Name

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    AD2XY

Purpose

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

Explanation

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    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.

Calling Sequence

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    AD2XY, a ,d, astr, x, y

Inputs

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    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

Outputs

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    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)

Notes

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      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').

Procedures Used

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      TAG_EXIST(), WCSSPH2XY

Revision History

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    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
    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
    Use CRVAL reference point for non-WCS transformation W.L. March 2007
    Use post V6.0 notation W.L. July 2009