In principle, the same shift-and-add correction applied to the bad pixel map, and employing the on-board tracking history, is also applied to the CCF flatfield before dividing out large scale sensitivity variations. In practice, to within measurement uncertainties, the telescope plus detector flatfield is consistent with unity.
Time invariant dark counts are present within images at the mean level of 2.6 x 10-4 counts s-1pix-1 and varies across the detector by up to 10%. Dark counts are not corrected before source extraction but since source and background extraction regions are small and from the same image locale, dark counts are not an issue for catalogue sources.
Due to the photon event nature of the detector, there is no CCD bias charge to subtract from images.
SAS task: omflatfield
CCF product: OM_LARGESCALESENS_000n.CCF