FOC Exposure Time Simulators

Objective Prism Image Simulation

FOCSIM handles point sources and normal extended sources well, unfortunately the capability 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 calculations or include background sources. Instead, it creates a simulated image for the given exposure 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. On-line help is available to answer questions about running the simulator. The associated 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.

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