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. For simplicity, in CURSA 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 12.
|field||data item, field||component|
|number of columns||number of fields||arity, degree|
|number of rows||number of records||cardinality|
In CURSA 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, however, 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.)
A CURSA catalogue which contains celestial coordinates in a restricted format which CURSA can interpret is called a target list. The applications which convert between celestial coordinates and plot finding charts operate on target lists. Target lists are described in Section 7.
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.
Columns have a number of attributes, such as their name, data type and units. A column’s attributes hold all the details which define its characteristics. The more important column attributes are described in Section 4.1, below.
Catalogues can also contain auxiliary information which applies to the entire catalogue. CURSA recognises two types of auxiliary information: parameters and textual information. A parameter is a single datum, such as the epoch or equinox of celestial coordinates stored in a catalogue. CURSA parameters are similar to FITS keywords (in fact, CURSA interprets named keywords in a FITS table as parameters). Parameters have attributes similar to columns.
Textual information is information, usually descriptive, associated with the catalogue and intended to be read by a human. For a FITS table the textual information is basically the contents of any ‘COMMENTS’ and ‘HISTORY’ keywords3.
In the jargon of relational database systems auxiliary information is often called metadata. In the context of CURSA the metadata for a catalogue comprises the details of the columns (name, data type, units, etc.), the parameters and the textual information.
In order to use CURSA you do not need to know the details of all the attributes of a column, but there are a few which you will probably encounter. These attributes are listed in Table 2 and are described briefly below.
|NAME||Name of the column|
|DIMS||Dimensionality: scalar or vector|
|SIZE||Size (number of elements) of a vector|
|UNITS||Units of the column|
|EXFMT||External display format|
|COMM||Comments describing the column|
hd_numberwould all refer to the same column.
xcatview(see section 11) or
catview(see section 12). The external format specifier should be a valid Fortran 77 format specifier for the data type of the column. COMM Explanatory comments describing the column.