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.

Mon Mar 1 17:52:09 GMT 1999