Name

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      HPRECESS

Purpose

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      Precess the astrometry in a FITS header to a new equinox

Calling Sequence

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      HPRECESS, HDR, [ yearf ]

Input-output

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      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

Optional Input

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

Method

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      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

Restrictions

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      The FK5 reference frame is assumed for both equinoxes.

Procedures Used

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

Revision History

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      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