This means that the flat-fielding is also effectively a gain correction and this is not reflected in the GAIN keyword in the image headers. I wonder how this is dealt with later on. This is preliminary because it is based on a small number of frames. This looks good. The plot at left shows an average of columns running the length of several of the chips in a flat-fielded R image.
Different colors are different chips.
This is typical of the first few images I checked. The flat-fielding using only twilight flats seems to be fairly good. These problems can't be corrected as far as I can see. Neither is a fatal problem but they are somtimes more than an annoyance because they complicate sky subtraction.