Guide stars for the WHT, INT or JKT autoguiders may be found from the HST's Guide Star Catalog, which is kept on disc and CD ROM at the Observatory. The basic catalogue contains about 19 million objects in the magnitude range 6--16, of which 15 million are stellar. It is based on two single-epoch, single passband Schmidt surveys with plate epochs between 1975 and 1983. Objects are classed as stellar or non-stellar. The limiting V magnitude is about 15.6 in uncrowded regions (brighter in crowded areas). Astrometry is based on the AGK3, SAOC and CPC catalogues. The density of stars in the GSC is sufficiently large that suitable guide stars can be found almost everywhere, except in cases where the autoguider viewing area is especially small ( e.g., WHT UES).
Absolute positional errors vary from 0.5arcsec at the original plate centres to 1.1--1.6arcsec at the N or S plate edges. Relative errors at 30arcmin separations vary from 0.3--0.8arcsec depending on hemisphere and magnitude. Hence the positions are not quite accurate enough to be used for reference stars for blind offsetting into narrow (1.0 arcsec) slits. These positions should only be used for blind offsetting into slits if a wide slit is being used or if some uncertainty in the final slit position can be tolerated ( e.g., offsetting onto an extended source).
The Guide Star Server (GSS) is a package which finds guide stars for the offset autoguiders at the different focal stations of the JKT, INT and WHT. It takes the target's position in any of the co-ordinate systems supported by the individual telescope's control system, together with the rotator ( i.e., spectrograph or camera) position angle, and outputs a list of suitable guide stars with positions and predicted coordinates of the autoguider probes.
Magnitude limits for selecting stars can be altered, and there is an option to produce a simple listing and plot of all the nearest Catalog stars (useful for making finding charts and for offsetting).
The GSS can be run from any account on the VAX cluster on La Palma, or on the Software Development Vax (GXSEG0) at Cambridge. The server can be run from the account GSS on GXSEG0; the password is available from software development group staff.
The guide star catalogue may also be accessed directly from the WHT Instrument Control Computer using the TAG I-task.
A User Guide is available from La Palma or Cambridge support staff, and it is kept in each telescope's control area.