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
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
Understand reversed X,Y (X-Dec, Y-RA) axes, W. Landsman October 1998
Correct algorithm when CROTA2 is in header W. Landsman April 2006
Correct sign error introduced April 2006, include CDELT values
when computing rotation of pole W. Landsman July 2007
Call hprecess/jprecess for 1950<>2000 W. L. Aug 2009