SCNSKY-Calculates a sky spectrum for a scanned CCD image

Description:

Uses the 'minimum median' algorithm to create a sky spectrum from an image without any lines clear of stars. Each column of the image is split into sections of a specified length and the median of each section is calculated. The sky element corresponding to the column is set to the value of the minimum median for that column (on the assumption that this is an uncontaminated value). This algorithm is suitable for exposures taken using a CCD in scanned mode, hence the name of the routine.

Parameters:

IMAGE

Name of image to obtain sky from.

SECTION

Length of each Y-section.

SPECTRUM

Name of spectrum to be generated.

LIMIT

Ignore pixels below a minimum value?

MINIMUM

Minimum valid pixel value"

Source comments:

 S C N S K Y

 Uses the 'minimum median' algorithm to create a sky spectrum
 from an image without any lines clear of stars.  Each column
 of the image is split into sections of a specified length and
 the median of each section is calculated.  The sky element
 corresponding to the column is set to the value of the minimum
 median for that column (on the assumption that this is an
 uncontaminated value).  This algorithm is suitable for exposures
 taken using a CCD in scanned mode, hence the name of the routine.

 Command parameters -

 'IMAGE'    The name of the image

 'SECTION'  The number of pixels in each section.

 'MINIMUM'  A minimum 'valid' pixel value.  A median value
            will be ignored if the range of pixels over which it
            was calculated contained pixels below this value.

 'SPECTRUM' The name of the resulting sky spectrum.

 Command keywords -

 'LIMIT'    Use the minimum pixel value to limit the median
            calculations - this is needed because there is no
            obviously illegal value for 'MINIMUM' which the
            program can take as an indication that it is to be
            ignored.  (If 'MINIMUM' is given in the command
            string explicitly, 'LIMIT' need not be set.)

 Input data -

 IMAGE is assumed to have a 2-D data structure.

 Output data -

 SPECTRUM is created with the same structure as IMAGE,
 except that the data will only have one dimension, and if
 IMAGE has an AXIS(2) structure, this will be omitted.  Any
 AXIS(1) structure will be copied unchanged.

                                 KS / CIT 25th May 1983