ACIS Subpixel Event Repositioning
Since the size of the core of the point spread function is smaller than the size of the ACIS pixels near the optical axis of the telescope, images of sources with arcsecond-scale features can be improved by using a subpixel algorithm (and by binning on scales smaller than a pixel).
One such algorithm, the Energy-Dependent Subpixel Event Repositioning (EDSER) algorithm of Li et al. (2004, ApJ, 610, 1204), was incorporated into the tool acis_process_events in CIAO 4.3 and into SDP version DS 8.4. Timed exposure mode datasets that were processed or reprocessed using these or later versions of the tool had the EDSER algorithm applied by default. Figures 1 and 2 show examples of datasets processed using subpixel algorithms.
The tool acis_process_events was modified in SDP version DS 10.4.2.1 (30 September 2015) and in CIAO 4.8 to make it possible to also use the subpixel algorithm for continuous-clocking mode datasets.
Note that the use a subpixel algorithm has little effect for sources that are far from the optical axis or that do not have small-scale features.
The ACIS Energy-Dependent Subpixel Event Repositioning why topic has more details.