.2.2 Mode_III_Target_Acquisition_-_Blind_Pointing_____________________

Mode III is the default acquisition procedure for the FOC. When no target acquisition
is specified in the Special Requirements section on the proposal exposure logsheets, the
telescope performs a straight-forward blind pointing on the coordinates provided by the
user. After the acquisition, no explicit verification of the target position in the fields of view
aperture is performed. No overhead time in addition to the guide stars lock time is charged
to the user for this acquisition.
Mode III is expected to be the standard acquisition procedure for IMAGE mode in
all instrument configurations, except for those formats with fields of view of less than 3
arcseconds square, providing the target has a coordinate position measured with the Guide
star selection system Astrometric Support Package (GASP) ensuring best accuracy with
respect to the guide star astrometric catalogue. The procedure will not be useful for point
sources in either OCC or SPEC modes, because blind pointing is unlikely to provide the
precision needed to accurately place a target on the slit of the new F/48 relay (width = 0:06
arcseconds) or on one of the coronographic fingers. However, for extended objects where
accurate pointing is not important, blind pointing can be used. Again, targets of magnitude
9 and brighter cannot be acquired on the fingers using blind pointing. In this case, see Mode
I.


5.2.3 EARLY_ACQuisition______________

When necessary, it is possible to take an acquisition image some time before the
scientific observation. The acquisition image can be used for a better identification of the
field, or a better evaluation of the source flux, etc. In order to update the observation
parameters, a minimum turnaround time of two months is necessary between the acquisition
and the science exposure.
Presently, the system does not have the capability to select for the science observation
the same pair of guide stars successfully used for the acquisition exposure, thus eliminating
the possibility of using the acquisition image to measure the target coordinates at the accu-
racy level required to perform, for example, a blind pointing on the new F/48 0.06 arcseconds
slit.
46 FOC Instrument Handbook Version 5.0



5.3 THE FOC TARGET ACQUISITION APERTURES

For convenience, a number of special formats to be used in the Mode I acquisition
exposures have been defined. These formats (listed in Table 7) have offsets that have been
chosen in order to optimize the small angle maneuvers necessary to move the targets to the
required locations. Their positions have been calibrated during SV and are being monitored
for stability and electronic distortions to ensure good pointing accuracy.
For example, in addition to the usual centered formats, a number of acquisition
formats have been defined which are conveniently located close to the 0.3 arcseconds and
0.5 arcseconds fingers of the new F/96 relay. Different sizes are available for each of these
combinations, but it is strongly recommended to use the larger images due to the initial
position error of the target's coordinates with respect to the Guide Stars (' 0.33 arcseconds).
For the acquisition to the spectrographic slit of the new F/48 relay, the 512 x 512 format is
recommended.
FOC Instrument Handbook Version 5.0 47

6.0 INSTRUMENT_PERFORMANCE____________________________



6.1 THE POINT SPREAD FUNCTION (PSF)

Before COSTAR, the HST PSF suffered from severe spherical aberration, which meant
that a circular aperture of 0.1 arcsecond radius contained only 15-18% of the light from a
star instead of the expected 70%. The principal effect of the spherical aberration was a loss
in sensitivity, because most of the light in the halo of a faint star is effectively lost in the
background noise. COSTAR has restored much of the OTA capability, in that the COSTAR-
corrected PSF contains more than 75% of the light within a radius of 0.1 arcsecond at visible
wavelengths while only losing less than 20% of the light to the two reflections at the two
extra mirror surfaces. The net increase in sensitivity is a factor of approximately 3_4 at
visible wavelengths. The correction COSTAR made to the PSF is illustrated in Figure 23,
which shows the radial profile of a aberrated image and a COSTAR-corrected image.
Figure 23. Radial profiles of pre-COSTAR aberrated PSF (dotted) and COSTAR-corrected
PSF (solid at Hfi).
48 FOC Instrument Handbook Version 5.0

In Table 8, the encircled energy fraction ffl (~ ) is tabulated for various circular
aperturesq_against__the_ number of pixels in the aperture and the effective radius (defined
as #pixels=ss ), with the definition that the encircled energy is 1.0 at a radius of 1 arc-
second (70 pixels). A more thorough discussion of the definition of the encircled energy
calculation and the rationale behind it is given in the Detective Quantum Efficiency section,
6.3. The improvement in performance over the aberrated PSF is shown in Figure 24, where
the encircled energy curve is compared to that of the aberrated OTA and with a perfect
diffraction-limited image from a 2.4m circular aperture with a 0.33 central obstruction. It
can be seen that the COSTAR-corrected FOC PSF approaches that of an ideal imaging
system in both encircled energy performance and in the FWHM of the PSF core.
Despite the outstanding performance of the OTA+COSTAR+FOC imaging system
in terms of encircled energy within small radii, the PSF appearance does not match a true
diffraction-limited simulation perfectly at all wavelengths. All of the COSTAR alignment
was done using the F486N filter, and it can be seen that the first diffraction ring shows a
non-uniform azimuthal intensity distribution (Figure 25a). There is also some residual coma
that varies with time, possibly due to some slack in the M1 tilt mechanism.
FOC Instrument Handbook Version 5.0 49


Figure 24. The encircled energy fraction and PSF profile for the COSTAR-corrected F/96
and pre-COSTAR F/96 relays compared to those expected from a perfect diffrac-
tion limited OTA.
In the ultraviolet, the PSF shows a fairly strong jet-like feature pointing approxi-
mately in the -V3 direction (Figure 25b). The strength decreases with increasing wavelength
but is still quite noticeable at 4000A. The cause of this feature and the asymmetry in the
first diffraction ring is unknown.
Three filters have been found to have artifacts. The F372M filter shows a strong
linear feature in the PSF wings, at approximately 45O to the OTA spider (Figure 25c). The
F501N and F502M filters both show a faint ghost image approximately 60 and 24 pixels
respectively from the PSF center.
A selection of PSF images is available on STEIS, the STScI archives. STEIS contains
proposal information, exposure catalogs, and STSDAS software releases (including the in-
strument filter and DQE tables) and is accessible via anonymous FTP, gopher, or WWW's
Mosaic interface at stsci.edu.
50 FOC Instrument Handbook Version 5.0




a. b. c.
Figure 25. Images of PSFs taken with the COSTAR-corrected F/96 camera. a. F486N filter
b. F120M filter c. F372M filter.
6.1.1 Image_quality_and_Field_Dependence_of_the_PSF________________________

The FOC was designed to image the HST focal plane in an off-axis position, 6.56
arcminutes from the optical axis. At this distance, the focal plane is tilted with respect to
the V1 axis by 10O . It is this plane that the FOC cameras image onto their photocathodes.
However, the focal surface produced by COSTAR is tilted with respect to the plane that the
FOC images. This results in a field-dependent focus variation of approximately 0.7mm over
the full field of the new F/96 relay. Similarly, the tangential and sagittal focal surfaces are
tilted with respect to each other, and this introduces field-dependent astigmatism. Both of
these effects increase linearly with distance from the fully-corrected field point.
The field-dependence of the PSF was investigated during SMOV, but the limited
observations do not allow detailed characterization of the performance as a function of field
position. It is clear from calibration observations that the PSF is visibly different away from
the central field point across the largest formats. However, this does not affect the encircled
energy within 0.1 arcsec radius by more than a few percent for any field position within
the 512X512 aperture. A more thorough evaluation of the field dependence of the PSF will
follow in a future Instrument Science Report.
FOC Instrument Handbook Version 5.0 51

6.2 DYNAMIC RANGE

If two or more photon events overlap during a given frame, the VPU detection logic
will only count one detected photon. This `undercounting', or non-linearity, sets a hard limit
on the maximum allowable photon rate for the FOC. This limit depends on the frame scan
time, which is proportional to the area in pixels of the selected format. Users can improve
the linearity performance by choosing a smaller imaging format (but at the cost of field of
view).
The linearity performance also depends on the image structure. If the illumination is
nearly uniform, then the non-linearity depends on the frame scan time and the photon event
size (typically 3 x 3 pixels). However, if the illumination comes from a perfect point source,
then the photon event size does not matter, since there are no neighboring photon events,
only those that arrive in the illuminated pixel. For FOC images, no source is truly pointlike,
but the linearity characteristics of astronomical point sources and flat fields are sufficiently
different that they are discussed separately. The situation for more complex image structure
(i.e. just resolved or linear) will of course be intermediate between these two limiting cases.


6.2.1 Uniform_Illumination___________

Here `uniform illumination' refers to the case where the intensity varies by less than
20% over scales of 20 pixels. The frame scan time is given by

z(S x L)
Tf = _____________sec
8:8 . 106
where z = 1 for normal and z = 2 for zoomed pixels. For the most widely-used format (512
x 512, normal pixels), this comes to 30 milliseconds. If, during a frame, more than half
of the format area is occupied by photon events, a further event will overlap one or more
existing events and will not be counted as a detected photon. This would predict a maximum
count rate for the 512 x 512 format of 0.05 counts/pixel in 30 milliseconds, or about 1.7
counts/sec/pixel. In practice the saturation level is reached at a lower level, because most of
the overlapping events are much larger than a single photon event and are classified as ion
events and rejected.
The flatfield nonlinearity was measured on-orbit during OV and SV using observations
of the internal LED calibration lamps. It was found that the intensity of the light from these
lamps is directly proportional to the commanded intensity level. The linearity curve was
measured for several different formats to verify the format dependence of the saturation, and
the dependence on zoom mode was also investigated. A plot of the measured linearity relation
for the 512 x 512 formats of the pre-COSTAR F/96 and pre-COSTAR F/48 detectors is
shown in Figures 26a and 26b, respectively. Superposed is a curve that describes the behavior
of the linearity relation for intensity values up to approximately 80% of the saturation value,
originally suggested by Jenkins (M.N.R.A.S., 226, 341 (1987)):

-ae
r = a(1 - exp( _____))
a
where r is the measured count rate, ae is the `true' count rate and a is a fitting parameter
that is identified as the asymptotic measured count rate.
When the FOC is configured for zoomed pixels, the linearity performance is slightly
different from what would be expected from scaling the results for normal pixels by the ratio
52 FOC Instrument Handbook Version 5.0

of frame scan times. This is because the event sizes and the VPU detector logic are different
for zoomed pixels. However, once the linearity performance of one format is calibrated for
each camera in zoom mode, the performance at other formats in zoom mode can be derived
by scaling by the ratio of format areas, as is the case for normal pixels. The validity of the
scaling assumption for both normal and zoomed pixels was checked and found to be true.

