Tycho-2 TDSC Merge Best Neighbour

The cross-match algorithm is not symmetric and searches Tycho2tdscMerge source counterparts in Gaia. The best neighbour is chosen among good neighbours as the one with the highest value of the figure of merit, which evaluates the ratio between two opposite models/hypotheses: the counterpart candidate is a match or it is found by chance.

Good neighbours are nearby objects in Gaia whose position is compatible within position errors with the external catalogue target. The cross-match algorithm is positional and exploits the full 5 parameter covariance matrix of the Gaia astrometric solution when available and the external catalogue positions and position errors. In addition it takes into account the Gaia environment using the local density.</p> <p>Please note that the cross-match algorithm is a trade-off between multiple requirements, in particular between completeness and correctness.