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Combining Dark Frames

  Similarly to the bias combination one can obtain an average dark current frame. However, one should consider whether the dark correction should really be applied: one needs a reasonable number of dark frames in order not to degrade the S/N, and obviously this takes telescope time. Because the dark level depends on the exposure time, weighting the input dark frames should be considered. Another possibility would be simply to taken the average dark value and to scale that number with the exposure time. Filter type is not important. Table gif show the default setting for combining dark frames. The CCD test command (see Section gif) provides tools to monitor stability characteristics and linearity of bias and dark frames.

  
Table: Keywords for combining dark calibration frames

If the bias is stable enough to allow taking averages, one might argue that it is not really needed. In that case, provided a good linearity of the CCD, one could do with the subtraction of an average dark frame. If the bias is unstable, one should be careful with simply combining bias frames. In that case the better solution might be an average of two bias frames, taken just before and after each flat of sky exposure.



Pascal Ballester
Tue Mar 28 16:52:29 MET DST 1995