Name

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      PRECESS_CD

Purpose

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

Explanation

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

Calling Sequence

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

Inputs/output

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      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.

Inputs

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      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

Input Keyword

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

Restrictions

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

Procedure

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      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.

Procedure Calls

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      PRECESS

Revision History

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      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
      Use B/Jprecess for conversion between 1950 and 2000 W.L. Aug 2009