Through the Looking-Glass and What Alice Found There
An astronomical catalogue is basically a table of values, consisting of the measurements of the same property for a set of objects, together with the auxiliary information necessary to describe this table. There are several different terminologies for describing the elements of such tables. In this manual a terminology which corresponds loosely to that used intuitively for the paper versions of astronomical catalogues is used:
Some of the other terminologies are shown for comparison in Table 11.
|field||data item, field||component|
|number of columns||number of fields||arity, degree|
|number of rows||number of records||cardinality|
In the CAT library each catalogue can contain only one table and the two terms can usually be used interchangeably without introducing any ambiguity. However, where it is necessary to differentiate between the two sorts of entities, table is used to denote the simple matrix of rows and columns and catalogue is used to denote the combination of a table and its associated auxiliary information. Note that this usage implies nothing about the contents of the catalogue; it may contain a published astronomical catalogue, a set of private astronomical results or, indeed, data which are entirely non-astronomical.
In CAT the auxiliary information which applies to the entire catalogue comprises an arbitrary number of parameters. Each parameter comprises a single datum. Examples might be the epoch or equinox of celestial coordinates stored in a catalogue. CAT parameters are similar to FITS keywords (in fact, CAT interprets named keywords in a FITS table as parameters).
Columns and parameters both have a number of attributes, such as their name and data type. A full list of the attributes of columns and parameters is given in Sections 6.7 and 6.9 respectively, though normally you will only need to manipulate a few of them. All columns have the same set of attributes (though they take different values, of course) and similarly all parameters have the same set of attributes.
Columns may either be scalars in which case each field comprises a single datum, or vectors, one-dimensional arrays where each field comprises a one-dimensional array of values. Parameters may only be scalars.
This section has introduced the terminology for the more important items in a CAT catalogue. Section 6 contains a more extensive discussion.