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Dark count

Dark frames are essential for some applications. They are advisable when using on-chip binning. For CCDs, the dark current may also be high if it has been exposed to too much light, or if power has been interrupted to a cold chip. A number of long exposure (30 to 60 mins) dark frames should be combined using a median filter, and the result scaled according to the object frame exposure time. This frame may then be subtracted from the object frame before reduction begins. If you wish to ensure that the error properties are accounted for then the dark frame should be copied into the object frame variance array.

Again for CCDs only, a further advantage of collecting long exposure dark frames is the option of deriving an independent estimate of the rate of cosmic ray events as a function of their intensity. This can provide a useful comparison with long exposure object frames and a check on how reliably cosmic ray removal has been.