Depending on the observation mode a certain fraction of the events recorded are due to out-of-time events (OoTE), i.e. are registered during readout of a CCD and thus will be assigned an incorrect position in RAWY and consequently an incorrect CTI correction. Typical values are 6.3% for full frame mode, and 2.3% for extended full frame mode (with FRAME_TIME_PARAMETER=3).
Out-of-time events have a much lower exposure per pixel per read-oiut frame, namely reduced by the number of pixel shifts during readout and by the abovementioned ratio of readout and cycle time. Therefore the likelihood for pile-up is extremely reduced (by more than 3 orders of magnitude). Extremely bright (and hard) sources can trigger the MIP rejection mechanism (see epframes) due to pile-up: if 2 (or more) photons hit the same pixel during one read-out frame the accumulated charge may exceed the limiting value of 3000 adu. Thus this “event” will be treated as MIP and this CCD column and the neighbouring ones get rejected in this read-out frame. This causes a spatial inhomogeneity in the exposure (a warning is issued by epframes in this case), and especially the number of recorded out-of-time events is significantly reduced in CCD columns in the core of the PSF (where they are usually strongest).
