Crayford focuser

The Crayford focuser is a simplified focusing mechanism for amateur astronomical telescopes. Crayford focusers are considered superior to entry-level rack and pinion focusers, normally found in this type of device. Instead of the rack and pinion, they have a smooth spring-loaded shaft which holds the focus tube against four opposing bearing surfaces, and controls its movement. It is named after the Crayford Manor House Astronomical Society, Crayford, London, England where it was invented by John Wall, a member of the astronomical society which meets there. The original Crayford Focuser is on display there.