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      Determine the sky level in an image


      Approximately 10000 uniformly spaced pixels are selected for the
      computation. Adapted from the DAOPHOT routine of the same name.
      The sky is computed either by using the procedure mmm.pro (default)
      or by sigma clipping (if /MEANBACK is set)

Calling Sequence

      SKY, image, [ skymode, skysig ,/SILENT, /MEANBACK, /NAN, CIRCLERAD= ]
        Keywords available when MEANBACK is not set (passed to mmm.pro):
        Keywords available when /MEANBACK is set:


      IMAGE - One or two dimensional array
      SKYMODE - Scalar, giving the mode of the sky pixel values of the
              array IMAGE, as determined by the procedures MMM or MEANCLIP
      SKYSIG - Scalar, giving standard deviation of sky brightness. If it
              was not possible to derive a mode then SKYSIG is set to -1

Input Keyword Parameters

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.
      /MEANBACK - if set, then the background is computed using the 3 sigma
            clipped mean (using meanclip.pro) rather than using the mode
            computed with mmm.pro. This keyword is useful for the Poisson
            count regime or where contamination is known to be minimal.
      /NAN - This keyword must be set to ignore NaN values when computing
              the sky.
      /SILENT - If this keyword is supplied and non-zero, then SKY will not
              display the sky value and sigma at the terminal
      The _EXTRA facility can is used to pass optional keywords to the programs
            that actually perform the sky computation: either mmm.pro
            (default) or meanclip.pro (if /MEANBACK) is set. The following
            keywords are available with the mmm.pro (default) setting
      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.
      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.
      /INTEGER - Set this keyword if the input SKY image only contains
                discrete integer values. This keyword is only needed if the
                SKY image is of type float or double precision, but contains
                only discrete integer values.
    If the /MEANBACK keyword is set then the following keywords are available
      CLIPSIG: Number of sigma at which to clip. Default=3
MAXITER: Ceiling on number of clipping iterations. Default=5
      CONVERGE_NUM: If the proportion of rejected pixels is less
          than this fraction, the iterations stop. Default=0.02, i.e.,
          iteration stops if fewer than 2% of pixels excluded.
      /DOUBLE - if set then perform all computations in double precision.
                Otherwise double precision is used only if the input
                data is double


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

Procedure Calls


Revision History

      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
      Added MEANBACK keyword, _EXTRA kewyord ,preserve data type in
            calculations W. Landsman November 2005
      Fix problem for very large images by requiring at least 2 pixels to
      be sampled per row. March 2007 W. Landsman
      Avoid possible out of bounds if /NAN set W. Landsman Jan 2008
      Use TOTAL(/INTEGER) June 2009

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