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For sources near the optical axis of the telescope, the size of the point spread function is smaller than the size of the ACIS pixels. Li et al. (2004, ApJ, 610, 1204) describe various sub-pixel event-repositioning algorithms that can be used to improve the image quality of ACIS data for such sources.

As of CIAO 4.3 (15 December 2010), it is possible to use the tool acis_process_events to

• apply the sub-pixel event-repositioning algorithm "EDSER" of Li et al. (2004),
• disable all pixel adjustments, or
• apply a pixel randomization.

Refer to the description of the parameter pix_adj for more details.

With the implementation of version DS 8.4 of the Standard Data Processing (SDP) code in the pipeline (28 June 2011), the default processing will apply the sub-pixel algorithm EDSER.

The ACIS Sub-Pixel Event Repositioning why topic has more details.

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