Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/ftp/instrument_news/FOC/Foc_handbook/v5/handbook_v5_ch08.ps Äàòà èçìåíåíèÿ: Fri Dec 9 19:44:33 1994 Äàòà èíäåêñèðîâàíèÿ: Sun Dec 23 20:15:18 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: zodiacal light

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 closely­spaced 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
COSTAR­corrected 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 COSTAR­corrected 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 N MAX , 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 on­line 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 up­to­date 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 signal­to­noise 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.