STScI Logo

Hubble Space Telescope
FOC Exposure Simulator - FOCSIM

The FOC Exposure Simulator, FOCSIM, provides the capability to simulate almost any FOC exposure and to calculate diagnostics about the image. FOCSIM, the FOC exposure SIMulator, uses the most current description of the FOC and its filters along with an input spectra (either from an on-line catalog or user-supplied file) to simulate an image from the FOC and provide the count rate within the image. FOCSIM provides not only a calculation of the exposure time needed to reach a desired S/N, but also statistics related to the count rate that is expected from the object and how it is spread across a spherically-aberrated source or across a COSTAR-corrected source.

FOCSIM

The WWW form allows you to run FOCSIM to simulate a 512x512 FOC image of an individual point-source, without creating the actual image. The results of each simulation are returned as a page which can be saved if desired.

How to run FOCSIM

The FOCSIM Beginner's Manual walks the user through their first FOCSIM session, step-by-step, then discusses more advanced uses of FOCSIM, including using user-supplied spectra.

How FOCSIM Works

FOCSIM starts with a given spectrum for the source and calculates the count rate based on the equations given in the FOC Handbook. Similarly, the count rates for the background sources the user specifies are calculated based on the equations in the FOC Handbook. For a point source, however, the count rate distribution over the detector area is specified by the Point Spread Function (PSF). There are two libraries of PSFs available to FOCSIM, one of theoretical mono-chromatic PSFs and one of observed PSFs. For the COSTAR-corrected FOC, a set of theoretical PSFs have been created based on the best descriptions available of COSTAR and the FOC. FOCSIM then utilizes files for the camera DQE based on in-flight calibration observations, a table with the measured reflectivities for the COSTAR FOC mirrors (for use in COSTAR-corrected FOC simu-lations), a library of measured filter throughputs kept in the Calibration Database System (CDBS), and files for the OTA's throughput taken from CDBS as well. This ensures the users of FOCSIM that the latest calibration data available has been incorporated into the simulation to provide as accurate a description of the FOC as possible.

What Sources can FOCSIM Simulate

There are three libraries of spectra available to FOCSIM: BPGS spectra, UCB spectra and CLB spectra. The BPGS spectra are observed spectra for many stellar standards used in astronomy, while the CLB spectra are spectra for objects used for calibrating the HST instruments. The UCB spectra are theoretical stellar spectra for over 75 MK classes. These provide a base for simulating a wide range of stellar observations, however not all observations are of stellar objects. FOCSIM also has the capability to create its own synthetic spectra using either power-law, black-body, or flat spectral models, normalized to whatever parameters the user gives. Finally, the user can supply their own spectra for the most accurate simulation, if it is available. These choices allows the user to examine almost any object as if viewed through the FOC.

What FOCSIM Calculates

The output from FOCSIM can include an image, several STSDAS tables and output report of the calculations. In general the most significant results are included in the standard output report, similar to:

Computed energy_fraction	        0.6947
Exposure time:	                        183.9 [Seconds]
ST MAG 	                                22.75
Source area for min exposure time	81.00 [Pixels]
Source rate in Source area	        0.6283 [Counts s-1]
Total backg Rate	                0.4883E-01
TOTAL SOURCE RATE	                0.9044
Rates in the Point Source Center Pixel:
    Point Source Rate                   0.7153E-01   [Counts pixel-1 s-1]

Using these results, a potential observer can determine whether or not his source object would suffer from any linearity or saturation problems and adjust the filters used accordingly.

Where do I go if something doesn't look or work right?

All materials for FOCSIM have been written by the STScI FOC group. Questions about running FOCSIM or about what FOCSIM can do for you can be directed through the STScI Help Desk.