Values of a for the most commonly-used formats are given in Table 9. The values
refer to dezoomed data in the case of formats that were originally zoomed. In practice, the
value of a depends somewhat on position in the image, since it is effectively a measure of
the photon event size, and this varies over the format due to slight focus quality variations.
To ensure that non-linearity does not compromise the science data, users are advised to
ensure that the count rate is kept below NM AX , which is the count rate that would give
10% nonlinearity, as given in the third column of Table 9. Correct and quantifiable operation
of the FOC at count rates exceeding NM AX cannot be guaranteed.


6.2.2 Non-Uniform_Illumination______________

When the illumination comes from a star, the FOC is able to count at a much higher
rate before saturation occurs. This is because photon events centered on pixels close to the
central pixel of a star are much less probable than in the flatfield case. However, because it
is difficult and time-consuming to obtain stellar images at a large number of intensity levels,
it was not possible to calibrate the point-source non-linearity relation for the pre-COSTAR
PSF to the same accuracy as could be achieved in the flatfield case. The dependence of the
core structure of the PSF on such factors as secondary mirror position, jitter and wavelength
also made such an investigation impractical.
FOC Instrument Handbook Version 5.0 53



a. b.
Figure 26. Flat-field linearity plots for the detectors in the pre-COSTAR F/48 and pre-
COSTAR F/96 relays, a and b respectively, based on 512 x 512 pixel flat-field
images of the internal LEDs at different intensities. The solid line in each plot is
the best-fit solution for the linearity function given in the text.


Figure 27. Linearity relation for point sources based on 512 x 512 pixel images taken with
the pre-COSTAR F/96 relay through the F342W filter.
54 FOC Instrument Handbook Version 5.0

During OV and SV it became clear that for the FOC pre-COSTAR F/96 camera
when used in the 512 x 512 format, the maximum obtainable count rate in the core of a
star was approximately 3 counts/second/pixel. Variations on this level occur because of
jitter, focus, etc. At count levels higher than this, the core of the star turns into a dark
`hole', and a bright crescent appears to one side of the core. Comparison of PSF's taken
with and without neutral density filters indicate that there is no noticeable deviation from
linear behavior for core count rates up to 1 count/sec in the brightest pixel. This produces
the simple guideline for observers: keep the count rate in the central pixel below 1
count/sec, and then any photometry method chosen will give results that are not biased
by non-linearity effects. If the central count rate is above 1 count/sec, one will need to use
other methods to determine the brightness of the star (such as measuring the intensity in
the bright halo). Table 8 give
An example of the non-linearity relation for point sources is shown in Figure 27. This
is a plot of the brightest pixel intensity for stars in two FOC pre-COSTAR F/96 512 x 512
images of the center of the globular cluster M15. One image was taken with F342W+F2ND
filters, while the other was taken with F342W+F2ND+F1ND filter. Apart from the large
scatter, which is understood, one can see that there is no measurable deviation from a linear
relation for central count rates up to 1.8 counts/sec. Clearly, any photometry method that
includes the brightness of pixels other than the brightest pixel will be even less susceptible
to non-linearity effects.
To put some hard numbers around these figures, note that a 20th magnitude A0V
star will give a total count rate with the F342W filter of 10.9 counts/sec. Referring to Table
8, one can see that for the new F/96 relay, the central pixel contains 8.7% of the total light,
therefore the expected count rate for this star is 0.95 counts/sec in the central pixel, which
is at the FOC linearity limit. To observe stars brighter than 20th magnitude, it is necessary
to either use a smaller format, neutral density filters or a narrower filter (e.g. F346M).


6.3 ABSOLUTE QUANTUM EFFICIENCY

Spectrophotometric standard stars were observed during SMOV and Cycle 4 Cali-
bration using a variety of filters to allow measurement of both the PSF characteristics and
the detector quantum efficiency. These measurements were made after the COSTAR mirror
tilts and DOB focus were set to optimize the imaging performance at 4860 Awavelength for
the new F/96 camera. To date, no attempt has been made to characterize the new F/48
camera, so this section refers to the new F/96 camera only.
The encircled energy and detector quantum efficiency are somewhat coupled since
the PSF does not have a well-defined edge; instead the flux drops steadily with distance
from the star center until it gets lost in the background noise. The flux in the wings is due
to scattering by dust and small imperfections in the OTA+COSTAR+FOC optical train,
and is more pronounced at shorter wavelengths. When constructing an encircled energy
curve, which is the curve of the fraction of light enclosed within a circular aperture of a
given radius as a function of radius, one naturally has to define how one measures the total
flux. In the past, this was done by choosing an aperture size that was large enough to
include the spherically aberrated PSF, or about 3.5 arcseconds radius. This aperture size
could comfortably fit inside the workhorse 512X512 imaging format before COSTAR was
installed.
FOC Instrument Handbook Version 5.0 55

56 FOC Instrument Handbook Version 5.0


Figure 28. Baseline overall (OTA + COSTAR + FOC) absolute quantum efficiency in counts
photon-1 as a function of wavelength for the three imaging modes and the four
long slit spectrograph orders.

With COSTAR, the magnified plate scale means that such a large aperture size cannot
be used for DQE measurements, particularly since most of the measurements of spectropho-
tometric standards were made using the 256X256 imaging format to improve the linearity
performance. For this reason, it was decided to define the encircled energy to be 1.0 at
a radius of 1.0 arcsecond (70 pixels) and to define the background as that value which
minimizes the scatter of the points in the encircled energy curve with 0.8" < r < 1.0". In
practice, this is equivalent to setting the background to the value measured at approximately
0.9" radius, and it does give encircled energy curves that are qualitatively in agreement with
what such a curve would look like: the encircled energy asymptotically approaches a constant
FOC Instrument Handbook Version 5.0 57

value at the last measured points. Users should be aware that there is some flux outside 1
arcsecond radius, especially in the ultraviolet, but this flux is not considered "useful" and
its contribution to the total DQE is not included. The net result of using this approach is
that the DQE appears to be lower than the values presented in the previous version of the
FOC Handbook, particularly at ultraviolet wavelengths. There are three reasons for this.
Firstly, a larger fraction of flux is scattered into the region between 1.0" < r < 3.5" and is
not counted towards the absolute sensitivity. This amounts to approximately 6% of the total
flux for the F140M filter. Secondly, when the background is determined at a radius of 0.9
arcseconds, one is effectively subtracting more background than would make the encircled
energy curve flatten asymptotically at larger radii. This effect amounts to about 7% of the
total flux for this filter. Lastly, the COSTAR reflectivities that had been used to predict the
DQE in the previous version of the Handbook were based on those measured just after the
COSTAR mirrors had been coated; in the year between coating and launch the reflectivity
had degraded by a few percent due to small amounts of molecular contaminants and some
dust covering introduced in the COSTAR vibration testing. The apparent "loss" compared
to the previous Handbook is offset by a corresponding increase in encircled energy at all radii,
since the total flux is made smaller. All of this sounds like a long-winded discussion, but it
is merely to explain why the DQE measurements presented here are significantly different
from those in the previous version of this Handbook. An even more thorough discussion is
given in FOC Instrument Science Report FOC-080.
The fluxes of the spectrophotometric standards within 1" were compared with syn-
phot predictions. The spectrophotometric standards had been recalibrated using the best
model of the white dwarf star G191B2B to redetermine the IUE sensitivity calibration (a
correction of approximately 10% in the 1200-2000 Awavelength range). It was found that
the observed/expected flux values depended on wavelength linearly for the reasons outlined
in the previous paragraph, so the DQE curve was transformed by this linear function to
derive the new DQE curve.
The overall (OTA + COSTAR + FOC) central absolute quantum efficiency Q(~)
in counts photon-1 with no filters in the beam is plotted and tabulated as a function of
wavelength in Figure 28 and Table 10 for the four FOC imaging and spectrographic con-
figurations. The data represent the product of in-orbit measurements for the new F/96
relay+OTA absolute quantum efficiency, and ground-based reflectance calibrations of the
COSTAR mirrors for the new F/48. The predicted loss of light from two reflections of MgF2
coated aluminum COSTAR mirrors amounts to a 20% loss in the visible and a 35% loss
in the ultraviolet. The loss due to the COSTAR mirrors is more than compensated by the
improvement in image quality, since the encircled energy performance is improved from 18%
within a 0.1" radius to '80% within the same area, based on theoretical PSFs.
The spectrograph efficiency is shown for the four orders of the grating (I, II, III and
IV) with no order sorting filters in the beam. For the new F/96 relay, errors do not exceed
20% while, for the others, errors in the 2000-6500A range for the imaging modes should
not exceed 20% and for wavelengths below 2000A they are expected to be of the order
50%. This latter uncertainty should be applied to all the spectrograph data especially in
the orders III and IV.
58 FOC Instrument Handbook Version 5.0

6.3.1 Format-dependent_Effects_____________

It has been found that the DQE is a function of detector format (see Instrument
Science Report FOC-075). The cause of this is not known. The relative sensitivities for each
format are given in Table 11, where the 512X512 format is set to 1.0 by definition. The DQE
values given in Table 10 and Figure 28 refer to the 512X512 format.
6.4 DETECTOR BACKGROUND

The detector background arises primarily from thermal electrons at the first photo-
cathode and high energy particles. In the 600 km altitude, 28O inclination orbit of HST,
substantial fluxes of magnetospheric electrons and protons are encountered in the South
Atlantic Anomaly (SAA). The more energetic of these particles are capable of generating
intense flashes of C^erenkov radiation in the MgF2 faceplate of the FOC intensifiers. Since
this noise source originates as photons at the very front end of the detector, the Video Pro-
cessing Unit of the FOC is not able to distinguish between real celestial photons striking the
cathode and C^erenkov generated photons.
The threshold energy for C^erenkov radiation in MgF2 is E > 220 keV for electrons
and E > 400 MeV for protons. Shielding of 4 mm aluminum or more was built into the
design of the FOC in order to prevent electrons of energies E < 3 - 5 MeV from reaching
the detectors from any direction.
The effects of the SAA on the FOC were extensively mapped during the commission-
ing phase. The FOC turned out to be considerably less sensitive to SAA electrons than had
been feared. This is presumably due to the additional shielding to electrons provided by the
rest of the HST spacecraft. The response of the FOC to SAA protons on the other hand
is in good qualitative agreement with the expectations_although the sensitivity of the two
FOC detectors differs somewhat.
The highest background rates (0.2 counts pixel-1 s-1 in the new F/48 and 0.02 counts
pixel-1 s-1 in the new F/96) are encountered over South America within the peak of the
FOC Instrument Handbook Version 5.0 59

