On-Orbit Simulations

It is important to keep in mind that these comparisons of the model suite to XRCF data serves only to refine and validate these models - their real purpose is to predict the behavior of AXAF on-orbit. Although we expect the CCDs to behave on-orbit very much the same as they did at XRCF, the HRMA will be somewhat different. SAOSAC already produces on-orbit rays so we can predict the on-orbit performance of the HRMA-ACIS system, but we must be careful to include the concept of spacecraft dither and the resulting aspect solution. Real events from ACIS on-orbit will not be delivered to the user in integer chip coordinates - rather they will have an aspect solution applied, with necessary errors, and be returned in real-valued ``sky'' coordinates. This added complexity will render some of the metrics used for XRCF comparison irrelevant and we must be careful to anticipate important changes to our assumptions. For example, the XRCF PSF is clearly undersampled by the ACIS pixels, at least marginally. Once aspect error is added, however, the PSF will broaden and it is likely that ACIS will achieve critical sampling of this more realistic PSF.