XY2AD

     Compute R.A. and Dec from X and Y and a FITS astrometry structure

     The astrometry structure must first be extracted by EXTAST from a FITS
     header.   The offset from the reference pixel is computed and the CD
     matrix is applied.     Of distortion is present then this is corrected.
     If a WCS projection (Calabretta & Greisen 2002, A&A, 395, 1077) is
     present, then the procedure WCSXY2SPH is used to compute astronomical
     coordinates.    Angles are returned in  degrees.
     XY2AD is meant to be used internal to other procedures.
     For interactive purposes use XYAD.

     XY2AD, x, y, astr, a, d

     X     - row position in pixels, scalar or vector
     Y     - column position in pixels, scalar or vector
           X and Y should be in the standard IDL convention (first pixel is
           0), and not the FITS convention (first pixel is 1).
     ASTR - astrometry structure, output from EXTAST procedure containing:
        .CD   -  2 x 2 array containing the astrometry parameters CD1_1 CD1_2
               in DEGREES/PIXEL                                   CD2_1 CD2_2
        .CDELT - 2 element vector giving physical increment at reference pixel
        .CRPIX - 2 element vector giving X and Y coordinates of reference pixel
               (def = NAXIS/2)
        .CRVAL - 2 element vector giving R.A. and DEC of reference pixel
               in DEGREES
        .CTYPE - 2 element vector giving projection types
        .LONGPOLE - scalar longitude of north pole
        .LATPOLE - scalar giving native latitude of the celestial pole
        .PV2 - Vector of projection parameter associated with latitude axis
             PV2 will have up to 21 elements for the ZPN projection, up to 3
             for the SIN projection and no more than 2 for any other
             projection
        .DISTORT - Optional substructure specifying distortion parameters

     A - R.A. in DEGREES, same number of elements as X and Y
     D - Dec. in DEGREES, same number of elements as X and Y

       Note that all angles are in degrees, including CD and CRVAL
       Also note that the CRPIX keyword assumes an FORTRAN type
       array beginning at (1,1), while X and Y give the IDL position
       beginning at (0,0).   No parameter checking is performed.

      AD2XY tests for presence of WCS coordinates by the presence of a dash
      in the 5th character position in the value of CTYPE (e.g 'DEC--SIN').

       TAG_EXIST(), WCSXY2SPH

       Written by R. Cornett, SASC Tech., 4/7/86
       Converted to IDL by B. Boothman, SASC Tech., 4/21/86
       Perform CD  multiplication in degrees  W. Landsman   Dec 1994
       Converted to IDL V5.0   W. Landsman   September 1997
       Understand reversed X,Y (X-Dec, Y-RA) axes,   W. Landsman  October 1998
       Consistent conversion between CROTA and CD matrix W. Landsman Oct. 2000
       No special case for tangent projection W. Landsman June 2003
       Work for non-WCS coordinate transformations W. Landsman Oct 2004

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