An error in the SASS processing of aspect time has been recognised in Rosat HRI observations prior to 17 Jan 1997. The error in calculating aspect time causes the position of events to be offset by upto 3.5 pixels This causes the PSF to be blurred by the order of 3".
An approximate solution, using an FTOOLS script, has been developed which is accurate provided that the roll angle is not changing too rapidly. A detailed description of this problem is provided at :
http://ledas-www.star.le.ac.uk/rosat-goc/rosstatus178
http://hea-www.harvard.edu/rosat/rsdc_www/aspfix.html