Name

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      GET_EQUINOX

Purpose

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      Return the equinox value from a FITS header.

Explanation

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      Checks for 4 possibilities:
      (1) If the EQUINOX keyword is found and has a numeric value, then this
              value is returned
      (2) If the EQUINOX keyword has the values 'J2000' or 'B1950', then
              either 2000. or 1950. is returned.
      (3) If the EQUINOX keyword is not found, then GET_EQUINOX will return
              the EPOCH keyword value. This usage of EPOCH is disparaged.
      (4) If neither EQUINOX no EPOCH is found, then the RADESYS keyword
              (or the deprecated RADECSYS keyword) is checked. If the value
              is 'ICRS' or 'FK5' then 2000 is is returned, if it is 'FK4' then
              1950 is returned.
      According Calabretta & Greisen (2002, A&A, 395, 1077) the EQUINOX should
      be written as a numeric value, as in format (1). However, in older
      FITS headers, the EQUINOX might have been written using formats (2) or
      (3).

Calling Sequence

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      Year = GET_EQUINOX( Hdr, [ Code ] )

Inputs

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      Hdr - FITS Header, string array, will be searched for the EQUINOX
              (or EPOCH) keyword.

Output

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      Year - Year of equinox in FITS header, numeric scalar

Optional Output

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      Code - Result of header search, scalar
              -1 - EQUINOX, EPOCH or RADECSYS keyword not found in header
              0 - EQUINOX found as a numeric value
              1 - EPOCH keyword used for equinox (not recommended)
              2 - EQUINOX found as 'B1950'
              3 - EQUINOX found as 'J2000'
              4 - EQUINOX derived from value of RADESYS or RADECSYS keyword
                  'ICRS', 'FK5' ==> 2000, 'FK4' ==> 1950

Optional Keyword Input

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      ALT - single character 'A' through 'Z' or ' ' specifying which
            astrometry system to use in the FITS header. The default is
            to use the primary astrometry or ALT = ''. If /ALT is set,
            then this is equivalent to ALT = 'A'. See Section 3.3 of
            Greisen & Calabretta (2002, A&A, 395, 1061) for information about
            alternate astrometry keywords.

Procedures Used

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      ZPARCHECK, SXPAR()

Notes

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      Technically, RADESYS = 'ICRS' does not specify any equinox, but can be
      assumed to be equivalent to J2000 for all but highest-precision work.

Revision History

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      Written W. Landsman STX March, 1991
      Don't use !ERR W. Landsman February 2000
      N = 1 for check of EPOCH keyword, not 0 S. Ott July 2000
      Added ALT keyword, recognize RADESYS along with deprecated RADECSYS
              W. Landsman Sep 2011