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      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. One might want to do this if (1)
      one needs to use software which does not recognize the GSSS astrometric
      parameters or (2) if the the image to be transformed, since the
      highly nonlinear GSSS solution does not transform easily.

Calling Sequence

      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.

Optional Inputs

      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.


      Images from the STScI server (http://archive.stsci.edu/dss/) contain
      both a GSSS polynomial plate solution and an approximate WCS tangent
      projection. The value of the WCSNAME keyword in the FITS header
      is 'DSS'. If WCSNAME = "DSS' then the more accurate DSS astrometry
      is extracted by EXTAST This procedure changes the value of WCSNAME
      to 'DSS_TANGENT' to indicate that the tangent solution should be used.
      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.

Procedures Used



      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
      Add WCSNAME W. Landsman Nov 2006

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