Converting between celestial coordinate systems

CURSA contains some limited facilities for converting between different celestial coordinate systems. Application catcoord can convert mean equatorial coordinates for a given equinox and epoch to mean equatorial coordinates for a new equinox and epoch, to Galactic coordinates⁹, or to de Vaucouleurs' supergalactic coordinates. catcoord uses the Starlink subroutine library SLA to convert between coordinate systems. The manual for this library, SUN/67[32], contains a brief introduction to the various celestial coordinate systems. Further details can be found in standard textbooks on spherical astronomy (see, for example, Spherical Astronomy by R.M. Green[15]).

catcoord creates a copy of the catalogue with the new coordinates added. It operates on a target list (see Section ). That is, it requires that the input catalogue contains columns of coordinates which it can interpret. The input catalogue must contain columns of Right Ascension and Declination for some equinox and epoch. Optionally it can also contain columns of proper motion in Right Ascension and Declination, parallax and radial velocity which permit more accurate conversions. It is not necessary that all four additional columns be present in order to use them. For example, if only columns of proper motion are present they can be used in isolation. These additional columns are usually only available in catalogues of relatively nearby and well-observed stars. In most catalogues the coordinates will simply comprise a Right Ascension and Declination for some equinox and epoch.

The coordinates computed by catcoord are suitable for plotting, display, pairing etc. However, for accurate work they are not suitable for further subsequent conversions to another equinox and epoch. This limitation arises because only new coordinates are computed; the proper motions etc. are not revised for the new equinox and epoch. Thus, in accurate work, new coordinates should always be computed from the original coordinates in the target list, not from intermediate coordinates created with catcoord. However, this caveat is only important when accurate coordinates are being computed.

catcoord offers only a limited set of conversions (converting mean equatorial coordinates to a new equinox and epoch, to Galactic coordinates or to supergalactic coordinates). Additional conversions, such as converting mean equatorial coordinates for some equinox and epoch to apparent coordinates, are available using the Starlink package COCO (see SUN/56[31]). To use COCO first use xcatview (see Section ) to save the coordinates as a text file in a suitable format and them import them into COCO.