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7.1.2 skyerr: Sky Matching with Per-Object Errors

matcher=skyerr values*='<ra/degrees> <dec/degrees> <error/arcsec>'
               params='<max-error/arcsec>'
               tuning='<healpix-k>'
values*:
  • ra/degrees: Right Ascension
  • dec/degrees: Declination
  • error/arcsec: Per-object error radius along a great circle
params:
  • max-error/arcsec: Maximum separation along a great circle - additional constraint to per-object errors
tuning:
  • healpix-k: Controls sky pixel size. Legal range 0 (60deg) - 20 (0.2"). k = log2(nside).

The skyerr matcher compares positions on the celestial sphere using error radii which can be different for each row. Rows are considered to match when the separation between the two ra, dec positions is smaller than both the fixed max-error value and the sum of the two per-row error values. If either of the error values is blank, then any separation up to max-error is considered a match.

According to these rules, you might decide to set max-error to an arbitarily large number so that only the sum of errors will determine the actual match criteria. However please don't do this, since max-error also functions as a tuning parameter for the matching algorithm, and ought to be reasonably close to the actual maximum acceptable separation. Note that if you set the healpix-k parameter explicitly, you may set max-error as large as you like.

As with sky matching, other longitude/latitude coordinate pairs may be used in place of right ascension and declination.


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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@bristol.ac.uk