The required conversions are between mean (FK4 and FK5) or apparent coordinates of arbitrary date and mean FK5 coordinates referred to equinox J2000 and the epoch of observation. The algorithms used are those in the COCO package (SUN 56), which is based on SLALIB (q.v. ) subroutines. Users should be aware of a subtle difference in the way that sky position angle is specified for the WHT and the other two (equatorial) telescopes. The WHT is an altazimuth telescope, and corrects continuously for field rotation. The instrument rotators are controlled in such a way that the position angle on the sky is defined in the coordinate system used for the target (B1950, J2000, etc. ). The GSC positions are in the J2000 system, and the GSS software therefore corrects the input position angle for the effects of precession before computing guide probe positions. By contrast, the instrument rotators on the INT and JKT are not designed for continuous movement, and are left in a fixed position, independent of coordinate system. This effectively corresponds to adopting a sky position angle defined in geocentric apparent coordinates, and the GSS operates on this assumption. Effects such as differential refraction and pointing model corrections are not included in the GSS software, since they require knowledge of the time of observation and the details of the telescope models. For most practical purposes, the definition of position angle is of little importance, and the assumptions made by the GSS will be the most appropriate. Applications in which precise positioning is required over a wide field may require a careful check to ensure that the position angles used by the TCS, GSS and any other astrometric software are self-consistent.