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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 COSTAR­corrected image.
Figure 23. Radial profiles of pre­COSTAR aberrated PSF (dotted) and COSTAR­corrected
PSF (solid at Hfi).

48 FOC Instrument Handbook Version 5.0
Table 8: Measured Energy Fraction e(l) for the New F/96 Relay
r Filter
n (arcseconds) F120M F140M F170M F210M F278M F346M F410M F486N F550M
1 0.000 0.039 0.061 0.055 0.094 0.093 0.087 0.071 0.064 0.054
9 0.024 0.214 0.294 0.319 0.415 0.452 0.409 0.385 0.380 0.345
21 0.037 0.315 0.421 0.463 0.555 0.621 0.528 0.543 0.548 0.532
37 0.049 0.393 0.499 0.558 0.626 0.712 0.601 0.627 0.613 0.611
69 0.067 0.491 0.577 0.652 0.688 0.774 0.722 0.744 0.697 0.670
97 0.080 0.536 0.619 0.696 0.722 0.802 0.773 0.799 0.769 0.727
137 0.095 0.578 0.657 0.739 0.768 0.832 0.809 0.830 0.831 0.807
177 0.108 0.599 0.682 0.763 0.798 0.858 0.833 0.844 0.855 0.851
225 0.121 0.626 0.703 0.781 0.821 0.886 0.856 0.858 0.870 0.870
293 0.139 0.653 0.724 0.798 0.841 0.909 0.877 0.877 0.886 0.881
349 0.151 0.664 0.736 0.808 0.851 0.919 0.896 0.888 0.897 0.888
421 0.166 0.682 0.749 0.819 0.859 0.928 0.916 0.905 0.909 0.898
489 0.179 0.696 0.762 0.829 0.867 0.934 0.927 0.922 0.917 0.906
577 0.194 0.712 0.777 0.841 0.873 0.942 0.938 0.934 0.928 0.915
665 0.209 0.723 0.790 0.850 0.877 0.948 0.944 0.941 0.941 0.921
749 0.222 0.734 0.799 0.855 0.883 0.951 0.948 0.945 0.950 0.929
861 0.238 0.747 0.810 0.863 0.889 0.954 0.952 0.950 0.955 0.942
973 0.253 0.750 0.821 0.871 0.896 0.958 0.956 0.953 0.958 0.949
1085 0.260 0.765 0.830 0.879 0.902 0.961 0.958 0.957 0.961 0.952
1201 0.281 0.774 0.839 0.886 0.909 0.964 0.959 0.961 0.964 0.955
1313 0.293 0.790 0.847 0.892 0.913 0.967 0.961 0.964 0.966 0.958
1457 0.309 0.803 0.856 0.900 0.917 0.970 0.964 0.967 0.969 0.961
1597 0.324 0.813 0.865 0.907 0.923 0.972 0.966 0.970 0.972 0.963
1741 0.338 0.822 0.875 0.914 0.929 0.974 0.968 0.973 0.975 0.964
In Table 8, the encircled energy fraction ffl (– ) is tabulated for various circular
apertures against the number of pixels in the aperture and the effective radius (defined
as
q
#pixels=ú), 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
diffraction­limited image from a 2.4m circular aperture with a 0.33 central obstruction. It
can be seen that the COSTAR­corrected 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
diffraction­limited 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
non­uniform 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 COSTAR­corrected F/96
and pre­COSTAR F/96 relays compared to those expected from a perfect diffrac­
tion limited OTA.
In the ultraviolet, the PSF shows a fairly strong jet­like feature pointing approxi­
mately in the ­V3 direction (Figure 25b). The strength decreases with increasing wavelength
but is still quite noticeable at 4000 š A. 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 45 ffi 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 COSTAR­corrected 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 off­axis position, 6.56
arcminutes from the optical axis. At this distance, the focal plane is tilted with respect to
the V1 axis by 10 ffi . 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 field­dependent 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 field­dependent astigmatism. Both of
these effects increase linearly with distance from the fully­corrected field point.
The field­dependence 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 non­linearity, 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 non­linearity depends on the frame scan time and the photon event
size (typically 3 \Theta 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
T f = z(S \Theta L)
8:8 \Delta 10 6
sec
where z = 1 for normal and z = 2 for zoomed pixels. For the most widely­used format (512
\Theta 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 \Theta 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 on­orbit 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 \Theta 512 formats of the pre­COSTAR F/96 and pre­COSTAR 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)):
r = a(1 \Gamma exp(
\Gammaae
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.
f
Table 9: Calculated Flat­Field Linearity Parameters
Camera
Format
(FâL)
Linearity Parameter
a
N MAX
(cs ­1 )
F/96 512zâ1024 0.11 0.04
512â1024 0.37 0.08
512â512 0.73 0.15
256â256 2.93 0.60
128â128 11.7 2.40
F/48 512zâ1024 0.065 0.03
512â1024 0.26 0.05
512â512 0.52 0.06
256â256 2.09 0.40
128â128 8.37 2.40
F/48 SPEC 256zâ1024 0.13 0.03
256â1024 0.52 0.10
Values of a for the most commonly­used 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 non­linearity does not compromise the science data, users are advised to
ensure that the count rate is kept below N MAX , 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 N MAX cannot be guaranteed.
