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The process of determining boresight offsets (systematic zero-point shifts found by comparing measured and known positions of catalogued objects) for the HRI has led to the conclusion that there are time-varying errors in the attitude solutions used to correct event positions. Although most known objects are detected at positions within 10'' of their catalog positions, residual discrepancies can be present even after application of the boresight corrections. This can be particularly evident in a comparison of observations of the same object taken six months apart (and hence at spacecraft roll angles differing by 180 ).
Compensation for such effects is possible whenever X-rays are detected from previously known objects whose positions have been accurately catalogued.