What was the aimpoint of the observation?
The aimpoint is the location in chip coordinates on the detector where the target lands. The aimpoint coordinates also depend on whether a Y or Z offset was specified. This is different than the focal point, which is the location of the sharpest (narrowest) PSF. The focal point may also be called the "on-axis position".
The nominal aimpoints are not explicitly given in the data products since they are only used during proposal planning. The aimpoint of the observation has sky coordinates of approximately (4096.5,4096.5) for ACIS observations, (16384.5,16384.5) for HRC-I observations, and (32768.5,32768.5) for HRC-S observations. The aimpoint is different each proposal cycle because of thermal relaxation of spacecraft structures.
For an ACIS-I observation taken in Cycle 12:
unix% dmcoords evt2.fits asol=asol1.fits op=chip chip_id=3 chipx=944 chipy=993 unix% pget dmcoords chip_id 3 unix% pget dmcoords x y 4087.435198975212 4062.54287101678 unix% pget dmcoords ra dec 00:42:43.330 +41:16:15.88
In this example the aimpoint was on chip 3, which is also known as ACIS-I3. Note that this approach is not guaranteed to work if the event file was created by reproject_events.
The FAQ on RA and DEC NOM, TARG, and PNT header keywords has related information on the observation aimpoint on the on-axis position.