Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/ftp/instrument_news/FOC/Foc_handbook/v6/handbookv6_ch08.ps Äàòà èçìåíåíèÿ: Wed Sep 6 00:04:30 1995 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 19:45:36 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: zodiacal light

FOC Instrument Handbook Version 6.0 85
8.0 FOC EXPOSURE TIME SIMULATORS
The general procedures to compute the required exposure times for any FOC observing con­
figuration and possible emission source outlined in the preceding section are perfectly adequate
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 integrals 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 example, 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. At present there are three ways to simulate FOC exposures: SYNPHOT in
the STSDAS package, FOCSIM, and SIMPRISM in the FOCPRISM package.
8.1 SYNPHOT
The STSDAS package of routines provided by STScI includes the SYNPHOT simulation
package. SYNPHOT uses a standard set of DQE and filter transmission tables ensuring that it uti­
lizes the most up­to­date throughput information in calculating count rates. In addition to the lat­
est throughput tables, SYNPHOT also takes into consideration the different sensitivities of each
format (described in Section 6.3.1).
SYNPHOT does not, however, have the capability of providing any spatial information for
any source as it does not work with either the PSFs or encircled energy tables. SYNPHOT outputs
the total count rate expected from any given source spectrum, then the results from T
able 8 can be
used to calculate the peak count rate for a point source. This value should follow the limitations
discussed in Section 6.2 to maintain photometric accuracy.
Background sources, such as zodiacal light, are not incorporated in the current version of
SYNPHOT. The calculation of the background and subsequent S/N for the image must then be
calculated by hand using the methods in Section 7.
8.2 FOCSIM
To fully simulate both simple and complex situations, an exposure time simulator for the FOC
has been developed by F. Paresce, Y. Frankel and W. Hack of STScI. This program, called FOC­
SIM, presently evaluates the exposure times and S/N ratio for all imaging exposures. FOCSIM
computes the expected spatial patterns of the FOC images using PSFs and, therefore, evaluates
the correct S/N for wide bandpasses and/or closely­spaced pairs of stars to simulate crowded field
conditions. Since FOCSIM only works with point sources or sources extended over the field of
view, it can not simulate the dispersed image created by the objective prisms. Instead, the task
SIMPRISM can be used to simulate these dispersed images (see Section 8.5).
FOCSIM 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. The user may select from a number of model spectra, have
FOCSIM create its own synthetic spectrum (black­body, power law or flat), or use his own file of
wavelengths and fluxes in appropriate units. The sources resident in the program include 77 simu­
lated stellar spectra covering a wide range of MK classes generated by the Kurucz (1979) stellar
atmosphere models and a number of UV standard stellar spectra from IUE (
Ap. J. Suppl., 40, 1,

86 FOC Instrument Handbook Version 6.0
1979). In addition, FOCSIM can create its own flat continuum between two wavelengths, line
spectrum with up to three emission lines, blackbody source of arbitrary temperature or power low
spectrum of arbitrary index. The normalization factors of flux and wavelength can all be specified
arbitrarily by the user. Furthermore, the diffuse background can also be calculated precisely by
FOCSIM using zodiacal light spectrum for UV air glow and the inherent detector background can
be accommodated with user supplied intensities. Unfortunately, FOCSIM currently only simu­
lates the normal 512X512 imaging format. When using different formats, the user must account
for format sensitivity increasing the expected count rate according to the values given in T
able 11
(see Section 6.3.1).
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 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 COSTAR­corrected FOC have
been added to the FOCSIM libraries. Using the updated DQE tables and PSFs allow FOCSIM to
simulate COSTAR corrected observations with accuracies dominated by the errors inherent in the
PSFs, primarily from small changes in focus or position in the COSTAR­corrected field of view.
Initial experience in Cycle 4 has indicated that errors of about 10% in the simulated count rates
for the 512X512 format 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 such as NMAX, data
on the magnitude of the red and blue leaks, and, of course, the resultant exposure times. 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 procedures.
FOCSIM will be made available at the STScI 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 sec­
ondly, it does not conform to IRAF's standards for software programming. A beginner's manual
is available upon request from STScI, 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 on­line help has recently been
added which can be accessed through the standard IRAF help facilities.
8.3 WWW Form for FOCSIM
An interface to FOCSIM has been developed for WWW as part of the FOC team's Instrument
Pages (see Chapter 9). This form allows the user to simulate a single point­source in the standard
512X512 FOC F/96 (pre­ and post­COSTAR) imaging format with average values for the back­
ground sources. The user can choose to calculate an exposure time needed to reach a desired S/N
or to calculate the S/N for a given exposure time using either observed or theoretical PSFs.
Finally, library spectra or synthetic spectra can be normalized to user specified values, although
user supplied spectra can not be used with this form.
The short, easy to use graphical interface afforded by the WWW browsers makes this simula­
tion quick to use with results being returned to the user in a very short time. The full ASCII output

