Name

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      XMEDSKY

Purpose

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      Subtract sky from an image as a 1-D function of X

Explanation

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      This procedure is designed to remove the sky from slitless spectra.
      The sky is assumed to vary with wavelength (along a row) but not with
      position (along a column). The sky is computed as the
      column-by-column median of pixels within 3 sigma of the image global
      median. This procedure is called by the cosmic ray rejection routine
      CR_REJECT

Calling Sequence

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      XMEDSKY, Image, Bkg, [ CLIP=[x0, x1, y0, y1], NSIG= ]

Inputs

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      Image: Input image for which sky vector is to be computed.
     

Input Keyword Parameters

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      CLIP: [x0, x1, y0, y1]: region of image to be used for all
              statistical computations. Default is to use the entire
              image. For STIS 1024 x 512 slitless spectra, the suggested
              value is CLIP = [32,1023,12,499]
      NSIG: Positive scalar giving the number of sigma a pixel must be above
              the global median to be reject. Default is 3 sigma.

Output Parameter

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      Bkg: Vector of sky values.
;

Modification History

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      Written by: R. S. Hill, Hughes STX, 20 Oct. 1997
      Converted to V5.0, use STDDEV() W. Landsman June 1998
      Check for valid WHERE, added NSIG keyword W. Landsman December 2000
      Assume since V5.1 so always use STDDEV W. Landsman Feb 2004
      Assume since V5.6 use DIMEN keyword to MEDIAN W. Landsman Jan 2008