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#### 7.1.3 `skyellipse`: Sky Matching of Elliptical Regions

    matcher=skyellipse values*='<ra/degrees> <dec/degrees> <primary-radius/arcsec>
                                <secondary-radius/arcsec>
                                <position-angle/degrees>'
                       params='<scale/arcsec>'
                       tuning='<healpix-k>'

> `values*`:
>   - `ra/degrees`: Right ascension of centre
>   - `dec/degrees`: Declination of centre
>   - `primary-radius/arcsec`: Length of ellipse semi-major axis
>   - `secondary-radius/arcsec`: Length of ellipse semi-minor axis
>   - `position-angle/degrees`: Position angle - measured from north pole to primary axis, in direction of positive RA
> `params`:
>   - `scale/arcsec`: Rough average of ellipse major radius; just used for tuning to set default pixel size
> `tuning`:
>   - `healpix-k`: Controls sky pixel size. Legal range 0 - 29. 0 is 60deg, 20 is 0.2".

The `skyellipse` matcher compares elliptical regions on the sky for overlap. Each row has to provide five values, giving the centre, the major and minor radii, and the position angle of an ellipse. Rows are considered to match if there is any overlap between the ellipses. The goodness of match is a normalised generalisation of the symmetrical case used by the `skyerr` matcher, in which the best possible match is two concentric ellipses, and the worst allowable match is when the circumferences just touch.

The calculations are approximate since in some cases they rely on projecting the ellipses onto a Cartesian tangent plane before evaluating the match, so for larger ellipses the criterion will be less exact. For objects the size of most observed stars or galaxies, this approximation is not expected to be problematic.

The `scale` parameter must be supplied, and should be a rough average value of the major radii. it is used only to set a sensible default for the `healpix-k` tuning parameter, and its value does not affect the result. If you set `healpix-k` directly, the value of `scale` is ignored.

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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk