Requirements

Obviously before running catpair you must have a primary and a secondary catalogue. The secondary catalogue must be sorted on the second column that is to be used for the pairing (usually this will be the $y$ or Declination coordinate). If your secondary is not sorted in this way then use catsort (see Section , above) to create a suitably sorted secondary catalogue.

You need to know the names of the columns in both catalogues which contain the coordinates which are to be used for the pairing (and whether they are Cartesian or spherical-polar coordinates). If you are in doubt about the columns in the catalogues use catheader (see Section , above) to obtain the details. If the coordinates are Cartesian then the coordinates in both input catalogues must be in the same system, with the same units,¹⁰ zero point and orientation. That is, a given value for the coordinates (say 23.5, 105.7) should correspond to the same position in both catalogues. If the coordinates are spherical-polar they must always be in units of radians. The coordinates in the two catalogues should be of the same type (equatorial, Galactic etc.) and if they are equatorial they should have the same system, epoch and equinox.

Finally you need to specify the critical distance, $D$, which determines whether two objects, one in each catalogue, are considered pairs or not. If the actual separation of the two objects is less than or equal to this distance then they are considered pairs; if it is greater then they are not. In catpair this critical distance may be either a constant, a column in the primary (so it varies for different objects in the primary) or an expression based on columns in the primary. In practice the value adopted for the critical distance is often derived from the errors associated with the positions in the catalogues. If you do not already know the errors on the positions in your catalogues, you could consult the textual information information associated with the catalogue, which will often contain these details. Again use catheader (see Section ) to access this information.