7.1.2 skyerr: Sky Matching with Per-Object Errors

matcher=skyerr values*='<ra/deg> <dec/deg> <error/arcsec>'
               params='<scale/arcsec>'
               tuning='<healpix-k>'

values*:

• ra/deg: Right Ascension
• dec/deg: Declination
• error/arcsec: Per-object error radius along a great circle

params:

• scale/arcsec: Rough average of per-object error distance; 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 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 no larger than the sum of the two per-row error values.

The scale parameter should be a rough average value of the error distances. It is used only to set a sensible default for healpix-k tuning parameter, and its value does not affect the result. If you set healpix-k directly, its value is ignored.

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

A variant form skyerr_q does the same thing but combines the two per-row error values in quadrature rather than by summation. In that case the separation between the two positions must be no larger than sqrt(error1²+error2²).

Note: the semantics of this matcher have changed slightly at version 2.4 of STILTS. In earlier versions the single parameter was named max-error and provided an additional constraint on the maximum accepted separation between matched objects. For most uses, the old and new behaviours are expected to give the same results, but in cases of difference, the new behaviour is more likely what you want.