If the exposure time of the first two raw screen flat fields in the
input set of frames is the same (within 4%), the difference frame is
computed. Then the whole readout port is divided into a chessboard of
16x16 boxes. For each one of these boxes, the median signal level from
the first raw frame is divided by the variance in the difference frame
scaled by 2. The median value of the 16x16 values obtained is the
accepted value for the conversion factor.

CONAD QC parameters are trended per readout mode for the R_BESS filter only.