% @(#)computsky.hlq	19.1 (ESO-IPG) 02/25/03 13:30:31 
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SECTION./COMP
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COMPUTE/SKY                                                 05-JUL-1989 AL/RHW
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COMPUTE/SKY infram1 infram2 caltab method sky_factor
	compute the sky background and restitute the frame
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Purpose:     Compute the sky background and restitute the frame
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Syntax:      COMPUTE/SKY infram1 infram2 caltab method sky_factor
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             infram1       = first input frame
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             infram2       = second input frame; if method = 3 then 
                             infram2 = infram1
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             caltab        = calibration table containing calibration 
                             coefficients for thr characteristic curve. This
                             table can be created with  CONVERT/DTOM
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             method        = method to be used:\\
                             1 = PDS scan after 14/4/1984 (descriptor O_TIME
                                 set to 1984.5 \\
                             2 = PDS scan before 14/4/1984\\
                             3 = only one input frame, second frame is equal
                                 to first frame; only desciptor SKYBGR is 
                                 update; this is the default;\\
                             0 = same as 1 but no pixel replacement
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             sky_factor    = sky_factor; default 1.4
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Note:        With the two input frames A and B (where B is a subset of A)
             and the calibration table, containing the coefficients the 
             following will be done:\\
             a. A flat sky background will be through the intensity level
                lower than the highest intensity level in the histogram of
                frame A multiplied by sky_factor. The background is determined
                inside 8 rectangualar areas surrounding frame B and within
                two concentric circles;\\
             b. get the PDS density population of the upper 5 and 10 percent 
                in the intensity distribution in B;\\
             c. Determine the centroid of these pixel distributions.
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             If infram1 = infram2 only part of the operations is done.
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Examples:
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             COMPUTE/SKY GAL1 GALSUB GALTAB 1 2.0
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