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Working the autoguider entails orchestrating the A&G, autoguider and telescope. There are three stages -- search for a guide star, start the autoguider following a star, and start the telescope acting on the tracking errors being sent by the autoguider.
In order to find guide stars, the autoguider probe can be driven in an
angular direction around a half-circle, and also has 40 mm of radial
travel. The commands for setting the position of the autoguider probe are
In general it is preferable to guide using an autoguider filter similar to that being used for the observations. Note however that this may make it more difficult to find an appropriate guide star. In order to see if a guide star is present, take an exposure. The autoguider will then search for a suitable guide star and mark it with a cursor. The commands to do all this are as follows:
If a suitable guide star is not found an error message will be returned, and it will be necessary to either change the filter, the integration time or the position of the guide probe. Note that bright guide stars (m < 12 or so) are unsuitable as they saturate the CCD in one second. In general it will be necessary to search a few guide probe positions to find a suitable guide star. In this case it will be useful to know that the chip subtends 13500 theta-units, and 7500 theta-units. The most effective procedure is to drive the probe to its furthest radial extent (r = 40000), see if an appropriate guide star is present, and if not, offset in theta by 7500 units and try again.
Once a suitable guide star has been located, it is necessary first to tell the autoguider to start generating guiding errors, and then to tell the telescope control system to start using the errors. It will also be necessary to tell the telescope control system the position of the guide probe. The commands to do this are:
Finally, stop guiding using the following commands: