Autoguidance is a technique used to minimize small but unfortunately significant errors in mounts. It is useful only when working with long exposures, perhaps in excess of 20 or 30 seconds, and therefore not necessary when photographing the Moon with relatively short exposures. The best quality mounts are engineered to high tolerances, but nevertheless exhibit small imperfections as a consequence of unwanted movement in gears, ball bearings and other mechanical components. The result of any such movement, particularly when working at high magnifications, is that point sources become slightly smeared or blurred. At lower magnifications, for example when using a wide-field refractor, such errors may be insignificant.
The concept of autoguidance is relativel simple. A second camera, typically a webcam, is attached to a supplementary guide scope focused on a particular star (usually known as the guide star). It is used to take a series of images at short regular intervals. The images are downloaded to a computer and the position of the guide star in the frame is compared to previous images in the sequence. When a change in position is detected, commands are sent to the mount to achieve the necessary corrections. Because the reference images of the guide star are repeated every few seconds, any change in position of the star being photographed is corrected very quickly, and hence not allowed to accumulate. This helps to achieve sharper images.
Software such as the free program GuideDog, provides the interfaces between the autoguider and the mount.