Combining Sky Frames

The procedure is similar to the flat field combining procedure, except that you may want to scale the sky level using the mean, median or the mode of the intensity distribution or taking into account the exposure time. Hence, in the combining, like in the case of combining flat fields, correct weighting of the frame is important. Combining should be done filter by filter. See Table 3.7 for the keywords.

 

Table 3.7: Keywords for combining sky calibration frames

Keyword	Content	Default	Description
CCD_IN	input name	?	table of input frames
SK_TYP	descriptor value	*	exposure type to check, if any
SK_SIG	yes|no	no	create sigma image
SK_MET	comb. method	avsigcl	type of combining operation
SK_DEL	yes|no	no	delete cal. frames afterwards
SK_STA	mean|median|mode	mode	correct. by Mode/Median/Mean
SK_EXP	yes|no	yes	scale by exposure time
SK_SCA	yes|no	no	scale by MMM
SK_OFF	yes|no	no	add offset obtained from MMM
SK_WEI	yes|no	yes	use a weighted average
SK_SEC	area	[<,<:>,>]	area for computing MMM
SK_RAN	real,real	-99999,99999	valid pixel range
SK_CLP	real	3.,3.	low/high sigma clipping factor
SK_NUL	real	NULL(2)	value for rejections