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      Obtain the X and Y coordinates of a line of constant declination


      Returns a set of Y pixels values, given an image with astrometry, and
      (1) A set of X pixel values, and a scalar declination value, or
      (2) A set of declination values, and a scalar X value
      Form (1) can be used to find the (X,Y) values of a line of constant
      declination. Form (2) can be used to find the Y positions of a set
      declinations, along a line of constant X.

Calling Sequence

      Y = CONS_DEC( DEC, X, ASTR, [ ALPHA ])


      DEC - Declination value(s) in DEGREES (-!PI/2 < DEC < !PI/2).
              If X is a vector, then DEC must be a scalar.
      X - Specified X pixel value(s) for line of constant declination
              If DEC is a vector, then X must be a scalar.
      ASTR - Astrometry structure, as extracted from a FITS header by the
              procedure EXTAST


      Y - Computed set of Y pixel values. The number of Y values is the
              same as either DEC or X, whichever is greater.

Optional Output

      ALPHA - the right ascensions (DEGREES) associated with the (X,Y) points


      Implemented only for the TANgent, SIN and CAR projections


      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 Greisen & Calabretta (2002, A&A, 395, 1077).
      These documents are available from

Revision History

      Written, Wayne Landsman STX Co. April 1988
      Use new astrometry structure, W. Landsman HSTX Jan. 1994
      Use CD matrix, add SIN projection W. Landsman HSTX April, 1996
      Converted to IDL V5.0 W. Landsman September 1997
      Fix case where DEC is scalar, X is vector W. Landsman RITSS Feb. 2000
      Fix possible sign error introduced Jan. 2000 W. Landsman May 2000
      Work for the CARee' projection W. Landsman May 2003

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