ii. Optical Source Associations
The 2MASS position reconstruction algorithms (cf. III.3.g) require matching infrared detections with objects in the primary astrometric reference catalog ( ACT) and the higher density USNOA-2.0 catalog. For each 2MASS source matched to these optical catalogs, the identifier, blue and red magnitudes, and 2MASS-to-optical positional offset information from the ACT or USNOA catalogs is included in the 2MASS Point Source Catalog records. It is emphasized that these are not identifications between the infrared and optical sources, but only associations.
Possible optical associations for the 2MASS sources are found using a simple closest positional-match algorithm. The closest optical source to each 2MASS source, within a maximum separation of approximately 6" is reported. A match to an ACT source takes precedence over USNOA match. No attempt is made to find the best pairings of 2MASS and optical sources in the event an optical catalog object can be matched to more than one 2MASS source. The number of possible optical matches for each 2MASS point source within the 6" search radius is provided for each association. Also listed is the number of 2MASS sources the associated optical catalog object might have been associated with. These last two parameters provide some measure of possible confusion in the associations.