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AD2XY

AD2XY

Name


    AD2XY

Purpose


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

Explanation


    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


    AD2XY, a ,d, astr, x, y

Inputs


    A - R.A. or longitude in DEGREES, scalar or vector.
    D - Dec. or longitude in DEGREES, scalar or vector
            If the input A and D are arrays with 2 or more dimensions,
            they will be converted to a 1-D vectors.
    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
    Fields added for version 2:
      .PV1 - Vector of projection parameters associated with longitude axis
      .AXES - 2 element integer vector giving the FITS-convention axis
              numbers associated with astrometry, in ascending order.
              Default [1,2].
      .REVERSE - byte, true if first astrometry axis is Dec/latitude
      .COORDSYS - 1 or 2 character code giving coordinate system, including
                'C' = RA/Dec, 'G' = Galactic, 'E' = Ecliptic, 'X' = unknown.
      .RADECSYS - String giving RA/Dec system e.g. 'FK4', 'ICRS' etc.
      .EQUINOX - Double giving the epoch of the mean equator and equinox
      .DATEOBS - Text string giving (start) date/time of observations
      .MJDOBS - Modified julian date of start of observations.
      .X0Y0 - Implied offset in intermediate world coordinates if user has
                  specified a non-standard fiducial point via PV1 and also
                  has set PV1_0a =/ 0 to indicate that the offset should be
                  applied in order to place CRVAL at the IWC origin.
                  Should be *added* to the IWC derived from application of
                  CRPIX, CDELT, CD to the pixel coordinates.
        .DISTORT - Optional substructure specifying distortion parameters

Outputs


    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


      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').

Common Blocks


      BROYDEN_COMMON - Used when solving for a reverse distortion tranformation
        (either SIP or TGV) by iterating on the forward transformation.

Procedures Used


      CGErrorMsg (from Coyote Library)
      TAG_EXIST(), WCSSPH2XY

Revision History


    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
    Allows use of Version 2 astrometry structure & optimised for
    large input arrays. Wrap test for cylindrical coords. J. P. Leahy July 2013
    Wrap test failed for 2d input arrays
                              T. Ellsworth-Bowers/W.Landsman July 2013
    Tweaked to restore shape of arrays on exit JPL Aug 2013.
    ..and make them scalars if input is scalar JPL Aug 2013
    Iterate when forward SIP coefficients are supplied but not the reverse
    coefficients. Don't compute poles if not a cylindrical system
              W. Landsman Dec 2013
    Evaluate TPV distortion (SCAMP) if present W. Landsman Jan 2014
    Support IRAF TNX projection M. Sullivan U. of Southhamptom Mar 2014
    No longer check that CDELT[0] differs from 1 W. Landsman Apr 2015
   



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