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.

 

Table 3.5: Keywords for combining dark calibration frames

Keyword	Content	Default	Description
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.