How are the photometric frames created?
Internal to the Maxim DL 5.x there is a routine that help the observer to clean his frames from dark, flat. The dark count is obtained from a portion of 4 shielded CCD rows in the KAF-401/1602 sensors; this is saved as the image parameter “BottomThermalCount”. The offset (or bias) level is determined by averaging 1000 “overscan” horizontal CCD pixels so that no thermal or photo-electrons are measured. For the Pyxis KAF-401 camera, the offset is typically 4000-6000 ADU; it is saved as the image parameter “BiasLevel”.
The procedure for creating the dark-count and flat-field frames is identical. The “BiasLevel” is subtracted when the images used to create the flat-field or dark frames are summed so that only the contribution from thermally or photo-generated electrons remains. The summed images are then scaled so that the maximum intensity in the sum-image is 65535; the maximum value for a 16-bit unsigned integer. The median value in the center of the image is computed and assigned to the “BiasLevel” parameter of the image.