PRECESS_CD

       Precess the CD (coordinate description) matrix from a FITS header

       The CD matrix is precessed from EPOCH1 to EPOCH2.  Called by HPRECESS

       PRECESS_CD, cd, epoch1, epoch2, crval_old, crval_new, [/FK4]

       CD - 2 x 2 CD (coordinate description) matrix in any units
               (degrees or radians).  CD will altered on output to contain
               precessed values in the same units.    On output CD will always
               be double precision no matter how input.

       EPOCH1 - Original equinox of coordinates, scalar (e.g. 1950.0).
       EPOCH2 - Equinox of precessed coordinates, scalar (e.g. 2000.0)
       CRVAL_OLD - 2 element vector containing RA and DEC in DEGREES
               of the reference pixel in the original equinox
       CRVAL_NEW - 2 elements vector giving CRVAL in the new equinox

       /FK4 - If this keyword is set, then the precession constants are taken
             in the FK4 reference frame.   The default is the FK5 frame.

       PRECESS_CD should not be used more than 2.5 centuries from the
       year 1900.

       Adapted from the STSDAS program FMATPREC.  Precession changes the
       location of the north pole, and thus changes the rotation of
       an image from north up.  This is reflected in the precession of the
       CD matrix.   This is usually a very small change.

       PRECESS

       Written, Wayne Landsman, ST Systems  February 1988
       Fixed sign error in computation of SINRA     March 1992
       Added /FK4 keyword                           Feb 1994
       Converted to IDL V5.0   W. Landsman   September 1997

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