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 take the average dark value and to scale that number with the exposure time. Filter type is not important. Table 3.5 shows the default setting for combining dark frames.

 

CCD_IN input name ? table of input frames

DK_TYP descriptor value * exposure type to check, if any

DK_SIG yes|no no create sigma image

DK_MET comb. method avsigcl type of combining operation

DK_DEL yes|no no delete cal. frames afterwards

DK_STA mean|median|mode mode correct. by Mode/Median/Mean

DK_EXP yes|no yes scale by exposure time

DK_SCA yes|no no scale by MMM

DK_OFF yes|no no add offset obtained from MMM

DK_WEI yes|no yes use a weighted average

DK_SEC area [<,<:>,>] area for computing MMM

DK_RAN real,real -99999,99999 valid pixel range

DK_CLP real 3.,3. low/high sigma clipping factor

DK_NUL real NULL(2) value for rejections

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 or sky exposure.