It should be noted that the characteristics of the OM detector imply that flat field correction has to be treated in a different way than in a normal CCD.
The physical pixels are subsampled by the centroiding algorithm, thus the concept of pixel to pixel variations changes since the real pixel cannot be recovered. A fixed pattern appears instead and it is corrected at a later stage in the processing.
It has been shown by studying the accuracy of the OM photometric calibration, which is based in observations of a large number of stars with OM and from the ground, and the statistical errors of the measurements, that the results obtained with OM are accurate to within a few percent without flat field correction. Furthermore, observations of the same stars in different parts of the detector show that large scale sensitivity variations are smaller than two per cent. Therefore, the SAS uses a unity flat field file for processing OM data. Studies of flat field correction in OM are being done to further demonstrate the validity of this approach.