ZOOM_XY

       Converts X, Y position on the image array to the the X,Y position
       in the current window.   (These  positions are identical
       only for an unroamed, zoomed image with with pixel (0,0) of the
       image placed at position (0,0) on the TV.)

      ZOOM_XY, Xim,Yim,Xtv,Ytv, [ OFFSET =, ZOOM = ]

      XIM - Scalar or vector giving X position(s) as read on the image
            display (e.g. with CURSOR,XIM,YIM,/DEVICE)
      YIM - Like XTV but giving Y position(s) as read on the image display.
      If only 2 parameters are supplied then XIM and YIM will be modfied
      on output to contain the converted coordinates.

      OFFSET - 2 element vector giving the location of the image pixel (0,0)
               on the window display.   OFFSET can be positive (e.g if the
               image is centered in a larger window) or negative (e.g. if the
               only the central region of an image much larger than the window
               is being displayed.
               Default value is [0,0], or no offset.
       ZOOM - Scalar specifying the magnification of the window with respect
               to the image variable.

      XTV,YTV - REAL*4 X and Y coordinates of the image corresponding to the
            cursor position on the TV display.   Same number of elements as
            XIM, YIM.

       If present, ZOOM_XY will use the TV and IMAGE common blocks which are
       defined in the MOUSSE software system (see
        http://archive.stsci.edu/uit/analysis.html)   If the user is not using
       the MOUSSE software (which keeps track of the offset and zoom in each
       window) then the common blocks are ignored.

       The integer value of a pixel is assumed to refer to the *center*
       of a pixel.

       Adapted from MOUSSE procedure of the same name W. Landsman HSTX Mar 1996
       Converted to IDL V5.0   W. Landsman   September 1997
       Properly include ZOOM keyword  W. Landsman   May 2000
       Put back common blocks for MOUSSE compatibility    September 2004

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