ASTRO-F/FIS Observing Simulation: Detection Limits for Point Sources

We describe the observing simulation software FISVI (FIS Virtual Instrument),which was developed for the Far-Infrared Surveyor (FIS) that will be on theJapanese infrared astronomy mission ASTRO-F. The FISVI has two purposes: one isto check the specifications and performances of the ASTRO-F/FIS as a whole; theother is to prepare input data sets for the data analysis softwares prior tolaunch. In the FISVI, special care was taken by introducing the "Compiled PSF(Point Spread Function)" to optimise inevitable, but time-consuming,convolution processes. With the Compiled PSF, we reduce the computation time byan order of magnitude. The photon and readout noises are included in thesimulations. We estimate the detection limits for point sources from thesimulation of virtual patches of the sky mostly consisting of distant galaxies.We studied the importance of source confusion for simple power-law models forN(>S), the number of sources brighter than S. We found that source confusionplays a dominant role in the detection limits only for models with rapidluminosity evolution for the galaxy counts, the evolution of which is suggestedby recent observations.