The main objective of the software described in this manual is to use the Space Telescope Guide Star Catalogue (hereafter GSC) to locate guide stars suitable for the ING Telescopes on La Palma. The GSC provides a large number of stars with (fairly) accurately-known positions. It will generally contain several stars in any of the guiding fields for all except the most perverse target positions (e.g. in dark clouds). The only exception is the WHT Nasmyth focus (through the derotation optics), where the field is too small to guarantee that a suitable star will be found. The software, therefore, accesses the GSC, finds suitable guide stars for a given position and telescope configuration and outputs their positions in a form suitable for the telescope, A&G and autoguider control software.
The software package is called GSS (Guide-Star Search) and runs on Vaxes under VMS. It is installed on the ING Vax Cluster and on the software development machines at RGO, and may be run from remote computers via the Internet. It provides output in the form of text files of guide star and probe positions and may be used to produce plots of the fields on a variety of graphics devices.
A more specialised application is the selection of guide stars suitable for closed-loop blind offsets. This is useful for slit spectroscopy of faint targets on either the WHT or the INT. The technique relies on the fact that the encoding of the guide probes is more precise than that of the telescope drives. A reference star whose position is known accurately with respect to the target is acquired on the spectrograph slit with the autoguider loop locked. The telescope and guide probe are then moved simultaneously by the known offset to the target, so that the same guide star remains in the autoguider field. When the guide loop is locked, the target should be accurately centred on the spectrograph slit. The fields accessible to the guide probes have peculiar shapes, so it is sensible to check in advance whether a suitable guide star will be visible both before and after the offset. If not, a different choice of reference star or slit position angle can be made.
Whilst GSS itself is an off-line application, selection and acquisition of guide stars for the WHT is integrated into the on-line control software via the TAG I-task. This uses the same catalogue, algorithms and applications code as GSS, but will generally require fewer steps to operate, and will reduce observing overheads significantly. TAG is recommended for most guide-star applications on the WHT (see the TAG Manual for more details). Even if you intend to use TAG at the telescope, it is worth running GSS to locate guide stars in advance if there is any doubt whether they will be accessible. The most obvious cases are:
The GSS software is supported from RGO Cambridge by Marion Fisher (mpf@ast.cam.ac.uk) and Robert Laing (rl@ast.cam.ac.uk). Please report any faults or suggestions for improvements via the Defect Database or directly to the authors.