CONS_RA

       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.

       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.

       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
       http://www.cv.nrao.edu/fits/documents/wcs/wcs.html

       Implemented only for the TANgent, SIN and CARtesian projections

       Written, Wayne Landsman  STX Co.        April, 1988
       Algorithm adapted from AIPS memo No. 27 by Eric Griessen
       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

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