How can one extrapolate the S/N with exposure time (deep imaging limits)?
It is safe to assume that the S/N for a given source will scale as the square root of the exposure time. Note that spatial variations in the detector sensitivity means that the effective exposure time for a given source may vary with parameters such as roll angle between orbits. Note that the GALEX exposure time calculator (ETC) relies solely upon photon counting statistics to compute the signal-to-noise ratio and does not include the effects of crowding. Due to the rather large GALEX PSF (5″ FWHM), crowding in deep exposures is a very acute problem. This may prove to be the limiting factor in the measurement of UV sources with magnitudes ~ 25. The GALEX science team have been able to reach a depth of at most NUV ~ 25.3 mag using PSF-fitting when relying upon prior information on the positions of sources from higher resolution optical data. This is true even in our deepest fields with exposure times of 240000 seconds in NUV. In principal, crowding is much less severe in the FUV and it
