ABSTRACT

Using a computer to automatically determine the best focus position improves image quality over extended periods and enables unattended operation too. In astrophotography, although people are working at infinity, it is by no means a fix-and-forget process. The extended imaging time during the night requires repeated monitoring and focus correction; in most cases, carried out in-between exposures. There are several different strategies and in practice, may use several, depending on circumstances: manual autofocus, triggered autofocus, automatic filter offsets, and automatic temperature compensation. In most cases though, the autofocus algorithm uses the imaging camera to take a short exposure. It then selects one or more stars and measures their diameter, moves the focuser, repeats and makes intelligent adjustments. Some algorithms require the image to be at approximate focus before running, others can work it out for themselves.