23 Importing CDS catalogues

A large collection of astronomical catalogues are available on-line at the Centre de Données astronomiques de Strasbourg (CDS) and can be retrieved by anonymous ftp (see Section 2). Most of these catalogues are available in two formats: FITS tables and simple ASCII text files. The recommended route to access these catalogues with CURSA is to retrieve the simple text file version. It is then usually possible to automatically construct an STL description of the file which correctly interprets the celestial coordinates in the catalogue. (If you retrieve the FITS table version the individual sexagesimal components of the celestial coordinates will be treated as separate columns, making the coordinates difficult to process.)

Each CDS catalogue usually comprises (at least¹⁸) two files: the data file containing the columns and rows of the catalogue and a ‘CDS description file’ detailing the contents of the catalogue. By convention this CDS description file has the file name ‘ReadMe’ (and consequently is known as the ‘ReadMe file’). The description of the catalogue which it contains is in a standardised form. CURSA application catcdsin will read a CDS ReadMe file and construct an equivalent CURSA STL description file from it. This STL description file will usually contain a description of the celestial coordinates in the catalogues which is fully compatible with CURSA.

catcdsin does not copy the CDS catalogue. It merely constructs an STL description file from the CDS description file. Both these files describe (using a different syntax, of course) the same catalogue text file. Because catcdsin does not have to copy the catalogue it executes quickly, irrespective of the size of the catalogue.

23.1 Running catcdsin

Unlike most other CURSA applications catcdsin is not an ADAM A-task (it is, in fact, a Perl script). Consequently, it handles parameters slightly differently to other applications. However, it never prompts for any parameters so the differences will not usually be important to you.

Suppose that you had the text version of a CDS catalogue and its corresponding ReadMe file in your current directory. You would simply type:

catcdsin generates the corresponding STL description, displays the name of the STL description file it has created and terminates. There are various options which can be specified. By default catcdsin copies the ReadMe file to the description file as textual comments. This behaviour can be suppressed by typing:

(This option is analogous to the usual mechanism for controlling the amount of textual information copied, which is described in Section 10.1.) The equinox and epoch of the celestial coordinates cannot be reliably determined automatically from the ReadMe file. You will need to read the ReadMe file yourself and decide what they are. They can then be specified by typing, for example:

Obviously, you substitute values appropriate to your catalogue. The equinox and epoch should have their usual CURSA syntax (see Section 7). Either, neither or both can be specified. If you wish to suppress the automatic interpretation of celestial coordinates, and instead have the sexagesimal subdivisions of angles treated as separate columns, type:

The input CDS description file does not have to be called ReadMe. For example, a file called cdsdesc.lis could be processed by typing:

These various options can be combined. For example, to process a file called cdsdesc.lis, specifying the equinox as J2000 and not copying the CDS file as textual information type:

CDS ReadMe files can (and often do) contain descriptions of more than one catalogue or table. Usually these catalogues or tables will be closely related; perhaps a main catalogue and a table of notes. catcdsin creates a separate STL description file for every catalogue found in the ReadMe file.

A few CDS catalogues do not contain celestial coordinates; spectral line wavelength lists are the obvious example. Occasionally the coordinates may be in a non-standard format which catcdsin does not interpret properly. In this case it may be possible to fix-up the STL description file generated by catcdsin by hand. See Appendices D and E for details of the STL format. Such occurrences seem to be rare.