The Autoguider then exposes the CCD for a predetermined time and displays the image on the screen. A region of the CCD called the acquisition window is then searched to find the centre of the brightest star, and then the next brightest star, and so on. This process is what the autoguider system regards as acquisition, resulting in a list of stars in order of brightness.
One of these stars is then chosen to guide on. This procedure is not foolproof, there are many sources of error: the telescope may not be pointing accurately enough, an error could have been made in positioning the Autoguider probe, or during inputting data. There may simply not be any stars bright enough to guide on within the Field Of View of the Autoguider CCD and this procedure will have to be repeated on a different area of sky. This process is complicated at UES by only having a small field of view from the Probe Fibre, and no direct light path to a large detector. Here, the coarse fibre is continuously read out and the telescope manually adjusted to get the guide star into the field of view of the Fibre.
The Guide Star Server Software is proving useful in making this process easier by automatically calculating the Autoguider probe position for a given telescope position and a guide star selected from the on-line guide star catalogue. This has made this process quite reliable.