Pairing two catalogues

catpair is provided to identify `corresponding' objects in two catalogues; objects are considered to correspond if they have similar positions. An output catalogue is generated from the list of corresponding objects.

In astronomical catalogues the `corresponding' rows in two catalogues are usually rows which contain data for the same astronomical object. Traditionally in relational database systems corresponding rows are identified by having identical values for some field, such as a name. For example, two rows might be considered to correspond if a name field in both catalogues adopted the value `NGC 1305' for both rows. This operation is usually called joining the two catalogues.

Figure: Two datasets for joining

In astronomical problems such joining by an exact match is relatively uncommon. A more common case is where corresponding objects are identified by similar positions in both catalogues. This situation is illustrated in Figure . The important point here is that, essentially because of measurement errors, the corresponding positions are merely similar, not an exact match. This circumstance makes establishing corresponding rows a much more complicated and problematic process. In practice the positions used are almost always some type of two-dimensional coordinates; usually celestial coordinates such as Right Ascension and Declination, or possibly Cartesian coordinates of some sort. In principle one, three or higher dimensional coordinates could be used though they are not important in practice. catpair only supports joining based on two-dimensional coordinates, though the coordinates may be either Cartesian or spherical-polar.

In CURSA this special sort of join based on an approximate match in two-dimensional coordinates is called pairing. Thus, in this usage, pairing is a special case of joining catalogues, albeit one which is important in astronomical practice.

catpair operates on two input catalogues, known as the primary and secondary catalogues. To fix ideas, think of the primary as being a small list of target objects which you have compiled, and the secondary as being a standard catalogue, such as the SAO star catalogue, one of the Durchmusterungen or Dreyer's New General Catalogue of non-stellar objects. The final result of the pairing is a new catalogue containing the paired objects; the output catalogue.

If you wish to pair several catalogues to create a single output catalogue you should invoke catpair several times, creating intermediate paired catalogues as appropriate.

Pairing is a relatively complicated process and you must answer several prompts to fully specify the operations to be performed. The following two sections, `Requirements' and `Running catpair' respectively describe the requirements for catpair and how to run it. You should read at least these two sections. Subsequent sections describe various aspects of the pairing process in greater detail. While it is not strictly necessary to read these latter sections, they may help you to understand what catpair is doing and hence to use it more effectively.