SAA proton density distribution. Since these rates are not high enough to cause damage
to the FOC detectors, the FOC is kept fully operational during SAA passages. However,
such high background rates do exclude useful scientific observations. A ground-track contour
delineating the observed region of high background has been installed within the HST ground
system in order that FOC observations not be scheduled within it. Users of the FOC need
therefore not concern themselves with avoiding the SAA under normal circumstances (i.e.,
periods not having unusually high solar activity). The typical detector background rates
experienced well outside the SAA are 7 x 10-4 counts pixel-1 s-1 in the detector for the new
F/96 relay and 2 x 10-3 counts pixel-1 s-1 in the detector for the new F/48 relay. Upward
fluctuations of a factor 3 from these minimum values are, however, seen throughout the
HST orbit. The minimum in-orbit background rates are, respectively, factors of 5 and 3
higher than the background rates measured during ground testing implying that the bulk of
detector background counts are particle induced.
In addition to the orbital variations in the background rates, the detector for the new
F/48 relay has exhibited a localized region of high background count rates seen as a 'white
spot'. These high background events are most likely related to a delamination in the potting
around the intensifier tube between the 8kV and 12kV supply lines. The region of high
background rates is found near the center of the full format with an arc extending across the
top of the full format as well.
The count rates in these regions vary, but are significantly higher than the nor-
mal background with rates ranging from 2-10 times the nominal rate. The regions can
be seen in Figure A5 in the Appendix where the white spot has a count rate of 5:7 x
10-3 counts s-1 pixel-1 and a background region far from the spot only has a count rate
of 2:83 x 10-3 counts s-1 pixel-1 . The count rates in the white spot and associated arc had
been seen to be increase in intensity with usage. Any increase in the count rate in the region
of the white spot and arc would eventually limit the usefulness of this relay for science.
The increase in background preceded the first failure of the F/48 camera to turn on,
which occurred in September 1992. The high voltage tripped during its ramping up, at the
beginning of an observation sequence. The F/48 camera was switched on again successfully in
October 1992, and a number of darks and flat fields were taken which confirmed background
values consistent with the ones previously measured. The next switch on attempt (January
1993) failed, but the F/48 was again successfully switched on December 22, 1993, before the
deployment of the COSTAR corrective optics. The camera remained on for the duration of
the observation. While the acquisition image showed a background level consistent with the
previous dark count images, a preliminary analysis of the following frames showed immedi-
ately that the background increased dramatically with time, eventually reaching saturation
levels approximately two hours after HV switch-on. Figure 29 shows the background count
rate for all the exposures, where the triangles are the estimated background levels at the
center of the image in counts s-1 pixel -1 . Whether this characteristic of increasing back-
ground is a permanent condition of the F/48 is not clear. As a consequence, the F/48
will not be made available to GOs during Cycle 5, pending further testing and
analysis.
60 FOC Instrument Handbook Version 5.0

Figure 29. A steady increase in the background count rate has been observed during the
most recent F/48 relay turn-on.

6.5 STRAY LIGHT

Normally, the FOC background is dominated by the detector, by zodiacal light in the
visible and by geocoronal Lyman alpha and diffuse galactic light in the far UV (see Section 7
for detailed calculations of these components). When a bright object such as the Sun, Moon
or the bright Earth limb is nearby, however, it may be dominated by stray light reaching
the OTA focal plane due to scattering from the baffle system, the OTA tube and dust on
the mirror. The expected brightness of stray radiation at the OTA focal plane due to the
proximity of the Moon or bright Earth limb in the daytime part of the orbit in V magnitudes
arcsec-2 as a function of the angle between the Moon or the limb and the OTA axis is shown
in Figure 30. The two curves shown correspond to the two values of the primary mirror's dust
coverage of 2% and 5% presently estimated to bracket the expected range of this parameter
in orbit. The spectral shape of the stray radiation in the case of the Earth can be assumed
to be, for most practical purposes, that of the Earth's average daylight nadir radiance given
in Figure 31.
The average zodiacal light background of 120 S10 corresponding to V ' 23 magni-
tudes arcsec-2 is reached at angles greater than 80O to the limb, approximately. For viewing
configurations in which the angle is less than this value, stray light will dominate in most
situations. One of the most interesting of these is that encountered when observing in the
continuous viewing zones (CVZ) which, in principle, allows for long uninterrupted integra-
tions of very faint sources. Due to the altitude of the spacecraft and the depression of the
FOC Instrument Handbook Version 5.0 61

horizon, the off-axis angle to the Earth limb in the CVZ will be in the range 20O - 44O ,
approximately. From Figure 30, the expected stray light illumination in this configuration
in the visible will be between 18th and 20th magnitudes arcsec-2 . This means that ob-
servations in the visible will be limited mainly by this source of background. Specifically,
assuming that one wishes to observe a B ' 25 magnitude A0V star with the new F/96 relay
and the F430W blue filter with an accuracy of 10%, a background of this type of average
brightness V = 19 magnitudes arcsec-2 in the CVZ requires an exposure time of 45 minutes
or almost the entire daylit part of the orbit. At night, such an accuracy would be obtained
in 16 minutes. Below 3000A, this effect will be negligible as shown in Figure 31. Non-CVZ
observations can also have bright limb approaches of 20O - 44O unless DARK TIME, which
is very inefficient, is specified.
This particular example also shows that there is no advantage in exploiting the CVZ
for long integrations or scheduling efficiency if the object is fainter than about 23rd magni-
tude since the gain in signal is more than offset by the increased background. Closing the
shutter during the daylight pass, in other words, is recommended in this scenario. It is, there-
fore, of more than passing interest to the observers to pay some attention to the maximum
allowable background they can tolerate for their specific observation and to communicate
this information to the ST ScI in the Phase II proposal submission.



Figure 31. The Earth's average daylight nadir radiance in Rayleighs A-1 i.e., 106 =4ss pho-
tons cm-2 sec-1 A-1 sr-1 as a function of wavelength.
62 FOC Instrument Handbook Version 5.0




Figure 30. Stray light illumination in V magnitudes arcseconds-2 at the OTA focal plane
due to the Moon and daylit Earth as a function of off-axis angle.

6.6 DETECTOR OVERLOAD

The FOC detectors described in Section 4.6 may be damaged by illumination levels
exceeding 107 photons s-1 pix-1 at the photocathode due to point sources and by an average
illumination from a diffuse source over the whole photocathode exceeding 104 photons s-1
pix-1 . Because of this danger, the 36 kV HV power supply on the 3 stage image intensifier
is set to trip off when the point source illumination exceeds the value given above or if the
average illumination from a diffuse source exceeds 200 photons s-1 pix-1 . Thus, for safety
reasons, no point source delivering more than 106 photons s-1 pix-1 at the photocathode or
a diffuse source delivering more than an average rate of 100 photons s-1 pix-1 over the whole
photocathode will be allowed to be imaged by the FOC. These values correspond to an 9th
magnitude blue star or a diffuse source of surface brightness 10 magnitudes arcsec-2 viewed
through the F430W filter with the new F/96 relay. Targets brighter than this limit can still
be observed by the FOC, provided that 1) enough neutral density filters are selected, so that
a linear countrate is observed by the FOC, 2) the Bright Object Acquisition procedure is
used to acquire the target onto the FOC aperture.
FOC Instrument Handbook Version 5.0 63

6.7 OVERHEAD TIMES AND MULTIPLE EXPOSURES

Assuming that the standard science data dump operations at the 32 kHz rate apply,
it will take a constant 3.9 minutes plus a variable component to transition from the absolute
time tag to stop an exposure to the absolute time tag to start the next one. The variable
component depends on the mode change required and can be up to another 3.5 minutes for
the worst case with the new F/96 relay ( 4 filter wheels) to 1.9 minutes for the worst case
with the new F/48 relay ( 2 filter wheels ). Thus, it could take up to a total of 7.4 and
5.8 minutes of time between successive exposures with an average of approximately, 6 and 5
minutes for the new F/96 and the new F/48 relays, respectively. In some specific situations,
it may be advantageous, in order to save time, to take multiple exposures without closing
shutter or dumping science data. Up to 11 consecutive exposures of this type can be made.
If no changes to FOC mechanisms are required, the time interval between exposures can be
reduced to 23 seconds total. This is the fastest rate at which the FOC presently can be run
provided the telescope can be slewed to a new position on the detector quickly enough to
permit it.
All the overhead times reported here are to be considered approximated, and should
not be used for an accurate calculation of the required time (See the Proposal Instructions
for a detailed and accurate description of all the relevant overheads).


6.8 GUIDING MODES WITH THE FOC

If no special requirements are placed on the guiding tolerance of the HST, (see HST
Phase II Proposal Instructions), the FOC will default to fine lock (estimated RMS jitter
0.005 arcseconds out of day/night transitions) for all configurations. These defaults can
be overridden with the guiding tolerance special requirements for situations which do not
strictly require the highest possible guiding accuracies. In this case the observer might
require coarse track (estimated RMS jitter of 0.015 arcseconds). Since this could prove quite
beneficial in terms of overhead time (20 minutes for fine lock and 0 minutes for gyro hold),
the user is encouraged to think carefully about his real requirements in this area. Gyro hold
with an absolute position error of 6000 arcseconds and a drift rate of 0.01 arcseconds s-1
is not expected to be used very often with the FOC but could find interesting applications
for purely photometric measurements.