6.2.2 Non­Uniform 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 time­consuming to obtain stellar images at a large number of intensity levels,
it was not possible to calibrate the point­source non­linearity relation for the pre­COSTAR
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. Flat­field linearity plots for the detectors in the pre­COSTAR F/48 and pre­
COSTAR F/96 relays, a and b respectively, based on 512 \Theta 512 pixel flat­field
images of the internal LEDs at different intensities. The solid line in each plot is
the best­fit solution for the linearity function given in the text.
Figure 27. Linearity relation for point sources based on 512 \Theta 512 pixel images taken with
the pre­COSTAR 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 pre­COSTAR F/96 camera
when used in the 512 \Theta 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 non­linearity 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 non­linearity relation for point sources is shown in Figure 27. This
is a plot of the brightest pixel intensity for stars in two FOC pre­COSTAR F/96 512 \Theta 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 non­linearity 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 š 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 well­defined 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
Table 10: Overall (OTA+FOC+COSTAR) Absolute Quantum
Efficiency Q(l) in 10 ­3 counts photon ­1
l(å) Q(F/48) Q(F/96) Q(SPI) Q(SPII) Q(SPIII) Q(SPIV)
1150 2.66 0.78 0.23
1160 6.59 1.61 0.37
1170 9.89 2.67 0.46
1180 13.17 3.92 0.55
1190 15.15 5.34 0.71
1200 16.47 6.79 0.54 0.81
1250 23.02 10.53 0.73 1.10
1300 26.18 12.64 0.81 1.25
1400 25.56 14.30 1.02 1.46
1500 25.35 15.58 1.22
1600 23.82 14.94 1.33
1700 22.10 14.24 1.56
1800 21.00 13.56 2.37 1.80
1900 24.26 15.19 3.12
2000 31.68 19.52 3.85
2200 47.74 30.33 5.34
2400 59.98 41.04 6.18
2600 62.70 49.89 6.44
2800 64.68 59.53 6.35
3000 65.36 70.32
3400 66.17 78.47 12.16
3800 60.02 78.61 12.34
4000 55.12 71.40 12.27
4500 41.15 44.91 11.88
5000 24.66 25.64 9.80
5500 10.39 12.65 5.62
6000 4.57 5.70
6500 0.43 0.53

56 FOC Instrument Handbook Version 5.0
Figure 28. Baseline overall (OTA + COSTAR + FOC) absolute quantum efficiency in counts
photon \Gamma1 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 long­winded 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 FOC­080.
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 1200­2000 š 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 \Gamma1 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 in­orbit measurements for the new F/96
relay+OTA absolute quantum efficiency, and ground­based reflectance calibrations of the
COSTAR mirrors for the new F/48. The predicted loss of light from two reflections of MgF 2
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
\Sigma 20% while, for the others, errors in the 2000--6500 š A range for the imaging modes should
not exceed \Sigma 20% and for wavelengths below 2000 š A they are expected to be of the order
\Sigma 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 Format­dependent Effects
It has been found that the DQE is a function of detector format (see Instrument
Science Report FOC­075). 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.
f
Table 11: Format­Dependent Sensitivity Ratios
Camera
Format
(FâL) Relative Sensitivity
F/96 512zâ1024 1.25
512zâ512 1.45
512â512 º1.0
256â256 1.20
128â128 1.23
F/48 512zâ1024 1.44
256zâ1024 1.28
512â1024 1.02
512â512 º1.0
256â256 0.85
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, 28 ffi 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 “
Cerenkov radiation in the MgF 2
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 “
Cerenkov generated photons.
The threshold energy for “
Cerenkov radiation in MgF 2
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 \Gamma 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 \Gamma1 s \Gamma1 in the new F/48 and 0.02 counts
pixel \Gamma1 s \Gamma1 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 ground­track 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 \Theta 10 \Gamma4 counts pixel \Gamma1 s \Gamma1 in the detector for the new
F/96 relay and 2 \Theta 10 \Gamma3 counts pixel \Gamma1 s \Gamma1 in the detector for the new F/48 relay. Upward
fluctuations of a factor ú 3 from these minimum values are, however, seen throughout the
HST orbit. The minimum in­orbit background rates are, respectively, factors of ú 5 and ú 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 2­10 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 \Theta
10 \Gamma3 counts s \Gamma1 pixel \Gamma1 and a background region far from the spot only has a count rate
of 2:83 \Theta 10 \Gamma3 counts s \Gamma1 pixel \Gamma1 . 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 switch­on. 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 \Gamma1 pixel \Gamma1 . 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 turn­on.
