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  Compute the distortion in a WFPC2 image and optionally return coordinates
  Uses the distortion solution of Anderson & King (2003, PASP, 115, 113)
  Pixel 424, 424 on each chip remains fixed, and other pixel positions are
  mapped to remove nonlinearities. If /GLOBAL is set, then all chips are
  put on the same reference frame where pixel 424, 424 in the WF3 chip
  remains fixed.

Calling Sequence

      WFPC2_METRIC, xin, yin, xout, yout, [ChipNum, HEADER=, /GLOBAL
                                          YEAR =, FILTER=
      WFPC2_METRIC, xin, yin, a, d, HEADER=, /RAdec, /GLOBAL ]


    XIN, YIN - X,Y positions (0-799) on a WFPC2 chip in
              IDL convention (first pixel is 0,0), scalar or vectors


    XOUT, YOUT - X,Y positions in the undistorted frame, same number of
                  elements as XIN, YIN
                          or if /RADEC is set
    AA, DD - Right ascension and declination (in degrees) corresponding
              to the input coordinates after distortion correction.

Optional Input

    ChipNum - Integer 1, 2, 3, or 4 specifying the WFPC2 chip number
            1-PC, 2-WF2, 3-WF3, 4-WF4. If not supplied, then WFPC2_METRIC
            will try to read the value from the DETECTOR in the FITS header.

Optional Inputs

    /GLOBAL - If set, then positions are returned in a master reference
              frame with pixel 424,424 of WF3 remaining fixed. Thus,
              information concerning the interchip separation and
              orientation (with a weak dependence on time and filter) is
    Header - FITS header with astrometry for a particular chip.
            If both /RADec and /Global are set, then the header must be
            from the WF3 chip.
    /RADec - If set, then astrometry information in the FITS header (which
            must be supplied as a keyword) is used to convert the output
            to Right Ascension and declination (both in degrees).
    FILTER - Filter name needed if /GLOBAL is set, must be either 'F300W'
            'F336W', 'F439W', 'F555W' or 'F814W'; otherwise the plate scale
            for F555W is assumed. WFPC2_METRIC will try to read this
            value from the FITS header if not supplied as a keyword.
    YEAR - Observation year including fraction (e.g. 1998.56) needed if
            /GLOBAL is set. WFPC2_METRIC will try to read this value from
            the FITS header if not supplied as a keyword. The time
            correction is currently applied through the year 2002; later
            dates will use the year 2002 correction.


    (1) Find the undistorted X,Y coordinates of position 682.3,234.2 on chip 1
        (the PC chip).
          IDL> WFPC2_METRIC, 682.3, 234.2, xout, yout, 1
            ==> xout = 681.13 yout = 235.05
    (2) Determine the RA and Dec of position 682.3, 234.2 on chip 1 on the
        WFPC2 image U2Z30201T
        IDL> WFPC2_READ, 'u2z30201t.c0h', im,h ;Get header for chip 1
        IDL> WFPC2_METRIC, 682.3, 234.2, aa, dd, header= h,/RADec
        IDL> print, adstring(aa,dd,2)
        05 20 53.572 -69 35 18.17
        Note that a chip number did not need to be specified since its value
        is in the FITS header
    (3) As above, but now compute coordinates in the global frame, needed
        for example, to compute the distance between stars on two different
        First get headers for chips 1 and 3
        IDL> WFPC2_READ, 'u2z30201t.c0h', im1,h1, im3,h3,num=[1,3]
        IDL> WFPC2_METRIC, 682.3, 234.2, aa, dd, 1, header=h3,/RADec,/GLOBAL
        IDL> print, adstring(aa,dd,2)
        05 20 53.513 -69 35 17.98
        Note that with /GLOBAL set, that the header must be for WF3, even
        though coordinates are being computed for chip 1. Also note that
        the time and filter will be read from the FITS header. Finally,
        note that the coordinates given in examples (2) and (3) differ
        slightly, because the chip separations incorporated in the FITS
        headers differ slightly from those in the Anderson & King solution.

Procedures Used


Revision History

    Written W. Landsman March 2003

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