Q: Given my source, how do I simulate an FOC image of it? A: Chapter 7 of the FOC Instrument Handbook provides the details of calculating the count rate from a given source, along with the background count rates that would be expected as well. From these formulae, you can determine how long you need to expose in order to reach a given S-to-N. Alternatively, you could look at using FOCSIM, the FOC Exposure Simulator. This program will compute the count rate for the source, the exposure time and S-to-N, among other diagnostic values using the latest calibration data available for the FOC. The FOC Beginner's Manual provides a step-by-step introduction to FOCSIM. The STSDAS package SYNPHOT can also be used to determine total count rates from a source. Q: Given a source spectrum, can I simulate an objective prism image? A: The iraf package hst_calib.foc.focprism contains a routine to simulate objective prism images using the latest calibrations for the prisms. These tasks will produce a simulated, noiseless image for any given exposure time which can then be analysed using any image analysis tasks to determine whether the count rates are linear in the spectral region of interest, or whether they are even observable at all.
