This keyword gives the exposure time of a single ACIS frame and is quantized in units of 0.1 s with the shortest EXPTIME being 0.2 s. It excludes the time during which charge is moved from the imaging region to the frame store region (0.04104 s).

The standard value of EXPTIME = 3.2 s for TIMED mode ACIS observations. If an observation uses interleaved mode then there are separate values of EXPTIME for the short and long frames (e.g. 0.2 s and 3.2 s, respectively).

The observation times entry and the chandra times help file explain how the various exposure keywords are related.