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XY2AD

XY2AD

Name


    XY2AD

Purpose


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

Explanation


    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. If 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.

Calling Sequence


    XY2AD, x, y, astr, a, d

Inputs


    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
    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
                 

Output


    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

Restrictions


      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.

Notes


      XY2AD 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


      TAG_EXIST(), WCSXY2SPH, SIP_EVAL(), TPV_EVAL()

Revision History


      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
      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
      Use CRVAL reference point for non-WCS transformation W.L. March 2007
      Use post V6.0 notation W.L. July 2009
      Some optimisation for large input arrays & use of version 2 astr
      structure, J. P. Leahy July 2013
      Evalue TPV distortion (SCAMP) if present W. Landsman Jan 2014
      Support IRAF TNX porjection M. Sullivan U. of Southamptom Mar 2014
      No longer check that CDELT[0] NE 1 W. Landsman Apr 2015



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