6.9 UNIFORMITY OF RESPONSE (FLAT FIELDING)

The extended format (512zx 1024) geometrically corrected flat fields for both of
the new relays are shown in Figures A1-A2. The new F/48 image shows the approximate
location of the new default 512 x 512 format, which is no longer at the center, but close
to the upper right quadrant. The flat fields were obtained from overlapped observations of
the inner region of the Orion Nebula and are at 3727 and 1360A respectively. The flat fields
show a number of various types of features, some more subtle than others. The more evident
features are the occulting fingers for the new F/96 relay, the slit finger for the new F/48
relay (used as a fiducial reference to the spectrograph slit) and the reseau marks. Because
of the geometric correction, the edges of the original raw images can be seen as curved edges
in these images, mainly on the left and right sides.
64 FOC Instrument Handbook Version 5.0

Because of the large amount of time necessary to obtain external flat fields for the
FOC, these two UV flat fields (one for F/96 and one for F/48) are currently the only UV
flat fields for the FOC. We plan to obtain another UV flat field at about 2200A during 1994.
In all images, regardless of format, a number of pixels at the beginning of the scan line
(i.e., starting at S = 1) are corrupted by defects in the beginning of the scanning sawtooth
waveform. The number of pixels corrupted depends on the detector and format. Generally
it is about 5% of the scan line for the new F/96 relay and relatively independent of format,
whereas for the new F/48 relay it gets progressively worse with smaller formats (for the 128
x 128 format it is as much as 25% of the scan line). The faint horizontal stripes seen at
small L values are due to a ripple instability of the coil drivers at the beginning of a frame.
The narrow line running from the bottom left corner to the upper right corner is due
to the read beam not being completely blanked when it is forced to fly back to S = 0; L = 0
at the end of the frame. This feature is more noticeable with the smaller formats. The narrow
horizontal features at the right edge, especially at L = 256; 512; 768, are due to noise glitches
on the scan coil driver caused by changes in the most significant bits of the line counter. For
both relays, the center 512 x 512 is seen outlined in larger formats. This effect is due to a
burn-in of that heavily used format in the camera target so that a charge discontinuity at
the edges of the format has appeared. The edges of a square baffle located just in front of the
detectors limit the extended field of the new F/96 relay at the upper and lower left corners,
the extended image field of the new F/48 relay on the upper left corner. The broad vertical
bands (bright and dark) seen near the beginning of the scan line arise from ripples at the
beginning of the scanning sawtooth waveform. The bands occur as a result of the varying
pixel size which is a consequence of the varying scan rate at the beginning of the scan. A
proper geometric correction would remove this effect; however, the distortion has a spatial
scale smaller than the reseau spacing and thus does not get corrected. This effect should be
corrected for in the near future. It must be noted that since the normal Routine Science Data
Processing (RSDP) calibration of the images always uses the appropriate section of a full
format flat field to flatten images obtained in all formats, these bands will not be flattened
out in smaller formats (this is not necessarily undesirable, as will be discussed later).
The remaining features fall into two categories, large scale and small scale features.
The large scale variations are due either to vignetting (significant only for the new F/48 relay)
or detector response. The expected vignetting for the full the new F/48 full field format is
shown in Figure 32a as a contour plot. Contours are shown as percentage transmission. The
expected vignetting has not been included in the flat field for the new F/48 relay shown
in Figure A1 so that features closer to the edge can be better seen. Figure 32b shows the
vignetting function along the long slit.
Contour plots of smoothed flat fields, including the effects of vignetting for the new
F/48 relay, are shown in Figure 33. A gaussian with a FWHM of 9 pixels was used to smooth
the image and the result was normalized to 100 at the center. Figure 34a and 34b show a
plot of row 300 of a UV flat field for the new F/48 relay and the new F/96 relay respectively
to give a better idea of the size of the flat field variations.
All previous indications are that the relative variations in large scale response as a
function of wavelength between 1300 and 6000A are weak; generally speaking, the large scale
response does not change more than 10% at all pixels except at the edges and corner of the
full format. Figure 35a and 35b show contour plots of the ratio of the Orion Nebula derived
FOC Instrument Handbook Version 5.0 65


a. b.
Figure 32. a. Contour plot of the vignetting function for the new F/48 relay across the
entire photocathode, with the location of the primary 512 x 512 imaging format
shown (dotted line). b. Plot of the vignetting function along the spectrographic
slit.



a. b.
Figure 33. a. Contour plot of the smoothed flat field for the new F/48 relay, including the
effects of vignetting. b. Contour plot of the smoothed flat field for the new F/96
relay.
66 FOC Instrument Handbook Version 5.0

flat fields to those obtained from the onboard LEDs for the new F/48 and the new F/96
relays respectively.
Beyond 6000A, the flat fields begin to change significantly, generally with poorer
relative sensitivity towards the corners. Although the changes in the large scale response with
wavelength are relatively minor, the changes in the fine scale features is more pronounced.
Scratches and other small scale defects deepen in the far UV; for the new F/96, some scratches
exhibit as much as a 30% decline in sensitivity in the far UV. Another source of fine scale
nonuniformity is the presence of patterns_unfortunately not fixed. Although not always
easily seen in low count extended areas or flat fields, there are two different patterns always
present. The more noticeable one is an approximately sinusoidal pattern with the peaks and
troughs oriented at an approximately 45 degree position angle and a period of 3.35 pixels for
the new F/96 relay. It is believed to originate from a moir'e effect between a TV tube grid
and the diode array on the target. The RMS amplitude of this pattern is approximately 5%
for the new F/96 relay and 2.5% for the new F/48 relay (the peak deviations from a flat
response due to this pattern are at least twice these values). This pattern becomes intensified
when count rates are in the nonlinear regime and thus is much more easily seen. In fact, it
is a quick way of recognizing serious nonlinearity in an image. The pattern noise disappears
at very low count rates.
A second pattern arises from some form of interference with an FOC digital timing
waveform that has a 4 pixel period. It shows up as vertically striped patterns on the flat fields.
Although very coherent in nature with regard to orientation and frequency, the details of the
modulation do not appear to remain constant from image to image. The RMS amplitude of
this pattern is approximately 2.5% for both relays.
There also appears to be an intrinsic granularity in the fine scale response, i.e., effec-
tively random pixel-to-pixel variations in response which has not yet been well characterized.
Some comments on flat field calibration are in order, especially with regard to the
routine pipeline calibration. Small drifts in distortion of the order of a pixel result in misreg-
istration of fine scale features such as scratches between the flat field and the science image.
Flat fielding the data in this case actually worsens the effects of the fine scale features by
correcting the wrong pixels. For this reason and because FOC flat fields are of relatively
low signal to noise (typically 300-500 counts per pixel), the flat fields used in RSDP are
heavily smoothed to eliminate most of the fine scale features. As a consequence, they are
not corrected for in the calibrated outputs of RSDP.
Unsmoothed flat fields are available from STEIS, and by using them, it is often
possible to improve the flat fielding by the appropriate registration of the flat field to the
science image. But this requires scientific judgment and must be applied on a case-by-case
basis.
As previously mentioned, the startup oscillations in the scanning waveform result
in uncorrected distortion at the beginning of the scan. The observer should keep in mind,
however, that although it appears to be a flat fielding issue since it manifests itself in flat
fields, it does not affect the total flux from an object_it just redistributes it. If one is doing
aperture photometry on stars, for example, one is likely to introduce more errors in flattening
out the effect than in leaving it alone. The ultimate solution would be the improvement in
the geometric correction files to remove the effect by means of geometric correction. It is
expected that this will be done by mid-1994.
FOC Instrument Handbook Version 5.0 67




a. b.
Figure 34. Plots across row 300 of the UV flat field for the new F/48 relay (a) and the new
F/96 relay (b). The effect of vignetting has not been included in plot (a).

a. b.
Figure 35. Contour plot of the ratio between external UV flat field and internal LED flat field
for the new F/48 relay (a) and the new F/96 relay (b) based on pre-COSTAR
data. The expected effects of vignetting on the ratio for the new F/48 relay are
not included. The center of each plot has been normalized to 1 with the contours
at intervals of 2.5%.
68 FOC Instrument Handbook Version 5.0

One final effect should be mentioned. Although it is not a flat field issue, it appears to
many at first glance to be one, and so it will be explained here. Many observers see a fringe
or fingerprint type of pattern in the background of their calibrated images where the fringes
are of relatively low spatial frequency_usually of periods of 20 or more pixels. It is a result
of the geometric correction applied to the data. It does not appear in the raw data. What is
being seen is not alternating areas of darker and brighter background, but rather, alternating
areas of higher and lower variance in the poissonian noise of the background. This effect
arises from the resampling algorithm used in the geometric correction_essentially what one
is seeing is the effect of the pixels in the output, geometrically corrected image drifting in
and out of phase with the corresponding pixels in the input, distorted image. Those pixels
mapping directly to the center of a pixel in the input image result in little or no effective
smoothing, while those which map to a point in between pixels in the input image will be
an average of the input pixels and thus have smaller variance in the noise. A small amount
of further smoothing to the geometrically corrected image will virtually eliminate the effect.
The pattern is identical in all images as long as they use the same geometric correction file.
The achievable relative photometric accuracy depends on many factors, of course, and
no simple rule of thumb will apply to all analysis. In many cases the accuracy depends on
the amount of work an observer is willing to do to calibrate his data. For RSDP calibrated
files, one should not expect the large scale accuracy to be better than 3-5% over the central
region of the format, and should expect errors as large as 10% closer to the edges (much
higher very close to the edges). Fine scale features can introduce large pixel-to-pixel errors
(i.e., scratches and reseau marks). Scratches and blemishes can be dealt with by careful flat
fielding. It is sometimes possible to remove the pattern noise with special techniques. Most
important is to avoid placing targets on or near areas with serious photometric problems if
possible. That is, keep targets of interest way from the edges of the format, burn-in regions,
A/D glitches and known blemishes if more accurate photometry is desired.


6.10 VISIBLE LEAKS

Although the FOC narrow and medium band filters are the very best present technol-
ogy can provide, they do exhibit a residual transmission of ' 10-3 - 10-4 between 5000 and
6000A where the detectors are still relatively sensitive. Consequently, indiscriminate use of
these filters to isolate faint UV features from a bright visible background can lead to serious
errors. The magnitude of the error is, of course, very sensitive to the precise shape of the
spectrum of the source to be observed throughout the sensitive range of the FOC. Thus, it
is not always sufficient to know only the expected flux of the source in the range ~0 ~=2
in order to estimate the expected count rate.
A striking example of a possible observing scenario that can be expected when imaging
a bright visible source in the UV is shown in Figure 36. In the example shown in this figure,
the source spectrum is assumed to increase sharply with increasing wavelength in the manner
expected from an M supergiant star. If this source is fed into the new F/96 relay with the
F231M filter on FW#3 in the beam, the resulting monochromatic count rate as a function of
wavelength through the entire OTA+COSTAR+FOC system is shown by the curve marked
F231M. The actual observed count rate in this configuration, of course, corresponds to the
integral of this curve.
FOC Instrument Handbook Version 5.0 69

Figure 36. The expected monochromatic count rate as a function of wavelength for the
new F/96 relay and the F231M filter or the F231M+F220W filters in the beam
for an extended source whose spectrum varies as the curve marked SOURCE
SPECTRUM. The source flux units are photons cm-2 sec-1 A-1 .

If the F231M filter alone is used in this endeavour, the contribution of the flux within
the band 2330 115A is only ' 18% of the total of 39 counts sec-1 . The counts originating
from the region ~ > 2580A represent, in contrast, 71% of the total. In this admittedly
extreme case, the thus derived UV brightness would be highly suspect, to say the least.
Solutions to this problem are not easy to find but, at least for the new F/96 relay, one simple
device would be to introduce a second cleverly selected filter into the beam in addition to
the original one. This selection should be geared towards maximizing the suppression of the
visible leak while minimizing absorption in the UV bandpass of interest.
In the case worked out in Figure 36, for example, the F220W filter on FW#2 is ideal
as shown by the curve marked F231M+F220W. Now, the in-band fraction of counts amounts
to 69% while the visible leak is only 5% or less of the total. The exposure time required
to reach a S/N=10 in this case increases by a factor of six mainly because of the effective
suppression of the visible counts.
Unfortunately, the new F/48 relay with its much smaller filter complement has far
less flexibility in this regard than the new F/96 relay. In this case, another possible solution
to the problem is to use the objective prisms to physically separate the UV from the visible.
This technique works best for point or, at least, compact sources where spatial and spectral
overlap is minimized. But even for extended sources, appropriate positioning of the target
70 FOC Instrument Handbook Version 5.0

with respect to the dispersion axis of the prism can work quite well. At that point, the only
remaining problem is to insure that the overload limit of the detector (described in Section
6.6) is not violated for the visible part of the image.


