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.

 

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