This section describes how multiple matches are handled by catpair. Multiple matches can arise because the pairing techniques are matching objects with similar rather than identical positions and an object in one catalogue can pair with several in the other catalogue. The terminology used in this section is:
That is, any match is potentially a pair and the pairing algorithm must prescribe which matches are considered pairs. There are three cases for multiple matches:
catpair is unsuitable for handling the third case, and should not be used with catalogues where it is likely to be important. There are, however, several options for handling the first two cases:
The third option is not practical in a general purpose program such as catpair because it relies on astronomical knowledge about the catalogues being paired. Either of the first two options may be appropriate, depending on the details of the pairing being performed. catpair provides both options separately for multiple matches in the primary and secondary, and you should choose the alternatives appropriate for your work.
An example might help to illustrate the difference between multiple matches in the primary and secondary. Suppose the primary was a private list of target objects and the secondary was the NGC catalogue. Table shows the equatorial coordinates for the triplet of galaxies NGC 3623, NGC 3627 and NGC 362812. Consider the following two cases.
CURSA Catalogue and Table Manipulation Applications