As mentioned earlier, autoguiders are available at both Prime and Cassegrain focus. They use the control computer for both signal processing and overall supervision.
When guiding is in progress, continuous velocity changes are made in both axes of the telescope in addition to the normal (open loop) tracking corrections in such a manner as to keep the guide star (whose centre is defined by the first moments of the brightness distribution) accurately centred on an electronically defined reference point.
The Prime Focus Camera is now guided by a commercial ST4 autoguider, though mounted on the same guide probe as the old image dissecting photomultiplier. A guide to its use has yet to be written.
The system has a bandwidth well matched to the telescope and high resolution, and is used in preference to manual guiding whenever possible.
When so commanded by the night assistant, the autoguider searches a square area in a raster scan. It senses the position of the brightest star and proceeds to guide on this star by sweeping an aperture in a cross pattern over the star image. The autoguider gives a display on the console monitor of the profile of the guide star. (The area searched depends upon which guider and/or which Cassegrain focal ratio is in use.)
The astronomer should be aware of the two ways in which guide stars can be acquired by the autoguider's image dissector. In the PROBE-TO-STAR mode, the telescope position at the time guiding begins is taken to be correct and the guider maintains the guide star at its initial position. If the guide star drifts with respect to the telescope, the telescope will be moved to bring the guide star back to its original. This continues until either OFF is selected or the loop is disabled by the operator, at which time the position is lost. Any subsequent guiding will begin by defining a new reference point based on the telescope's position during the first few seconds after re-acquisition. Temporary loss of the guide star because of cloud will not cause the guiding position to be lost unless OFF is selected.
In the STAR-TO-PROBE mode the autoguider will shift the telescope as it acquires the guide star, to bring the star to a standard position on the photocathode (this nominally corresponds to the centre of the guiding eyepiece graticule). Where a choice is possible, e.g. for direct photography, the STAR-TO-PROBE mode is preferable as there is no risk of a double exposure if the guiding position is lost. When the telescope has been accurately set to a particular position, however, (e.g. with the program star centred in an aperture) the PROBE-TO-STAR mode must be used to maintain the telescope position as initially set.
A detailed account of the way the autoguider works can be found in PASP 99, 80 (1976), and AAO TM 7 also contains a simplified description.
This Page Last updated: Feb 21, 1996, by Chris Tinney.