Pileup can be defined as the coincidence of two or more photon events per detection cell per ACIS frame. Pileup signatures appear in both the spatial and spectral information extracted from ACIS, and the strengths of these signatures increase with smaller spot size and larger fluence (counts per frame). In this section we describe the obvious signatures of pileup in the spatial and spectral domains. A further discussion of pileup can be found in http://asc.harvard.edu/cal/Links/Acis/acis/Cal_prods/pileup/pileup.html
