Focal plane geometry  

This section describes work in progress concerning ACIS ray tracing. The problem we want to solve consists of the following: given a photon emerging from the HRMA from a given direction, we want to determine which CCD the photon will hit and the pixel coordinates of the event. Our input information is the direction of the incoming photons (in terms of three direction cosines) from an arbitrary point outside the mirror. This information will be provided by the ASC program SAOSAC (see the XRCF Phase 1 report ). The problem consists of finding the (unique) intersection of the ray trajectory with the CCD surface, taking into account the tilt of the latter with respect to the HRMA optical axis.