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Acquiring guide stars

Guide star acquisition is facilitated by the Guide Star Server (GSS), with which it is possible to predict the probe co-ordinates for a star from the Hubble Space Telescope Guide Star Catalogue using software running on a Vaxstation in the La Palma cluster. The catalogue itself is on two CD-roms attached to this Vaxstation. To run the GSS, star a session on any VAX computer and type:--

@DISK$USER3:[GSS]GSS_INIT
GSS

This will invoke the GSS programme, which is characterised by the prompt GSS>. The following string of commands input to the GSS programme will generate a file, OUTPUT.GS, with a list of suitable guide stars and their predicted AUTORADIAL and AUTOTHETA values:-

CONFIG WHT CASS
ROTATOR <rotatorposition>
SEARCH <objectposition>
EXIT

Where the rotator position is the sky position angle in degrees, and the object position is specified as RA, Dec and Equinox in the same format as is used by the Telescope Computer. The file OUTPUT.GS can then be typed or printed, and the guide star acquired.

To find a guide star, search the autoguider field by typing:--

FIELD at the ICL interface, or:
1 FIELD at the autoguider interface.

The autoguider will scan the field three times, then look for a star on the final readout. If a suitable star is found then a white cursor appears on the star image. It is then possible to guide on that star. If the star appears too faint it is possible to increase the guiding integration time by typing:--

GUIINT <time> at the ICL interface, or:
<time> GUIINT at the autoguider interface,

where the time is in milliseconds. Guiding integration times can be as short as half a second, and a sensible maximum is 10 seconds.

If no sufficiently bright star is found in the field the procedure is to move guide probe with the ICL AUTOTHETA or AUTORADIAL commands by about the field size. The field size is approximately 8000 millidegrees in AUTOTHETA (it depends somewhat upon the radial position), and about 10000 microns in AUTORADIAL. Beware of low values of AUTOTHETA; at values below about 35000 the guide probe vignettes the slit viewing optics.

Once a suitable star has been found by the autoguider, you can start the autoguider guiding and sending error signals to the TCS by typing:

FON at the ICL interface, or:
GUIDE ON at the autoguider interface.

The TCS needs the guide probe co-ordinates in order to convert these errors into corrections in Altitude and Azimuth, so to start guiding you need to type:

PROBE <autoradial> <autotheta>
AUTOGUIDE ON

at the TCS console, or the ICL interface. There is a programmed function key on the TCS terminal for AUTOGUIDE ON.

To stop guiding at the end of an exposure type:

AUTOGUIDE OFF at the TCS or ICL terminal:
FOFF at the ICL interface, or GUIDE OFF at the autoguider interface.