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      Form a square array where each value is its distance to a given center.


      Returns a square array in which the value of each element is its
      distance to a specified center. Useful for circular aperture photometry.

Calling Sequence



      N = either a scalar specifying the size of the N x N square output
              array, or a 2 element vector specifying the size of the
              N x M rectangular output array.

Optional Inputs

      XCEN,YCEN = Scalars designating the X,Y pixel center. These need
              not be integers, and need not be located within the
              output image. If not supplied then the center of the output
              image is used (XCEN = YCEN = (N-1)/2.).


      IM - N by N (or M x N) floating array in which the value of each
              pixel is equal to its distance to XCEN,YCEN

Optional Input Keyword

      /DOUBLE - If this keyword is set and nonzero, the output array will
              be of type DOUBLE rather than floating point.


      Total the flux in a circular aperture within 3' of a specified RA
      and DEC on an 512 x 512 image IM, with a header H.
      IDL> adxy, H, RA, DEC, x, y ;Convert RA and DEC to X,Y
      IDL> getrot, H, rot, cdelt ;CDELT gives plate scale deg/pixel
      IDL> cdelt = cdelt*3600. ;Convert to arc sec/pixel
      IDL> dist_circle, circle, 512, x, y ;Create a distance circle image
      IDL> circle = circle*abs(cdelt[0]) ;Distances now given in arcseconds
      IDL> good = where(circle LT 180) ;Within 3 arc minutes
      IDL> print,total( IM[good] ) ;Total pixel values within 3'


      The speed of DIST_CIRCLE decreases and the the demands on virtual
      increase as the square of the output dimensions. Users should
      dimension the output array as small as possible, and re-use the
      array rather than re-calling DIST_CIRCLE

Modification History

      Adapted from DIST W. Landsman March 1991
      Allow a rectangular output array W. Landsman June 1994
      Converted to IDL V5.0 W. Landsman September 1997
      Add /DOUBLE keyword, make XCEN,YCEN optional W. Landsman Jun 1998

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