HEULER

       Change the coordinate system of a FITS header or astrometry structure

       Converts a FITS header or a astrometry structure containing WCS (world
       coordinate system) information between celestial, ecliptic, and
       Galactic coordinates

       HEULER, hdr, [/GALACTIC, /CELESTIAL, /ECLIPTIC, ALT_IN = , ALT_OUT=]
                      or
       HEULER, astr, /GALACTIC, /CELESTIAL, /ECLIPTIC

       hdr - FITS header (string array) containing WCS information
                            or
       Astr - Astrometry structure as extracted from a FITS header
             by extast.pro (See EXTAST for more info).
       Header or astrometry structure will be modified by the program to
       contain astrometry in the new coordinates system.

       One of the following exclusive keywords is *required*
       /GALACTIC - Convert the header to Galactic coordinates
       /CELESTIAL - Convert the header to celestial (RA & Dec) coordinates
       /ECLIPTIC - Convert the header to ecliptic coordinates

      The following two keywords apply if the FITS header contains multiple
      WCS keywords. See Section 3.3 of Greisen & Calabretta (2002, A&A, 395,
      1061) for information about alternate astrometry keywords.
      ALT_IN -  single character 'A' through 'Z' or ' ' specifying an
          alternate astrometry system present in the input FITS header.  The
          default isto use the primary astrometry or ALT = ' '.   If /ALT_IN
          is set, then this is equivalent to ALT_IN = 'A'.
      ALT_OUT - single character specifying the alternate WCS keywords
          to write the *output* astrometry.    If not specified, then ALT_OUT
          is set equal to ALT_IN.

       Currently assumes that celestial and ecliptic coordinates are in
       J2000.   Use HPRECESS if this is not the case.
       ST Guide Star (DSS) image headers are first converted to a standard
       tangent projection, prior to the coordinate conversion

       The algorithm used is described in Section 2.7 of Calabretta & Greisen
       (2002, A&A, 395, 1077).    The CRVAL coordinates are transformed
       directly using EULER.    The new LONPOLE and LATPOLE values are then
       determined by transforming the pole of the new system to the old, and
       converted to native coordinates using WCS_ROTATE.

       A FITS header, hdr, has a standard tangent projection WCS information.
       Add an alternate 'G' Galactic projection.    Note that the original
       WCS information will be left unchanged
       IDL> heuler, hdr, /Galactic, alt='G'

       EULER, EXTAST, GSSS_STDAST, PUTAST, SXADDHIST, WCS_ROTATE

       Written    W. Landsman                  June 2003
       Use PV2 tag in astrometry structure rather than PROJP1 W. L. May 2004
       Use double precision to compute new North pole  W.L. Aug 2005

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