There was a shift in the Chandra aimpoint of about 8 arcsec, caused by a
change in alignment of the aspect camera with respect to the HRMA and
focal plane science instruments (ACIS & HRC detectors). All aimpoints
are affected, particularly ACIS-S: the direction of the shift moves
targets positioned at the ACIS-S default aimpoint closer to the node
boundary, increasing the risk of apparent loss of events at that boundary.
There are no permanent science impacts from the aimpoint shift, but
users with ACIS-S observations made from 11 July 2011 through 29 August
2011 may need to take special care in the data analysis, as outlined in
the Chandra Aimpoint Shift data caveat.
http://cxc.harvard.edu/ciao/caveats/aimpoint.html
Further details about the aimpoint shift were released in Chandra
Electronic Announcement #75.
http://cxc.harvard.edu/announcements/announce_75.html
Elizabeth Galle
for the CIAO team
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