The aimpoint of the observation has sky coordinates of (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 dmcoords tool can be used to find out on which chip the aimpoint of an ACIS observation falls:

unix% dmcoords infile=acisf01838N001_evt2.fits asolfile="pcadf084244404N001_asol1.fits" \
      x=4096.5 y=4096.5 opt=sky
unix% pget dmcoords chip_id
7

In this example the aimpoint was on chip 7, which is also known as ACIS-S3. Note that this approach is not guaranteed to work if the event file was created by reproject_events.

For more details on the ACIS focal plane layout, see the POG.