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      Obtain the X and Y coordinates of a line of constant right ascension


      Return a set of X pixel values given an image with astrometry,
      and either
      (1) a set of Y pixel values, and a scalar right ascension (or
          longitude), or
      (2) a set of right ascension values, and a scalar Y value.
      In usage (1), CONS_RA can be used to determine the (X,Y) values
      of a line of constant right ascension. In usage (2), CONS_RA can
      used to determine the X positions of specified RA values, along a
      line of constant Y.

Calling Sequence

      X = CONS_RA( RA, Y, ASTR, [ DEC] )


      RA - Right Ascension value in DEGREES (0 < RA < 360.). If Y is a
              vector, then RA must be a scalar
      Y - Specified Y pixel value(s) for line of constant right ascension
              If RA is a vector, then Y must be a scalar
      ASTR - Astrometry structure as extracted from a FITS header by the
              procedure EXTAST


      X - Computed set of X pixel values. The number of elements of X
              is the maximum of the number of elements of RA and Y.

Optional Output

      DEC - Computed set of declinations (in DEGREES) for X,Y, coordinates


      The algorithm (and notation) is based on AIPS Memo 27 by Eric Greisen,
      with modifications for a coordinate description (CD) matrix as
      described in Paper II of Calabretta & Greisen (2002, A&A, 395, 1077).
      These documents are available from


      Implemented only for the TANgent, SIN and CARtesian projections

Revision History

      Written, Wayne Landsman STX Co. April, 1988
      Algorithm adapted from AIPS memo No. 27 by Eric Greisen
      New astrometry structure
      Converted to IDL V5.0 W. Landsman September 1997
      Added SIN projection W. Landsman January 2000
      Fix possible sign error introduced Jan. 2000 W. Landsman May 2000
      Work for the CARee' projection W. Landsman May 2003
      For TAN projection ensure angles between -90 and 90 W. Landsman Jan 2008

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