In most imaging instruments, a combination of telescope and detector spatial response combines to give several detector resolution elements across a point response function. In X-ray instruments, this point response is usually well calibrated before launch, and reasonably stable in flight. Due to the lack of atmosphere, departure from the point response is usually due to the intrinsic surface brightness distribution of non-point sources.
ASTERIX represents this a priori information about the instrumental point response using the PSF system (see Appendix A), providing the mechanism by which instrument independent applications may be written. The model used by the system is two dimensional and can handle variation in both energy and time.
This relatively simple situation is in contrast to many forms of ground based imaging, where atmospheric distortions degrade the point response over timescales (until very recently) too short to be corrected. In addition, most optical/infrared detectors can saturate for bright sources, whereas X-ray detectors (which are usually photon counting devices) simply suffer higher dead-time losses. These problems dictate the use of techniques other than that of comparing the observed source surface brightness distribution with a pre-computed model, limiting the usefulness of the PSF system with these data.