SAO sent MPE the corrected code in late Dec 1998. MPE released SASS7_B with the fix incorporated in 1999 April. Thereafter, this bug will not be present in SASS products. Two history cards will be added to the fits header and another card will be added that makes it easier for software parsing:
ASP_FIX= 'T' HRI ASPECT CORRECTION performed HISTORY HRI Aspect bug fix incorporated in these data HISTORY executed by SASS 7_B; 1999 Feb ; *OR LATER VERSIONS*
MPE has developed a full aspect solution in MIDAS/EXSAS as a byproduct of their work with the WFC star tracker. This approach takes the raw detector coordinates, performs the degapping and aspecting (including boresight offsets), and applies the necessary conversion to celestial x,y.
At SAO, F. Primini has developed an approximate solution which is accurate if the roll angle is not changing rapidly (i.e. for normal observations). Since the standard SASS output includes the aspect table (in the ...anc.fits file), we create a 'bad' version of the table by replacing the aspect times with bad times (calculated simply by dividing the fractional part of the aspect time by 128). For each event, we then read (interpolate) RA,DEC from both the good (original) aspect table and from the bad table. Taking the differences, we then find a correction directly for x,y. Finally, we add the new keyword plus two history cards to the header.
ASP_FIX= 'T' HRI ASPECT CORRECTION performed HISTORY HRI Aspect bug fix incorporated in these data HISTORY executed by FTOOLS HRI Aspect Fix SAO-Vs1.0/1999feb
The scripts for running the ftools version are available at the RSDC/SAO web site
(http://hea-www.harvard.edu/rosat/aspfix.html).To obtain the scripts via ftp:
ftp -i sao-ftp.harvard.edu then cd to pub/rosat/aspfix