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      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.

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
              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 position angle to have its
              standard meaning.

Optional Inputs

      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.

Optional Input Keyword

      /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

Revision History

      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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