6.11 GEOMETRIC DISTORTION AND STABILITY

Because of the nature of the detectors, and the off-axis location of the instrument, the
raw FOC data suffers from geometric distortion, i.e., the spatial relations between objects on
the sky are not preserved in the raw images produced by the FOC cameras. This geometric
distortion can be viewed as originating from two distinct sources. The first of these, optical
distortion, is external to the detectors and derives from the off-axis nature of the instrument
apertures. The second, and much more significant source of distortion is the detector itself.
Geometric distortion is a fact of life when dealing with detectors containing image
intensifiers, primarily because intensifiers rely on an electric field for accelerating, and a
magnetic field for focusing the photoelectrons. Any variation in the uniformity of either
results in image distortion within the intensifier. Photon positions are then further distorted
in the process of "reading-out" the TV tube's target, firstly because the read-out beam is
performing an angular sweep across a plane target, and secondly because of non-uniformities
in the scanning rate of the beam. For this reason, each video format has individual distortion
characteristics, and so unfortunately, the distortion measured for one format cannot be used
to correct the distortion of an exposure taken in another format.
In order to measure and correct for the detector distortion, fiducial reference points
(reseau marks) are etched onto the first of the bi-alkali photocathodes in the intensifier tube.
(Since these reseau marks only transmit about 10for all practical purposes they cannot be
flat fielded out.) These reseau marks form an orthogonal grid of 17 rows and 17 columns
with a separation of 1.5 mm (60 pixels), each reseau being 75 microns square (3 x 3 pixels).
The detector distortion can be determined by illuminating the photocathode with an internal
light source, (i.e., an internal flat field). The observed positions of the reseau marks, when
compared to the expected positions, provide a map of the detector distortion across the
field. The optical component of the distortion is determined independently from ray-tracing
models of the HST and FOC optics, and is applied to the reference reseau grid to give the
`expected' positions.
Although the geometric distortion arises from several sources, the correction of images
is carried out in a single step using a flux-conserving algorithm which maps values from the
raw, distorted image into a geometrically corrected image. A two-dimensional polynomial
transformation, which combines both the optical and detector distortion components, is
used to provide the mapping of distorted pixel coordinates to corrected coordinates. Figures
37 and 38 show as an example the magnitude of the distortion field as determined for the
pre-COSTAR F/48 and pre-COSTAR F/96 cameras using their 512zx 1024 formats, from
inflight calibrations.
The squares show where a regular grid of points on the sky (using a 60 pixel spacing)
should have appeared if there were no distortion; the ends of the line segments show where
the grid points actually appear in the distorted image. (The lines have been multiplied by
a factor of 2 to make them more easily visible, e.g., a line length of 50 pixels represents a
distortion displacement of 25 pixels). The pixel coordinates shown refer to normal, square
pixels, rather than the rectangular, zoomed pixel mode the images were obtained in.
FOC Instrument Handbook Version 5.0 71



Figure 37. The 512zx 1024 format distortion field for the new F/48 relay.
Figure 38. The 512zx 1024 format distortion field for the new F/96 relay.
72 FOC Instrument Handbook Version 5.0

In order to carry out geometric correction of FOC data, i.e., to recover an image in
which the spatial relationships between objects are restored, a necessary requirement is that
the geometric distortion field, shown in Figures 37 and 38, must be stable. By this we mean
that there must be no significant change in the observed reseau positions with time.
Short term variation of the geometric distortion pattern occurs during the period
immediately following FOC high voltage switch-on. During this time the observed reseau
positions show an RMS deviation from the stable positions of approximately 1-2 pixels. This
period however, extends for only about 40 minutes, by which time the reseau position have
stabilized to within 0.5 pixels, which is considered adequate for imaging purposes. In order to
avoid this period of instability, the scheduling software automatically inserts a delay interval
immediately following high voltage switch-on which prevents exposures being taken during
this time. Long term variations in the geometric distortion were expected to occur as a
result of desorption and out-gassing in the OTA and instruments, however given the time
since launch, the desorption curve is now considerably flatter and out-gassing should be very
near stable. Consequently, effects on distortion are much smaller and are taking longer to
materialize. Based on our experience;
a. The new F/96 relay continues to be very stable, the geometric variation in the new
F/96 relay has shown only about 1-2 pixels of movement over this period, (which theoretically
should be the worst time).
b. The situation with the new F/48 relay is somewhat less certain since monitoring
of the pre-COSTAR F/48 relay has shown it to be considerably less stable, with several
large, unexplained changes in the geometric configuration. Any future new F/48 data must
be considered to be poorly characterized with respect to geometric distortion.
Future determination of the new F/96 relay's geometric distortion will be based on
observations of dense starfields and thus should be able to correct for the high spatial fre-
quency variations in scan rate much better than the existing geometric corrections which are
derived from the reseau grid.


6.12 PLATE SCALE

The plate scale (i.e., the pixel size in arcseconds) has been determined for the two
cameras in the FOC before the deployment of COSTAR. This was done by taking a series
of images of a pair of astrometric stars, moving the telescope between exposures by a pre-
determined angular offset. The measured distances (in pixels) between the astrometric stars,
combined with the known separation (in arc seconds) then give us the plate scale.
For the new F/96 relay the plate scale is 0.01435 arcsec pixel-1 ( 0:0002) based on
observations of an astrometric star field with the post-COSTAR F/96 relay. Extrapolating
from the new F/96 results, the plate scale for the new F/48 should be 0.02825 arcsec pixel-1
( 0:0002).
These values are "radial" plate scales and are within a few percent of the nominal
values, which will be vis. 0.014 arcsec pixel-1 for the new F/96 relay, and 0.028 arcsec
pixel-1 for the new F/48 relay.
FOC Instrument Handbook Version 5.0 73

7.0 OBSERVER'S_GUIDE(PRESCRIPTION_FOR_ESTIMATING_EXPOSURE_TIMES)______________________________________________________________

The first step consists in specifying the required signal to noise ratio S/N or the
relative accuracy ffiN=N = (S=N )-1 of the measurement. Then, the exposure time required
to attain that accuracy is given, in general, for Poisson statistics, by:


t = (S=N )2 (RS + 2RB )R-2S (1)


where RS is the source rate and RB the background rate in an appropriate resolution element
in counts sec-1 . The problem then simply reduces to properly estimating RS and RB .

For a point source in the FOC field of view and for a count rate per pixel much less
than NMAX calculated from Table 9, the source rate is given by:

ss 2 Z 1
RS = __D (1 - p) ffl(~)F (~)Q(~)T (~)d~ (2)
4 o

where:

D = diameter of the ST primary = 2.4 meters
p = ratio of obscured area to total area of primary mirror = 0.138
ffl(~) = fraction of energy intercepted by the appropriate resolution element
F(~) = source flux at ST in photons cm-2 sec-1 A-1
Q(~) = FOC+OTA response function for T(~) =1 in counts photon -1
T(~) = transmission of filters or efficiency of dispersing elements
The terms in eq. (2) can be assumed to be appropriate averages over the pixel to pixel
variations in the instrument response function. Q(~) and T(~) are plotted in Figures 28 and
11 through 15.
The background rate, on the other hand, can be expressed, in general, as:
~ ss Z 1 ~

RB = nz Bp + __ D2 (1 - p) p I B (~)Q(~)T (~)d~ (3)
4 o
where:

n = number of normal (z=1) or zoomed (z=2) pixels in appropriate resolution element
Bp = inherent detector background count rate per normal pixel
IB (~) = specific intensity of diffuse background at ST in photons cm-2 sec-1 sr-1 A-1
p = solid angle subtended by a normal FOC pixel in steradians.
Equations (2) and (3) can be evaluated numerically or by approximating them by
assuming that the spectral passband is sufficiently narrow. This permits the following sim-
plifications:


RS ' 3:9 x 104 ffl(~0 )F (~0 )Q(~0 )T (~0 ) ~ (4)

" #
6:9 x 10 -10 B
RB ' nz Bp + ____________ I (~0 )Q(~0 )T (~0 ) ~ (5)
K

where all the relevant functions are evaluated at wavelength ~0 of peak response and ~ is
the FWHM bandpass of the instrument in Angstroms. The latter two parameters are listed
in Table 3. K takes on the numerical values 1 and 4 for the new F/48 and the new F/96
relays, respectively.
74 FOC Instrument Handbook Version 5.0

For an extended source, the size of the resolution element nz is determined by the user
according to his application. For a point source, the encircled energy tabulated in Table
8 should be used to determine ffl(~) and nz for each specific case. The precise area to be
used depends in general on the S/N ratio. If it is very high, one can afford to increase the
size of the resolution element nz to collect more photons, if it is low, nz should be kept as
small as possible. For any particular situation, there is an optimum nz at which the S/N is
maximum for a given exposure time t or at which t is minimum for a given S=N ratio. A
few quick calculations should be enough to locate this condition once the background has
been properly defined as indicated in the next paragraphs.
At least two sources of diffuse background have to be considered in estimating IB (~0 )
in eq. (5). The first is residual airglow above the ST altitude of 500-600 km. For the FOC
bandpass of 1200-6000A only two features need to be considered: the HI, Lyman ff line at
1216A and the OI, 1304A triplet. The latter feature need only be considered for daytime
observations. Their contribution to RB can be evaluated via the graphs shown in Figure
39. In this graph, the second term in the brackets in eq. (5) is evaluated for the three FOC
relays for the condition T(~0 ) = 1 as a function of spacecraft position in the orbit and for
a zenith oriented line of sight. Solar zenith angle 0O corresponds to local noon, 180O local
midnight. Lyman ff intensities can be expected to increase approximately a factor of 40%
if the line of sight drops to the horizon. RB can be determined by multiplying the data on
Figure 39 by the appropriate T(1216A) or T(1304A) and nz and adding to Bp .
The second source of background is zodiacal light, which can be an important con-
tributor to RB in the 3000-6000A range. This contribution as a function of wavelength is
plotted in Figure 40 for the three relays. An intensity of 90 S10 units (' 3 x 10-4 photons
cm-2 sec-1 sr-1 A-1 ) and a standard solar spectrum is assumed in these calculations. This
corresponds to a line of sight direction of ecliptic latitude fi = 40O and helioecliptic longitude
~ - ~ =85O . Thus, RB for the zodiacal light can be computed by multiplying the results
shown in Figure 40 by the appropriate nz T(~0 ) ~ and by the factor S/90 where S can be
computed for any target position by means of the data tabulated by Levasseur-Regourd and
Dumont (Astr. Ap., 84, 277, 1980) and reprinted here for convenience as Table 12.


