AD2XY

     Compute X and Y from native coordinates and a FITS  astrometry structure

     If a WCS projection (Calabretta & Greisen 2002, A&A, 395, 1077) is
     present, then the procedure WCSXY2SPH is used to compute native
     coordinates.   If distortion is present then this is corrected.
     In all cases, the inverse of the CD matrix is applied and offset
     from the reference pixel to obtain X and Y.
     AD2XY is generally meant to be used internal to other procedures.   For
     interactive purposes, use ADXY.

     AD2XY, a ,d, astr, x, y

     A -     R.A. or longitude in DEGREES, scalar or vector
     D -     Dec. or longitude in DEGREES, scalar or vector
     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 increment at reference point in
               DEGREES/PIXEL
        .CRPIX - 2 element vector giving X and Y coordinates of reference pixel
               (def = NAXIS/2) in FITS convention (first pixel is 1,1)
        .CRVAL - 2 element vector giving coordinates of the reference pixel
               in DEGREES
        .CTYPE - 2 element vector giving projection types
        .LONGPOLE - scalar longitude of north pole (default = 180)
        .PV2 - Vector of additional parameter (e.g. PV2_1, PV2_2) needed in
               some projections
        .DISTORT - Optional substructure specifying distortion parameters

     X     - row position in pixels, scalar or vector
     Y     - column position in pixels, scalar or vector
     X,Y will be in the standard IDL convention (first pixel is 0), and
     *not* the FITS convention (first pixel is 1)

      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(), WCSSPH2XY

     Converted to IDL by B. Boothman, SASC Tech, 4/21/86
     Use astrometry structure,  W. Landsman      Jan. 1994
     Do computation correctly in degrees  W. Landsman       Dec. 1994
     Only pass 2 CRVAL values to WCSSPH2XY   W. Landsman      June 1995
     Don't subscript CTYPE      W. Landsman       August 1995
     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 October 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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