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The main boresightings were performed separately for the PSPC and HRI during the PVC phases of the mission (see also App. C. These comprised of 5-9 successive pointings at a bright X-ray target for which the celestial position is accurately known, and intended to determine the actual alignment matrices between the star trackers (Sect. 1.2.5) and the XRT. The results were derived at MPE and communicated to GSOC\ for inclusion in the GSOC attitude solution S/W. The basic (pre-launch) plans for the boresight procedure, residual errors, and accuracy specifications are described in [Endrass1987, Hefler and Endrass1989, Hefler1989].
In principle, the boresight parameters should remain stable for the whole mission, however the accuracy of determination of the necessary boresight correction was improved with experience, consequently the boresight correction folded into SASS has changed with time.