7.1 POINT SOURCES

How all this works in practice is best illustrated by some examples.


7.1.1 Imaging_____

How all this works in practice is best illustrated by some examples. Say that you are
interested in observing an unreddened A0V star of mu =24 with an accuracy of 10% using the
F342W filter, and the new F/96 relay with normal sized (unzoomed) pixels for which z = 1.
From Table 3 for the F346M filter, you find that ~0 =3480A, ~ =434A, QT(~0 ) =0.045.
The stellar flux F (~0 ) = 8:0 x 102 x 10-0:4x24 = 2:0 x 10-7 photons cm-2 sec-1 A-1 .
Inserting these values into eq. (4), you get RS = 0:15 x ffl(~0 ) counts sec-1 . This is the total
count rate from that star spread out over a certain number of pixels corresponding to the
ffl(~0 ) chosen from Table 8. If the star is reddened by a given total extinction AV , you should
use a standard or average reddening curve (see Savage and Mathis, Ann. Rev. Astr. Ap.,
17, 73, 1979 for an example) to deduce the appropriate A~0 . Then, RS can be multiplied
by 10-0:4A~0 to take this effect into account in the simplest possible way. The possible
FOC Instrument Handbook Version 5.0 75

inaccuracies introduced by this method are probably not worse than the uncertainties on
the validity of the reddening curve itself and/or the prediction of the continuous flux to be
observed.
Next, calculate RB from eq. (5) using the data in Figures 39 and 40. Far ultraviolet
airglow is not going to be a factor in the B bandpass. The zodiacal light background per
pixel can be estimated by means of the data graphed in Figure 40 and Table 12. Suppose
the star is viewed at fi = 15O and ~ - ~ = 120O for which S=120 S10 units. Then, this
contribution is 2:3 x 10-7 x 120=90 x 0:58 x 434 = 8 x 10-5 counts sec-1 per normal pixel
so that, assuming Bp = 7 x 10-4 counts sec-1 per normal pixel, eq. (5) can be written as:


RB =n ' 7 x 10-4 + 8 x 10-5 = 8:0 x 10-4 counts s -1 pix -1


Then, the required exposure time can be easily computed from eq. (1) and the data in
Table 8 for ffl(~o ). Using the data in the column marked F346M, one obtains:
76 FOC Instrument Handbook Version 5.0


Figure 39. Residual 1216 and 1304A airglow contribution to the FOC background counting
rate with no filters in place in counts sec-1 per normal pixel as a function of
the solar zenith angle at the spacecraft at 500km altitude. The line of sight is
assumed to be oriented towards the zenith.
FOC Instrument Handbook Version 5.0 77

Figure 40. Zodiacal light contribution to the FOC background counting rate with no filters
in place in counts sec-1 A-1 per normal pixel as a function of wavelength. The
zodiacal light intensity is assumed to be 90 S10 units.

Thus, integrating under the PSF out to a radius of 0.036 arcseconds at n = 21 provides
enough flux for the required S/N to be achieved in a minimum exposure time of 1800 seconds.
If the background rate for some reason had been 5 times higher, the minimum exposure time
would have been 3500 seconds at n = 9.
The accuracy of this approximation is, of course, a sensitive function of the shape of the
instrument bandpass and is, therefore, expected to be highest for the narrow, well-defined
passband filters with negligible red and/or blue leaks. It will certainly only give rough order
of magnitude estimates for the wide band pass filters for which a numerical integration of
eq. (2) is required for higher confidence predictions. If there are other point sources within
a few Airy radii of the primary source, their contribution to the background RB must be
evaluated by means of the appropriate system point spread function. It should also be kept
78 FOC Instrument Handbook Version 5.0

in mind that some background sources may vary in intensity during an exposure. This will
be the case for the airglow or scattered light emission sources for exposures lasting a good
fraction of an ST orbit (see Section 6.5). In this situation, it is advisable to pick the worse
case intensity to evaluate the required exposure time.
Particular attention has to be paid, in any case, to the expected count rate since it may
violate the assumption that RS n-1 NMAX (see Table 9 and Section 6.2.2). If it does for
the particular format chosen as indicated in Table 6, either the format must be changed or a
neutral density filter inserted in order to drop the expected rate below the threshold. This,
of course, will also result in an increase in the exposure time required to reach the required
S/N ratio.


7.1.2 Spectroscopy_______

Similar computations can be carried out for a point source in the slit of the spectrograph
for the new F/48 relay except that, of course, the long slit spectrograph efficiencies plotted
in Figure 28 have to be used in equations (4) and (5). The bandpass ~ is now naturally
limited by the projected slit width of 0.06 arcseconds corresponding to 4, 2, 1.3 and 1A
for first, second, third and fourth order, respectively. The transmission of the order sorting
filter also has to be taken into account with special attention devoted to possible higher
order confusion if the filter has an appreciable near uv and visible leak and the source has
appreciable emission in these regions. This confusion can be eliminated completely for point
or pseudo point objects with the use of the objective prism FOPCD as the cross disperser.
In this case, the transmission and the dispersion of the prism given in Table 5 have to be
factored into the calculations.
For the case of the objective prisms, eq. (4) can be rewritten in the form:


RS (~) = 3:9 x 104 "(~)F (~)Q(~)T op (~)T (~)ffi~


where T op is the transmission of the prism tabulated in Table 5 and ffi~ is the wavelength
interval in A corresponding to the FOC spatial resolution. This interval can be expressed
simply as:
ffi~ = 2r(~)D(~)P S-1

where D(~) is the prism dispersion in A mm-1 tabulated in Table 5, r(~) is the radius of
the circle enclosing the required energy ffl(~) in arcseconds given in Table 8 and PS is the
plate scale of the appropriate relay in arcseconds mm-1 given in Section 3. Then, the source
count rate around ~ is:


RS (~) = 7:8 x 104 ffl(~)r(~)D(~)F (~)Q(~)T op (~)T (~)P S-1 (7)


Equations (3) and (5) for the noise calculations remain the same except that some simpli-
fication can be introduced due to the fact that the overwhelming sources of background in
the case T(~) = 1 are the system integrated zodiacal light and the geocoronal Lyman ff line.
Thus, in this case, eq. (5) can be written as:

" #
2:7 x 10-5 S c -4 kR
RB = nz Bp + __________________ + __(7:7 x 10 )I (8)
K b
FOC Instrument Handbook Version 5.0 79

where c = 0 for the NUVOPs and the FUVOP and FOPCD in the new F/48 relay, c = 1
for the FUVOPs on the new F/96 relay, b = 1 for the new F/48 relay, and b = 2 for the new
F/96 relay. S is the intensity of the zodiacal light in S10 units and I kR is the intensity of
the Lyman ff airglow in kilorayleighs.
To see how this works, suppose you want to observe a 20th visual magnitude QSO with
a -2 spectrum and you want to compute the required exposure time to obtain a S/N=10
at 1700A with the FUVOP of the new F/96 relay. In this case, F(1700A)=10-5 photons
cm-2 sec-1 A-1 . From the data tabulated in Table 8, you find r(1700A)=0.08 arcseconds
for ffl(1700A) = 0:7 and, from the data in Table 5, D(1700A) = 11:72=24 x 10-3 = 488A
mm-1 and T op (1700A) = 0:88 while Q(1700A)=0.014 from Table 10. This means that the
source rate from eq. (7) at 1700A is 0.46 counts sec-1 . The count rate is spread over n = 97
pixels for z = 1 from Table (8). Assuming that S = 120 S10 for the zodiacal light, I kR = 5
kilorayleighs, Bp = 7 x 10-4 counts sec-1 pixel-1 , and K=4, c=1, b=2, eq. (8) gives:

" #
2:7 x 10-5 x 120 7:7 x 10-4 x 5
RB = 97 7 x 10-4 + _________________________+ _____________________ =
4 2

i j
97 7 x 10-4 + 8:1 x 10-4 + 1:9 x 10-3 = 0:33 counts sec -1


Finally, the required exposure time is:

100(0:46 + 2 x 0:33)
t = _____________________________= 529 seconds:
0:462


7.2 EXTENDED SOURCES

The prescription for an extended source deviates only slightly from the formulation
discussed so far provided RS is redefined as:

ss 2 p Z 1
RS = nz __D (1 - p) IS (~)Q(~)T (~)d~ (9)
4 o


6:9 . 10-10 Z 1
= nz _______________ IS (~)Q(~)T (~)d~
K o
where n is now the chosen number of normal (z = 1) or zoomed (z = 2) pixels in the required
resolution element and IS (~) is the specific intensity of the extended source in photons
cm-2 sec-1 sr-1 A-1 . Equations (3) and (5) for RB need not be modified. Conversion
of other specific intensity units into photons cm-2 sec-1 sr-1 A-1 can be executed via the
following relations:

Units___ Photons_cm-2___sec-1___sr-1___A-1___________


U magnitudes per arcseconds squared =3.2x1013 x 10-0:4U at 3600A
B magnitudes per arcseconds squared = 6.4x1013 x 10-0:4B at 4470A
V magnitudes per arcseconds squared = 4.3x1013 x 10-0:4V at 5560A
1 Rayleigh A-1 = 8.1x104
1 erg cm-2 sec-1 sr-1 A-1 = 5 x107 ~ (A)
1 Wm-2 Hz-1 sr-1 =1.5x1029 [~(A)]-1
80 FOC Instrument Handbook Version 5.0

1 S10 =333

Suppose, for example, you want to observe a Lyman ff aurora above the limb of Jupiter
of intensity 20 kiloRayleighs with a spatial resolution of 0.28 arcseconds with a S/N = 10
with the new F/96 relay. You will be using 400 new F/96 pixels for this purpose. You
should use the F120M filter because it has the highest transmission at Lyman ff and the
lowest transmission at the longer wavelengths where the disk Rayleigh scattering spectrum
may overwhelm any far uv auroral features.
From Figure 13, you find that at ~ =1216A, the F120M filter has T=0.1 and from
Table 10 you deduce that Q(1216A)=0.008. Then, since in this case the Jovian emission
line of width 1A is much narrower than the instrumental bandpass of 86A, eq. (9) can be
written simply as:



