In this tutorial, we’ll se how to correct our CCD frames taken during the night to obtain scientific data, reducing errors derived from the optical configuration, CCD sensor’s dust and scratches and filters.
Flat fielding is correcting of the combined optical-system and CCD throughput for each pixel, so it make that each pixel on the CCD would respond equally to a source with the same photon flux. The Flat field correction removes the effect of the pixels sensitivity variations across the array and the effect of dust or scratches on the CCD sensor. A flat field frame must be produced for each filter combination is used to obtain good results.
The flat-field frames are obtained by shooting an uniform illuminated target very close to the telescope (so, out of focus) or by taking frames in the sky near dusk or dawn. The easy technique using an illuminated led-flat white with the possibility to control the light level. Best solution is to obtain several frames (at least ten or, better, twenty) and median these to reduce the noise. Is also important take an equivalent number of dark frames with the same exposition.
The CCD Flat frames should be operated well away from saturation where the response may be nonlinear. In most of the cases, for a commercial CCD camera, always below 40,000 ADU can be perfect.
Generate the Master Flat-field
A single Flat field frame should be something like in fig.1: an almost white and plane figure where is possible to see all the imperfections between the chip and the sky.
Fig. 1: detail of a common Flat field frame