SKY

       Determine the sky level in an image using the the procedure MMM

       Approximately 10000 uniformly spaced pixels are selected for the
       computation.  Adapted from the DAOPHOT routine of the same name.

       SKY, image, [ skymode, skysig, HIGHBAD= , READNOISE= ,/SILENT
                                      /NAN, CIRCLERAD= ]

       IMAGE - One or two dimensional array

       SKYMODE - Scalar, giving the mode of the sky pixel values of the
               array IMAGE, as determined by the procedure MMM.
       SKYSIG -  Scalar, giving standard deviation of sky brightness

       CIRCLERAD - Use this keyword to have SKY only select pixels within
               specified pixel radius of the center of the image.  If
               CIRCLERAD =1, then the radius is set equal to half the image
               width.   Can only be used with square images.
       HIGHBAD - scalar value of the (lowest) "bad" pixel level (e.g. cosmic
                rays or saturated pixels) If not supplied, then there is
                assumed to be no high bad pixels.
       /NAN - This keyword must be set to  ignore NaN values when computing
              the sky.
              Note that the CIRCLERAD, HIGHBAD and /NAN are not exclusive, e.g.
               one can set both /NAN and CIRCLERAD
       /SILENT - If this keyword is supplied and non-zero, then SKY will not
               display the sky value and sigma at the terminal
       READNOISE - Scalar giving the read noise (or minimum noise for any
                pixel).     Normally, MMM determines the (robust) median by
                averaging the central 20% of the sky values.     In some cases
                where the noise is low, and pixel values are quantized a
                larger fraction may be needed.    By supplying the optional
                read noise parameter, MMM is better able to adjust the
                fraction of pixels used to determine the median.

       A grid of points, not exceeding 10000 in number, is extracted
       from the image array.  The mode of these pixel values is determined
       by the procedure MMM.   In a 2-d array the grid is staggered in each
       row to avoid emphasizing possible bad columns

       MMM, DIST_CIRCLE

       Written, W. Landsman   STX Co.            September, 1987
       Changed INDGEN to LINDGEN                 January, 1994
       Fixed display of # of points used         March, 1994
       Stagger beginning pixel in each row, added NSKY, READNOISE, HIGHBAD
          W. Landsman        June 2004
      Adjustments for unbiased sampling  W. Landsman June 2004
      Added /NAN keyword, put back CIRCLERAD keyword W. Landsman July 2004

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