400 x 6:9 x 10 -10 4 4
RS ' _____________________ x 2 x 10 x 8:1 x 10 x 0:008 x 0:1 = 0:09
4


counts sec -1 per resolution element

The background rate RB will be dominated by the detector background and the geo-
coronal Lyman ff airglow if the observation is carried out at night. From the curve marked
the new F/96, 1216A in Figure 39 for a typical observing configuration of 150O local solar
zenith angle, you obtain 2.2 x 10-3 counts sec-1 pixel-1 looking towards the zenith. This
implies that, for Bp = 7 x 10-4 counts sec-1 pixel-1 , you have:

h i
RB = 400 7 x 10-4 + 2:2 x 10-3 x 0:1 = 0:37

counts sec -1 per resolution element

This means that S/N=10 for this Jovian aurora and resolution can be reached in:

75(0:09 + 2 x 0:37)
t = ___________________________ = 2562 seconds
0:0922
Observations at higher spatial resolution would require correspondingly longer exposure
times.
If this same aurora is to be observed against a planetary disk background of Lyman ff
emission of 15 kilorayleighs with the same accuracy, the relevant background rate becomes:


" #
6:9 x 10 -10 4 4
RB = 400 7 x 10-4 + 2:2 x 10-4 + ____________ 1:5 x 10 x 8:1 x 10 x 0:008 x 0:1
4
h i
= 400 7 x 10-4 + 2:2 x 10-4 + 1:7 x 10-4 = 0:44 counts sec -1 per res: el:
so that:



25(0:09 + 2 x 0:44)
t = ___________________________ = 2994 seconds
0:0922
FOC Instrument Handbook Version 5.0 81

In this case, however, you might be looking onto the visible disk of the planet and the
visible leak will dominate the count rate. To estimate the visible leak contribution notice
that at '5000A, the F120M filter has a residual transmission of 10-4 and assume the Jovian
spectrum to be solar with an intensity of ' 2 x 106 Rayleighs A-1 at 5000A. Thus, you can
approximate the effect by spreading this intensity over '1500A where Q(~) '0.03. Then,
with these assumptions:


" #
6:9 x 10-10 6 4 -4
RB ' 0:44 + 400 _________________2 x 10 x 8:1 x 10 x 0:03 x 10 x 1500
4

' 0:44 + 50 counts sec -1 per resolution element:


A solution to this problem would be to insert another filter into the beam to suppress
the visible contamination. A good choice would be F140W for which T(1216A) = 0.05 and
T(5000A) = 3 x 10-4 and:


RS = 0:09 x 0:05 = 4:5 x 10-3 count sec -1 per resolution element
" #
6:9 x 10-10 6 4 -4 -4
RB = 0:44 x 0:05 + 400 ________________x 2 x 10 x 8:1 x 10 x 0:03 x 10 x 3 x 10 x 1500
4


= 0:02 + 0:015 = 0:035 counts sec -1 per resolution element


235(4:5 x 10-3 + 2 x 0:035) 4
t = ______________________________________ = 9:2 x 10 seconds = 26 hours
(4:5 x 10-3 )2


Obviously, this hypothetical program cannot be accomplished with the FOC. To reduce
the exposure time to physically realistic levels one needs to, say, reduce the required accuracy
and/or spatial resolution. For example, halving both the accuracy and the resolution yields
a more acceptable exposure time of 1.6 hours.
Finally, suppose you wish to image an extended object (a planetary nebula, for exam-
ple) with the new F/96 relay at the highest possible resolution in the zoomed configuration
for the biggest possible field of view. Suppose the object exhibits a line spectrum with a
surface brightness at Hfi of 5 x 10-13 ergs cm-2 sec-1 arcsec-2 and you wish to use the
F486N interference filter to isolate the line to an accuracy of 10%. From the data shown
in Figures 12 and 28 you find that at 4861A, T = 0:6, and Q = 0:03. From the conversion
relations on page 78, you note that Is (4861) = 5 x 10-13 x 4:25 x 1010 x 5 x 107 x 4861 =
5:16 x 109 photons cm -2 sec -1 sr -1 . Thus, eq. (9) becomes for n = 1, z = 2:


2 x 6:9 x 10-10 9 -1
RS = _______________________x 5:16 x 10 x 0:6 x 0:03 = 0:03 counts s per zoomed pixel
4



From the data shown in Figure 40 and a zodiacal light brightness of 90 S10 and Bp = 7x10-4
counts s-1 per normal pixel, eq. (5) becomes:


RB = 2[7 x 10-4 + 3 x 10-7 x 0:63 x 34] = 1:4 x 10-3 counts s -1 per zoomed pixel
82 FOC Instrument Handbook Version 5.0

because T (~o ) = 0:63 and ~ = 34A for the F486N filter from the data in Table 3. In
consequence, finally:


100(0:03 + 2 x 1:4 x 10-3 )
t = ______________________________________ = 3644 seconds :
0:032
FOC Instrument Handbook Version 5.0 83

8.0 THE_FOC_EXPOSURE_TIME_SIMULATOR,_FOCSIM_________________________________________

The general procedures to compute the required exposure times for any FOC observing
configuration and possible emission source outlined in the preceding section are perfectly ad-
equate for most purposes including proposal preparation and feasibility verification. There
are cases, however, where it is useful to have the means to evaluate more precisely the in-
tegrals in eqs. (2), (3) and (9). This capability is especially important when the emission
source spectrum is not well behaved outside the wavelength range of interest (see, for exam-
ple, the situations described in sections 6.10 and 7 concerning visible leaks), when the precise
spatial distribution of counts in the image is important as in crowded fields and for more
precise planning envisaged in Phase II of proposal preparation. For these and other possibly
more complex situations, an exposure time simulator for the FOC has been developed by
F. Paresce, Y. Frankel and W. Hack of ST ScI. This program, called FOCSIM, presently
evaluates exactly the exposure times and S/N ratio for any imaging exposure. It also al-
lows computation of the actual expected spatial patterns of the FOC images and, therefore,
evaluation of the correct S/N for wide bandpasses and/or closely-spaced pairs of stars to
simulate crowded field conditions.
FOCSIM, as presently configured, is a menu driven interactive FORTRAN program
which runs under IRAF. It accepts user input describing the FOC observing configuration
and the physical characteristics of an astronomical source to be observed and computes
count rates, background levels and exposure times consistent with those inputs. The user
may select from a number of synthetic spectra the radiation sources for the program or use his
own file of wavelengths and fluxes in appropriate units. The sources resident in the program
include 77 simulated stellar spectra covering a wide range of MK classes generated by the
Kurucz (1979) stellar atmosphere models, a number of UV standard stellar spectra from IUE
(Ap. J. Suppl., 40, 1, 1979), a flat continuum between two wavelengths, up to three emission
lines, a blackbody source of arbitrary temperature and a power low spectrum of arbitrary
index. The normalization factors of flux and wavelength can all be specified arbitrarily by
the user. Any of these sources can be made artificially extended by an appropriate change in
scale and normalization factors, if so desired. Furthermore, the diffuse background can also
be calculated precisely by FOCSIM. Presently, user supplied intensities of zodiacal light, for
UV airglow, and inherent detector background can be accommodated.
In support of COSTAR, FOCSIM has been upgraded to allow for automatic selection
of the COSTAR imaging modes. The latest DQE tables are available for use with FOCSIM
for both of the new relays. These can be selected in the FOCSIM setup and can be found in
the FOCSIM auxiliary directory. In addition to the new DQE tables, theoretical PSFs have
been produced to simulate COSTAR corrected FOC PSFs and observed PSFs taken with the
COSTAR-corrected FOC will soon be added to the FOCSIM libraries. Using the updated
DQE tables and PSFs will allow FOCSIM to simulate COSTAR corrected observations with
accuracies dominated by the errors inherent in the PSFs, either from modelling errors in
the TIM PSFs or from small changes in focus or position in the COSTAR-corrected field of
view. Initial experience in Cycle 4 has indicated that errors of about 10% in the simulated
count rates should be expected.
The output of FOCSIM includes all relevant information on the input parameters
selected, the appropriate instrumental parameters and subsidiary data such as the individual
components of background, the monochromatic count rate shown in Figure 36, restrictions
84 FOC Instrument Handbook Version 5.0

such as NMAX , data on the magnitude of the red and blue leaks, and, of course, the resultant
exposure times for the required accuracy. The user can also request that FOCSIM output
the transmission curves for the filters and the source spectra as IRAF SDAS tables, which
can subsequently be plotted using IRAF graphing procedures.
FOCSIM will be made available at the ST ScI to interested users of the FOC who have
local accounts. Unfortunately, FOCSIM is not available for general distribution along with
STSDAS for a couple of reasons: first, it uses additional libraries which are prohibitively
large, and secondly, it does not conform to IRAF's standards for software programming. A
beginner's manual is available upon request from ST ScI, either as a POSTSCRIPT file or
a printed version. It describes the basic steps necessary for running FOCSIM by walking
through a sample session. The manual also provides a list of the catalogs of spectra that
are available for use with FOCSIM and samples of the output which FOCSIM produces.
Additional on-line help has recently been added which can be accessed through the standard
IRAF help facilities.
The STSDAS package of routines provided by ST ScI includes the SYNPHOT sim-
ulation package. FOCSIM and SYNPHOT share the same DQE and filter transmission
tables ensuring that both packages utilize the most up-to-date throughput information in
calculating count rates. However, SYNPHOT does not have the capability of providing any
spatial information for any source as it does not work with either the PSFs or the encircled
energy tables. Furthermore, SYNPHOT only works with source information and does not
incorporate background sources, such as zodiacal light, into the calculations, resulting in
the necessity of calculating the signal-to-noise and background count rate of an exposure
by using the methods in Section 7. Although more limited than FOCSIM in the output
products, SYNPHOT produces the same results as FOCSIM for the same input conditions.
Therefore, either SYNPHOT or FOCSIM can be used for calculating the expected source
count rate, but FOCSIM will automatically provide more information about the expected
image.
FOC Instrument Handbook Version 5.0 85

9.0 LIMITING_MAGNITUDES_____________________

FOCSIM can be used to predict the limiting magnitude of any observing configuration.
An example of this type of calculation is shown in Figures 41 and 43 for scenarios matching
the observed average in-flight conditions. Studies of calibration images have been used to
determine an average zodiacal light intensity of S10 = 191 units and a detector background
for the new F/96 relay of Bp = 6 x 10-4 and for the new F/48 relay Bp = 2 x 10-3 counts
s-1 pixel-1 . Figure 41 shows the predicted exposure time in seconds needed to reach a S/N
= 10 for a specified visual magnitude of a B5V star through the F342W (U) filter for the
two cameras. With average in-flight conditions, we should expect to detect a B5V star of
V = 28 with the U filter in about 10 hours of exposure time with the new F/96 relay. The
limiting magnitude is V = 27:5 if a S/N=5 in a 5 hour exposure is deemed sufficient. Figure
42 illustrates the results of calculations using extended sources. For this case, the specific
intensity of the source is expressed in terms of visual magnitudes per arcseconds squared and
the spectrum is assumed to have the shape of a B5V star. The spatial resolution in this case
is chosen to be 0.112 arcseconds, which corresponds to binned regions of 4 and 8 pixels on a
side for the new F/48 or new F/96 relays respectively. One should be able to detect a source
of intensity = 23:5V magnitudes per arcseconds squared at S/N = 10 and 0.112 arcsecond
resolution in 10 hours of exposure with the U filter under the average in-flight conditions
described earlier.
86 FOC Instrument Handbook Version 5.0

Figure 41. Exposure time required to reach a S/N = 10 on a B5V star with the U filter in
an average observing condition with Bp = 6 x 10-4 counts sec-1 pixel-1 and a
zodiacal light intensity of 191 S10.