FOC Instrument Handbook Version 6.0 87
of FOCSIM gives all the calculated count rates for the source and background and can be saved
by Mosaic to the users local disk for later reference.
Along with the fill­out form for running FOCSIM, the FOCSIM Beginners Manual is also
available on WWW. Both the Beginners Manual and the FOCSIM form can be found at URL
http://www.stsci.edu/ftp/instrument_news/FOC/Foc_tools/focsim/focsim.html
8.4 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 43 and 44 for scenarios matching the
observed average in­flight conditions. Studies of calibration images have been used to determine
an average zodiacal light intensity of S10 = 120 units and a detector background for the F/96 relay
of B p = 7â 10 ­4 . These are the same values that are used when the FOCSIM WWW form is used.
Figure 43 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 with the F/96 camera. With average
in­flight conditions, we should expect to detect a B5V star of V=28.75 with the U filter in about
10 hours of exposure time with the F/96 relay. The limiting magnitude is V=28.25 if a S/N=5 in a
5 hour exposure is deemed sufficient.
Figure 44 illustrates the results of calculations using extended sources. For this case, the spe­
cific 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 cho­
sen to be 0.112 arcseconds, which corresponds to binned regions of 8 pixels on a side for the F/96
relay. One should be able to detect a source with an intensity of 23.5 V 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 in­flight conditions described earlier.
8.5 Objective Prism Image Simulation
FOCSIM handles point sources and normal extended sources well, unfortunately the capabil­
ity for simulating the dispersed point sources from an objective prism image was not included.
Instead, the related IRAF package FOCPRISM was created to simulate and analyze objective
prism images based on calibrated dispersion curves for each objective prism. This package uses
the same filter transmission curves and DQE curves as STSDAS and FOCSIM, insuring reliable
count rate calculations. Since it uses routines related to SYNPHOT, it can also take into account
the different formats for the FOC. However, unlike FOCSIM, it does not output count rate calcu­
lations or include background sources. Instead, it creates a simulated image for the given expo­
sure time and source spectra.
This simulated image does not suffer from linearity effects like the FOC, thus the user should
use this to make sure the desired image does not exhibit count rates greater than 1 count/sec/pixel
for an emission line of interest or about 0.5 counts/sec/pixel for continuum regions of interest.
These count rates serve as guidelines for maintaining photometric accuracy in the final objective
prism image. On­line help is available to answer questions about running the simulator. The asso­
ciated routine, OBJPRISM, takes an objective prism image, extracts and calibrates the spectra
given the exposure time and undispersed position of the star. An observer can not only simulate
an objective prism image now but also analyze the image after it has been taken using the same
software that was used to calibrate the prisms in the first place.

88 FOC Instrument Handbook Version 6.0
Figure 43. Exposure time required to reach a S/N = 10 on a B5V star with the U filter in an aver­
age observing condition with B p = 7 â 10 ­4 counts sec ­1 pixel ­1 and a zodiacal light
intensity of 120 S10.
21 22 23 24 25 26 27 28
1
10
100
1000
Apparent Visual Magnitude [V]
F/96
Figure 44. 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
43.
18 19 20 21 22 23 24
10
100
1000
F/96