Crayford focuser

The Crayford focuser was devised by John Wall of Crayford Manor Astronomical Society, and have proved very popular, now being available commercially. The principle is very simple: the drawer-tube rides against two pairs of small bearings and is adjusted via a rod which is held in tension on the opposite side. The Crayford has several advantages over the more conventional rack and pinion focuser. Tension is adjustable so preventing heavy eyepieces from causing the drawer-tube to move; there is zero backlash at the focusing rod; motorised versions avoid vibration at the eyepiece allowing very precise focus to be achieved.

The picture below is of a Crayford that I made. It is 1.1/4" fitting and constructed from aluminium alloy (requiring the use of a small lathe). It is motorised, using a low voltage geared DC motor which is housed inside the bracket which carries the focusing rod and operated from the handset shown. Transmission to the rod is via a pair of small nylon spur gears.

Machining facilities are not essential for making a Crayford. An excellent alloy model is described in detail here.

With a little ingenuity versions can be made using meccano, or whatever bits and pieces you can find that can be suitably adapted.