DIST_ELLIPSE

       Create a mask array useful for elliptical aperture photemetry

       Form an array in which the value of each element is equal to the
       semi-major axis of the ellipse of specified center, axial ratio, and
       position  angle, which passes through that element.  Useful for
       elliptical aperture photometry.

       DIST_ELLIPSE, IM, N, XC, YC, RATIO, POS_ANG, /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
               M x N rectangular output array.
       XC,YC - Scalars giving the position of the ellipse center.   This does
               not necessarily have to be within the image
       RATIO - Scalar giving the ratio of the major to minor axis.   This
               should be greater than 1 for postion angle to have its
               standard meaning.

       POS_ANG - Position angle of the major axis, measured counter-clockwise
               from the Y axis.  For an image in standard orientation
               (North up, East left) this is the astronomical position angle.

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

       IM - REAL*4 elliptical mask array, of size M x N.  THe value of each
               pixel is equal to the semi-major axis of the ellipse of center
                XC,YC, axial ratio RATIO, and position angle POS_ANG, which
               passes through the pixel.

       Total the flux in a elliptical aperture with a major axis of 3', an
       axial ratio of 2.3, and a position angle of 25 degrees centered on
       a specified RA and DEC.   The image array, IM is 200 x 200, and has
       an associated FITS header H.
       ADXY, H, ra, dec, x, y       ;Get X and Y corresponding to RA and Dec
       GETROT, H, rot, cdelt        ;CDELT gives plate scale degrees/pixel
       cdelt = abs( cdelt)*3600.    ;CDELT now in arc seconds/pixel
       DIST_ELLIPSE, ell, 200, x, y, 2.3, 25  ;Create a elliptical image mask
       ell = ell*cdelt(0)           ;Distances now given in arcseconds
       good = where( ell lt 180 )   ;Within 3 arc minutes
       print,total( im(good) )      ;Total pixel values within 3'

       The speed of DIST_ELLIPSE 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_ELLIPSE

       Written    W. Landsman             April, 1991
       Somewhat faster algorithm          August, 1992
       Allow rectangular output array     June, 1994
       Converted to IDL V5.0   W. Landsman   September 1997
       Added /DOUBLE keyword   W. Landsman   July 2000

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