DIST_CIRCLE

      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.

      DIST_CIRCLE, IM, N, [ XCEN, YCEN,  /DOUBLE ]

      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.

      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

       /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

       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

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