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DIST_ELLIPSE

## Name

DIST_ELLIPSE

## Purpose

Create a mask array useful for elliptical aperture photemetry

## Explanation

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

DIST_ELLIPSE, IM, N, XC, YC, RATIO, POS_ANG, /DOUBLE

## Inputs

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.

## 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.

## Output

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.

## Example

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
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'

## Restrictions

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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