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.
It is possible to use the data in table 6.4 of the POG and the tool dmcoords to convert the chip coordinates of nominal aimpoints to ra,dec and x,y and to determine what chip the aimpoint was on.
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.
For more details on the ACIS focal plane layout, see the Chapter 6 of the POG. There are also a number of talks from the Chandra Calibration Workshops.
The FAQ on RA and DEC NOM, TARG, and PNT header keywords has related information on the observation aimpoint on the on-axis position.