Algorithms

This section describes the parameters the user can use for cross-matching catalogues. For more information on the cross-match service internals, see Pineau et al. (2011a) and the associated ADASS 2010 presentation.

Cross-match criteria

By position

This cross-match is a simple cross-match: for each source of the catalogue A, it returns the sources of the catalogue B lying at an angular distance less than Radius (\theta) from the catalogue A source.

In other words, for each source of the catalogue A, it returns the sources of the catalogue B which are inside the cone of aperture 2\theta having for apex (also called vertex) the center of the ICRS frame and for axis the direction of the catalogue A source in the ICRS frame.

By position including error

This cross-match is a cross-match based on the positional uncertainties of both catalogue A and catalogue B sources. The selection of the candidates is the same as described in section 3.1.1 of Pineau et al (2011b).

Sigma corresponds to k_{\gamma} in eq. 3 of Pineau et al (2011b).
Max. distance is a safeguard preventing from correlating sources having a too high angular separation.

To look for candidates, we first estimate the maximum of the semi-major axis of errors on the positions of catalogue B sources by a boxplot: e_{B max} = q_3 + 1.5 (q_3 - q_1) where q_1 and q_3 are the lower and upper quartiles respectively.

For each source of the catalogue A, the search radius we use to look for catalogue B source candidates is the minimum between k_{\gamma} \sqrt{e_A^2 + e_{B max}^2} (in which e_A is the semi-major axis of the error on the source of the catalogue A), and the watchdog Max. distance. We then keep only candidates satisfying eq. 3 of Pineau et al (2011b).

For USNO-B1.0: a systematic error of 0.2 arcsec is quadratically added to positional errors on both e_RAJ2000 and e_DEJ2000.

Warnings:

Sources having for positional error a NULL (or a NaN) value are ignored.

Because of the use of a boxplot on positional errors to avoid outliers, the result of the cross-match of 2 catalogues is not necessarily symmetric: the number of associations between A and B can be different from the number of associations between B and A.

Cross-match area

All-sky

All sources of the catalogue A are cross-matched.

Cone

Only the sources of the catalogue A lying at an angular distance less than Radius from the given position (Center) are cross-matched.

Healpix Cell

Only the sources of the catalogue A which are inside the HEALPix pixel of given Index and Nside are cross-matched. The given Index is the index defined by the HEALPix Nested scheme in the ICRS frame. For more information on HEALPix, see http://healpix.jpl.nasa.gov or Gorski et al. 2005.