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      Sky adjust the planes of a datacube.


      When removing cosmic rays from a set of images, it is desirable that
      all images have the same sky level. This procedure (called by
      CR_REJECT) removes the sky from each image in a data cube.

Calling Sequence

      Datacube: 3-D array with one image of same field in each plane.
                  Returned with sky in each plane adjusted to zero.

Output Arguments

      Skyvals: Array of sky values used on each plane of datacube.
                  For a scalar sky, this parameter is a vector
                  containing the sky value for each image plane. For a
                  vector sky, this parameter is a 2-D array where each
                  line corresponds to one image plane.

Input Keyword Parameters

      REGION - [X0,X1,Y0,Y1] to restrict area used for computation
                  of sky. Default is 0.1*Xdim, 0.9*Xdim, 0.1*Ydim,
                  0.9*Ydim. If INPUT_MASK is specified, the two
                  specs are combined, i.e., the intersection of the
                  areas is used.
      VERBOSE - Flag. If set, print information on skyvals.
      NOEDIT - Flag. If set, return sky values without changing
      XMEDSKY - Flag. If set, return vector sky as a function of X.
      SELECT - Array of subscripts of planes of the cube to process.
      EXTRAPR - Applies only in XMEDSKY mode.
                  Subregion to use for polynomial extrapolation of sky
                  vector into portions excluded by REGION parameter.
                  (Default=first and last 10% of pixels; set to zero
                  to defeat extrapolation)
      EDEGREE - Applies only in XMEDSKY mode.
                  Degree of polynomial for extrapolation (Default=1)
      INPUT_MASK - Cube of flags corresponding to data cube. If used,
                  the sky computation is restricted to the smallest
                  contiguous rectangle containing all the pixels flagged
                  valid (with 1 rather than 0).


      Uses astronomy library "sky" routine for scalar sky and
      column-by-column median for vector sky.

Modification History

  10 Jul. 1997 - Written. R. S. Hill, Hughes STX
  20 Oct. 1997 - 1-D sky option. RSH
    7 Aug. 1998 - SELECT keyword. RSH
    6 Oct. 1998 - Extrapolation. RSH
    7 Oct. 1998 - INPUT_MASK added. RSH
  21 Oct. 1998 - Fallback to 3-sigma clipped mean if mode fails. RSH
  22 Mar. 2000 - Combine mask with region rather having mask
                    override region. Improve comments. RSH
  16 June 2000 - On_error and message used. Square brackets for array
                    subscripts. EXTRAP included in this file.
                    WBL & RSH, 16 June 2000

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