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## B.39 `tskymatch2`: Crossmatches 2 tables on sky position

`tskymatch2` performs a crossmatch of two tables based on the proximity of sky positions. You specify the columns or expressions giving right ascension and declination in degrees for each input table, and a maximum permissible separation in arcseconds, and the resulting joined table is output.

If you omit expressions for the RA and Dec, an attempt is made to identify the columns to use using column Unified Content Descriptors (UCDs) or names. First columns bearing appropriate UCD1 or UCD1+ values (`POS_EQ_RA`, `POS_EQ_RA_MAIN`, `pos.eq.ra` or `pos.eq.ra;meta.main` and their equivalents for declination) are sought. If these cannot be found, columns named something like "RA" or "RA2000" are sought. If either is found, the column units are consulted and radian->degree conversions are performed if necessary (degrees are assumed if no unit value is given). If nothing likely can be found, then the command will fail with an error message. This search logic is intended as a convenience only; it is somewhat ad hoc and subject to change. To make sure that the correct angle values are being used, specify the `ra` and `dec` position parameters explicitly.

`tskymatch2` is simply a cut-down version, provided for convenience, of the more general matching task `tmatch2`. If you want more match options or otherwise more configurability, you can probably find it by using `tmatch2`.

- B.39.1 Usage
- B.39.2 Examples

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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