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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
 


next up previous contents
Next: Combine Example Up: Preparing Your Calibration Frames Previous: Combining Flat Fields
http://www.eso.org/midas/midas-support.html
1999-06-15