ACIS header keyword whose value represents the static exposure time for a frame plus the time required to transfer charge from the image region to the frame store region (0.04104 s):

  TIMEDEL = EXPTIME + 0.04104 s. 

For the standard TIMED mode ACIS observation where EXPTIME = 3.2 s, TIMEDEL = 3.24104 s. If an observation uses interleaved mode, then there are separate values of TIMEDEL for the short and long frames (e.g. 0.24104 s and 3.24104 s, respectively).

The Observation Times entry explains how the various exposure keywords are related.