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 \Gamma2 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 \Gamma2 is reached at angles greater than 80 ffi 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 off­axis angle to the Earth limb in the CVZ will be in the range 20 ffi \Gamma 44 ffi ,
approximately. From Figure 30, the expected stray light illumination in this configuration
in the visible will be between 18th and 20th magnitudes arcsec \Gamma2 . 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 \Gamma2 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 3000 š A, this effect will be negligible as shown in Figure 31. Non­CVZ
observations can also have bright limb approaches of 20 ffi \Gamma 44 ffi 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 š A \Gamma1 i.e., 10 6 =4ú pho­
tons cm \Gamma2 sec \Gamma1 š A \Gamma1 sr \Gamma1 as a function of wavelength.

62 FOC Instrument Handbook Version 5.0
Figure 30. Stray light illumination in V magnitudes arcseconds \Gamma2 at the OTA focal plane
due to the Moon and daylit Earth as a function of off­axis angle.
6.6 DETECTOR OVERLOAD
The FOC detectors described in Section 4.6 may be damaged by illumination levels
exceeding 10 7 photons s \Gamma1 pix \Gamma1 at the photocathode due to point sources and by an average
illumination from a diffuse source over the whole photocathode exceeding 10 4 photons s \Gamma1
pix \Gamma1 . 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 \Gamma1 pix \Gamma1 . Thus, for safety
reasons, no point source delivering more than 10 6 photons s \Gamma1 pix \Gamma1 at the photocathode or
a diffuse source delivering more than an average rate of 100 photons s \Gamma1 pix \Gamma1 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 \Gamma2 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 \Sigma60 00 arcseconds and a drift rate of 0.01 arcseconds s \Gamma1
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 (512z \Theta 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 \Theta 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 1360 š A 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 2200 š A 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
\Theta 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 \Theta 512 is seen outlined in larger formats. This effect is due to a
burn­in 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 6000 š A 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 \Theta 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 6000 š A, 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 pixel­to­pixel 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 300­500 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 case­by­case
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 mid­1994.

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 pre­COSTAR
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 3­5% 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 pixel­to­pixel 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, burn­in 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 \Gamma3 \Gamma 10 \Gamma4 between 5000 and
6000 š A 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 \Sigma \Delta–=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 \Gamma2 sec \Gamma1 š A \Gamma1 .
If the F231M filter alone is used in this endeavour, the contribution of the flux within
the band 2330 \Sigma 115 š A is only ' 18% of the total of 39 counts sec \Gamma1 . The counts originating
from the region – ? 2580 š A 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 in­band 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 off­axis 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 off­axis 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 ``reading­out'' the TV tube's target, firstly because the read­out beam is
performing an angular sweep across a plane target, and secondly because of non­uniformities
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 bi­alkali 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 \Theta 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 ray­tracing
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 flux­conserving algorithm which maps values from the
raw, distorted image into a geometrically corrected image. A two­dimensional 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
pre­COSTAR F/48 and pre­COSTAR F/96 cameras using their 512z \Theta 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
200 400 600 800 1000
200
400
600
800
1000
PIXEL NUMBERS
LINE
NUMBERS
diff file : dist75.tab
run entry : day835 , dated Tue 13:50:59 05­Apr­94 , history* V *
ref file : optref.tab
ref entry : optref , dated Tue 13:50:59 05­Apr­94 , history* V *
magnif. : 2.
plotted : Tue 16:34:47 05­Apr­94
Figure 37. The 512z \Theta 1024 format distortion field for the new F/48 relay.
200 400 600 800 1000
200
400
600
800
1000
PIXEL NUMBERS
LINE
NUMBERS
diff file : zlrg.tab
run entry : respos , dated Tue 13:50:59 05­Apr­94 , history* V *
ref file : optref.tab
ref entry : optref , dated Tue 13:50:59 05­Apr­94 , history* V *
magnif. : 2.
plotted : Tue 13:56:01 05­Apr­94
Figure 38. The 512z \Theta 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 switch­on. During this time the observed reseau
positions show an RMS deviation from the stable positions of approximately 1­2 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 switch­on 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 out­gassing in the OTA and instruments, however given the time
since launch, the desorption curve is now considerably flatter and out­gassing 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 1­2 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 pre­COSTAR 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 \Gamma1 (\Sigma0:0002) based on
observations of an astrometric star field with the post­COSTAR F/96 relay. Extrapolating
from the new F/96 results, the plate scale for the new F/48 should be 0.02825 arcsec pixel \Gamma1
(\Sigma0:0002).
These values are ``radial'' plate scales and are within a few percent of the nominal
values, which will be vis. 0.014 arcsec pixel \Gamma1 for the new F/96 relay, and 0.028 arcsec
pixel \Gamma1 for the new F/48 relay.