HPRECESS

       Precess the astrometry in a FITS header to a new equinox

       HPRECESS, HDR, [ yearf ]

       HDR - FITS Header, must contain the CRVAL astrometry keywords,
               and either an EPOCH or EQUINOX keyword.
               HDR will be modified to contain the precessed astrometry

       YEARF - Scalar, giving the year of the new (Final) equinox.
               If not supplied, user will be prompted for this value.

       The CRVAL and CD (or CROTA) keywords are extracted from the header
       and precessed to the new equinox.  The EPOCH or EQUINOX keyword in
       the header is  updated.  A HISTORY record is added

       The FK5 reference frame is assumed for both equinoxes.

       EXTAST, GET_EQUINOX(), SXADDPAR, SXADDHIST, PRECESS, PRECESS_CD
       PUTAST, ZPARCHECK

       Written  W. Landsman        STX              July, 1988
       CD matrix precessed -                        February, 1989
       Update EQUINOX keyword when CROTA2 present   November, 1992
       Recognize a GSSS header                      June, 1994
       Additional Noparams value recognize for storing CDs.  RSH, 6 Apr 95
       Converted to IDL V5.0   W. Landsman   September 1997
       Understand reversed X,Y (X-Dec, Y-RA) axes,   W. Landsman  October 1998

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