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#### 7.1.1 `sky`: Sky Matching

    matcher=sky values*='<ra/degrees> <dec/degrees>'
                params='<max-error/arcsec>'
                tuning='<healpix-k>'

> `values*`:
>   - `ra/degrees`: Right Ascension
>   - `dec/degrees`: Declination
> `params`:
>   - `max-error/arcsec`: Maximum separation along a great circle
> `tuning`:
>   - `healpix-k`: Controls sky pixel size. Legal range 0 - 29. 0 is 60deg, 20 is 0.2".

The `sky` matcher compares positions on the celestial sphere with a fixed error radius. Rows are considered to match when the two (`ra`, `dec`) positions are within `max-error` arcseconds of each other along a great circle.

In fact this matching is not restricted to equatorial coordinates - the `ra` and `dec` parameters may represent any longitude-like and latitude-like coordinates in degrees, since the spherical geometry for the matching is unchanged under such transformations.

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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