WFPC2_METRIC

   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.

      WFPC2_METRIC, xin, yin, xout, yout, [ChipNum, HEADER=, /GLOBAL
                                           YEAR =, FILTER=
                            or
      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 distorted 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.

     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.

     /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
              incorporated.
     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
         chips.
        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.

     LINTERP, SXPAR(), XYAD, YMD2DN()

     Written     W. Landsman         March 2003

[Home Page] [Software, Documentation] [IDL Documentation] [Quick Reference] [Feedback]