Figure 42. Exposure time required to reach a S/N = 10 on a B5V spectrum extended source
with 0.1 arcsecond resolution with the U filter in the observing condition listed
in Figure 41.
FOC Instrument Handbook Version 5.0 87

10.0 FOC_DATA_ANALYSIS_AND_PRODUCTS_________________________________



10.1 PIPELINE PROCESSING

All data taken by the FOC are automatically processed and calibrated by the Routine
Science Data Processing (RSDP), also called the "pipeline". It is possible to repeat, off line,
the calibration part of the pipeline processing by using an IRAF/STSDAS task called CAL-
FOC (CALibration of FOC data), used automatically by the pipeline. For every observation,
the user will receive two sets of data coming out of the pipeline: the input and output files
to CALFOC. The input files to CALFOC are:
1) the raw image,
2) a mask image characterizing the location of known bad pixels, reseaux, and likely data
errors determined by online processing of transmitted data called the Data Quality
File,
3) a file containing astronomical information related to the observation called the Stan-
dard Header Packet, and
4) a file containing engineering data related to the observation called the Unique Data
Log.
The output files from CALFOC are:
1) the geometrically corrected image,
2) the geometrically and photometrically corrected image, and
3) a trailer file (*.trl) containing a log of the pipeline processing.
The data processing flow chart for normal imaging and spectrographic images is shown
in Figure 43. CALFOC assumes that the processing parameters are in the image header,
either directly from RSDP preprocessing, or inserted by task "cloadrsdpx" or "loadrsdpx".
The processing parameters govern which correction steps are to be performed, and which
calibration files are to be used. For normal imaging observations, the following steps are
performed in order:
o dark count subtraction (not done at this time)
o format dependent photometric correction (using ITF reference file) (not done at this
time)
o unzoom the zoomed image
o absolute calibration affecting header parameters only
o geometric distortion correction involving data interpolation and requiring a new mask
image
o relative calibration or flat field correction (removing instrumental sensitivity variations)
using UNI reference file, which is a reciprocal of a flat field.
For spectrographic (long-slit) observations, the final step is different i.e.:
o spectrographic relative and absolute calibrations with flux and wavelength calibrations
affecting both data and headers.
In the spectrographic mode, several orders may overlap. The pipeline does not deliver
separately calibrated data sets for each order, leaving line identification and order deconvo-
lution to the user. The pipeline delivers the raw image and data sets corresponding to the
results of the tasks as indicated in Figure 43. Detailed information on FOC calibration pro-
cedures and algorithms can be found in the "Requirements Section" of the "Design Manual,
88 FOC Instrument Handbook Version 5.0

DRD-SOGS-SE-06-1," available from STScI. Some more general information is contained in
the Calibration HST Data Set manual. See also the STSDAS Calibration Guide.


10.2 GENERAL PROCEDURES

All delivered images are REAL*4 datatype, to avoid integer rounding. The actual
counts in each pixel are preserved as accurately as possible to permit immediate visualization
of counting statistics and noise.


10.2.1 Dark-Count_Subtraction_____________

The dark-count reference file multiplied by the exposure time is subtracted from the
input science file. The dark-count file is a full-frame image (512 x 1024), so if the science
file is smaller than full frame then only the appropriate section of the dark-count file is used.
Use "imarith" or "darkx" IRAF tasks. Standard processing does not apply the dark-count
subtraction to images at this time.


10.2.2 Format-Dependent_Photometric_Correction_(ITF)__________________________

These reference files are called ITF (Intensity Transfer Function) files for historical
reasons. There is one such file for each format. The format-dependent correction is applied
by multiplying the image from the previous step (i.e. the dark-count subtracted image) by
the appropriate ITF file. Use "imarith" IRAF task. Standard processing does not apply the
photometric correction to images at this time.


10.2.3 Correct_For_Zoom_Mode______________

If the image was taken in zoom mode, the next step is to split the data values along
the first image axis (the sample direction). The length of the first axis (NAXIS1) is doubled,
and the length of the second axis (NAXIS2) is not changed. Use "dezoomx" IRAF task.


10.2.4 Compute_Absolute_Sensitivity_______________

This does not affect the data values. The inverse sensitivity, pivot wavelength and RMS
bandwidth of the optical mode selected are computed and stored in the header of the output
image. The zero-point magnitude and the observation mode are also saved in the output
header. Multiplying the data numbers in the image by the value of the header parameter
PHOTFLAM and dividing by the exposure time converts to flux density F in units of ergs
per square centimeter per second per angstrom. Use "evalband" IRAF task.


10.2.5 Geometric_Correction___________

A raw FOC image is distorted by a few percent for two reasons, the optics and the
detector. Both distortions are comparable in magnitude. The optical distortion was com-
puted by ray tracing, and the detector distortion is measured by taking flat-field images and
observing the positions of reseau marks that are uniformly spaced on the photocathode. A
geometric correction reference file includes both optical and detector distortion. It gives the
location in the input (distorted) image of each corner of every pixel of the output (corrected)
image. The geometric correction is performed for each output pixel by adding up the counts
in the corresponding region in the input image. This procedure rigorously preserves flux.
Use "newgeom" IRAF task.
FOC Instrument Handbook Version 5.0 89


Figure 43. Flow diagram of FOC imaging data through the Routine Science Data Processing
System.
90 FOC Instrument Handbook Version 5.0

10.2.6 Relative_Calibration_or_Flat_Field_Correction_(normal_images_only)______________________________

This correction is called the uniform detective efficiency (UNI) correction, and removes
the nonuniform detective efficiency of the detector. It is applied by multiplying the image by
the UNI reference file (format independent), which is the reciprocal of a flat field. The UNI
files are full-frame in size, which is 1024 x 1024 because the image is dezoomed. As with the
dark-count correction, if the science image is smaller than full-frame then only a subset of
the UNI file is used. Since the sensitivity of the detector depends on wavelength, six different
UNI files are provided for different wavelengths. For the new F/48 configuration, lambda
= 3345 and 4800 Angstroms are provided; for the new F/96 configuration, lambda = 1360,
4800, 5600, and 6600 Angstroms are provided. The file to select is determined by comparing
wavelengths of the UNI files with the pivot wavelength of the optical mode. Use "imarith"
IRAF task.


10.2.7 Spectrographic_Detective_Efficiency_Correction_____________________

The SDE correction is only applied to spectrographic images. It includes both the flat-
field correction and a conversion from counts to flux density. It is applied after geometric
correction because the absolute sensitivity depends on wavelength, and a major function of
the geometric correction for spectrographic images is to align the spectrum with the axes and
set the dispersion. The correction is applied by multiplying by a spectrographic detective
efficiency reference file. The use of an order-selecting filter can change the location of a given
wavelength on the photocathode, so there are several reference files; the appropriate one is
selected based on the filters used. These files are full-frame (1024 x 1024), so only a subset
will be used if the science image is smaller than this. Use "imarith" IRAF task. Standard
processing does not apply the SDE correction to images at this time.
FOC Instrument Handbook Version 5.0 91

11.0 ACKNOWLEDGMENTS______________________

This handbook could not have been written without the expert advice and assistance
of our colleagues on the FOC team at the ST ScI. In particular, we are indebted to F.Paresce,
D. Baxter, P.Greenfield, P. Hodge, M. Miebach and W. Baggett for supplying us with all of
the information presented here.
The FOC has been brought to its present status by the devoted efforts of many groups
including the ESA/ST Project Office Staff, the FOC Investigation Definition Team (IDT)
and various industrial contractors (especially British Aerospace, Matra-Espace and Dornier
System GmbH). The authors are particularly grateful to a number of people in these and
other organizations that gave generous amounts of their time to assist us in producing this
handbook. These are, in particular, the entire IDT, Richard Hook of ST/ECF, M. Saisse of
the LAS/Marseille, and our colleagues P. Bely, C. Burrows, C.Cox, J. Crocker, R.Doxsey,
G.Hartig, O. Lupie, P.Stanley at the ST ScI.
The FOC/IDT members are: R. Albrecht, C. Barbieri, J. C. Blades, A. Boksenberg, P.
Crane, J. M. Deharveng, M. Disney, P. Jakobsen, T. Kamperman, I. R. King, F. Macchetto
(Principal Investigator), C. D. Mackay, F. Paresce, and G. Weigelt.
 92 FOC Instrument Handbook Version 5.0




Figure A1. Extended format (512zx 1024) de-zoomed image taken with the pre-COSTAR
F/48 relay under uniform external illumination. This image does not show the
effects of vignetting that will be present after the installation of COSTAR. The
slit finger is just visible at the right center edge inside the default 512 x 512
imaging format which is outlined with the solid line.
 FOC Instrument Handbook Version 5.0 93




Figure A2. Extended format (512zx 1024) de-zoomed image taken with the pre-COSTAR
F/96 relay under uniform external illumination. The occulting fingers and clip-
ping of the frame due to the baffle are clearly visible.
 94 FOC Instrument Handbook Version 5.0


Figure A3. Extended format (512zx 1024) de-zoomed negative image taken with the pre-
COSTAR F/48 relay in spectrograph mode of an extended external object. The
area in the upper left corner suffers serious vignetting, limiting the wavelength
coverage for slitless spectroscopy.
 FOC Instrument Handbook Version 5.0 95




Figure A4. Central 256 x 512 pixels of the full 256 x 1024 pixel negative image taken with
the COSTAR-corrected F/96 relay showing a star at the undispersed position
overlayed with the image showing its Far-UV prism spectrum.
 96 FOC Instrument Handbook Version 5.0




Figure A5. Dezoomed extended (512zx 1024) negative reproduction of an pre-COSTAR
F/48 image showing the high background features. This image is displayed with
a very high contrast grayscale to accentuate the shapes of the localized features.