### 6 Celestial coordinates

Most astronomical catalogues contain columns of celestial coordinates of some sort: usually Right Ascension and Declination for some equinox and epoch, or perhaps Galactic or ecliptic coordinates. The storage, manipulation and presentation for display of celestial coordinates in the computer-readable version of astronomical catalogues is something of a vexed topic which has caused a deal of confusion and difficulty, much of it, in principle, unnecessary.

For preexisting catalogues, such as those described in Section 2, the format of the celestial coordinates will already be fixed and CURSA will simply display the columns in whatever way is possible. For example, many catalogues contain the hours, or degrees, minutes and seconds which comprise a coordinate as separate columns; a form which is singularly inconvenient for further processing. However, CURSA has some special facilities for processing and displaying coordinates, and catalogues that have been specifically prepared for CURSA can take advantage of these.

CURSA can store columns of coordinates as radians but automatically present them as sexagesimal hours or degrees when they are listed by the browsing applications xcatview (see Section 11) or catview (see Section 12). The advantages of this approach are that internally within CURSA the coordinates remain in radians, which is the most convenient form for computations, but they are presented to the user, and he interacts with them, as sexagesimal hours or degrees, which is the way that he naturally thinks about them.

Also, it is possible within xcatview or catview to interactively alter the precise way that a coordinate is formatted for display. These facilities are described in detail in Appendix B. Similarly, while displaying coordinates in units of hours or degrees formatted as sexagesimal values is usually the required behaviour, occasionally you may want to display angles as simple decimal numbers expressed in radians (as they are represented internally). Both xcatview and catview provide this facility.

CURSA application catgscin (see Section 24) reformats coordinates in regions of the HST Guide Star Catalog to a format which is fully compatible with CURSA. Similarly, catremote (see Section 25), the application for extracting subsets from remote on-line catalogues, returns coordinates which are fully compatible. Also, catcdsin (see Section 23) will usually reformat the text versions of CDS catalogues into STL format catalogues containing CURSA-compatible coordinates. Finally, the CURSA ‘home page’ (see page 8 for the URL) contains a list of catalogues which have been converted to have coordinates which are fully compatible with CURSA.