GSSSEXTAST

      Extract IDL astrometry structure from a ST Guide Star Survey FITS header

      This procedure extracts the astrometry information from a ST Guide
      Star Survey FITS header and places it in an IDL structure for
      subsequent use with GSSSxyad and GSSSadxy.

      GSSSExtast, hdr, astr, noparams

      h - the GSSS FITS header

      astr  - Structure containing the GSSS Astrometry information
               .CTYPE  =  ['RA---GSS','DEC--GSS']
               .CRVAL = plate center Ra, Dec (from PLTRAH, PLTRAM etc.)
               .XLL,.YLL = offsets lower lefthand corner
               .AMDX, .AMDY = 12 transformation coefficients
               .XSZ,.YSZ = X and Y pixel size in microns
               .PLTSCL = plate scale in arc sec/mm
               .PPO3, .PPO6 - orientation coefficients

      Most users should use EXTAST rather than this procedure.   EXTAST will
      call GSSSEXTAST if supplied with GSSS FITS header.

      SXPAR() - Extract parameter values from a FITS header

       01-JUL-90 Version 1 written by Eric W. Deutsch
       Code derived from Software by Brian McLean
       20-AUG-91 Modified to Double Precision Variables.  E. Deutsch
       June 94 Change astrometry tags to better agree with EXTAST  W. Landsman
       Converted to IDL V5.0   W. Landsman   September 1997
       29-JUN-99 Added support for AMD[X,Y]1[2-3] for DSS images by E. Deutsch
       Eliminate use of obsolete !ERR  W. Landsman    February 2000

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