CONS_DEC

       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
            either
       (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.

       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.

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

       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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