GSSS_STDAST

      Insert the closest tangent projection astrometry into an GSSS Image

       This procedure takes a header with GSSS (ST Guide Star Survey)
       astrometry and writes a roughly equivalent tangent projection
       astrometry into the header.

       GSSS_STDAST, H, [Xpts, Ypts]

       H -  FITS header (string array) containing GSSS astrometry.
       GSSS_STDAST will write the roughly equivalent tangent projection
               astrometry solution into H.

       xpts, ypts -- Vectors giving the X and Y positions of the three
               reference points used to find approximate tangent projection.
               Default is Xpts = [0.2,0.8,0.5], Ypts = [0.2, 0.4, 0.8]

       The procedures GSSSXYAD is used to exactly determine the RA and Dec
       at 3 reference points.    STARAST is then used to find the tangent
       projection astrometry that best matches these reference points.

       Some early GSSS images (before the 1994 CD-Rom) used keywords CRPIXx
       rather than CNPIXx.    The GSSS astrometry in these images could be
       corrupted by this procedure as the CRPIXx values will be altered.
       The tangent is only a approximation of the nonlinear GSSS astrometry,
       but is generally accurate to about 0.1 pixels on a 1024 x 1024 image.

       GSSSEXTAST, GSSSXYAD, STARAST, PUTAST, SXADDHIST, SXDELPAR

       13-AUG-91 Version 2 written from MAKEASTGSSS  Eric Deutsch (STScI)
       Delete CDELT* keywords from header   W. Landsman      May 1994
       Remove call to BUILDAST  W. Landsman                  Jan, 1995
       Added optional Xpts, Ypts parameters   E. Deutsch     Oct, 1995
       Converted to IDL V5.0   W. Landsman   September 1997

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