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It has recently been realised that there is a systematic error in the roll angle with respect to the astronomical coordinate frame of the PSPC and HRI. The effect of this roll error is not included in the current SASS\ processing. (It will be included in the REV2 reprocessing of the entire dataset planned for the near future.)
The error is small, being for the PSPC. This corresponds to a position error of 6'' at the radius of the support ring.
In principle, users can correct for the roll angle error by rotating an image about the centre of the field of view. However, there is also a known attitude solution error which causes X-ray position offsets of order 6''. The origin of this attitude solution error is not known. Before rotating an image to correct for the roll angle error, the image should be shifted to correct for the attitude solution error. As this cannot be done without accurate positions for a number of the X-ray sources in the field of view, users will make the roll correction only rarely.