Crayford focuser
A friction-based focuser invented by John Wall for amateur telescopes.
The Crayford focuser is a streamlined focusing system built for amateur telescopes. It improves on basic rack and pinion designs by doing away with gears. Instead, a smooth, spring-loaded shaft presses the focus tube against four bearing surfaces, moving it without the slop or backlash common in cheaper focusers. The design comes from John Wall, a member of the Crayford Manor House Astronomical Society in London, England. He first showed it to the society, then published plans in the *Journal of the British Astronomical Association* (February 1971), *Model Engineer* (May 1972), and *Sky & Telescope* (September 1972). Wall chose not to patent it, giving the idea freely to amateur astronomers. The original focuser remains on display at the society’s meeting place.
The Crayford gained popularity among telescope makers because it could be built without high-precision machining yet still deliver accurate, backlash-free focusing. As commercial versions became available at low cost, they outperformed most rack and pinion units. They soon became the standard on many telescopes above entry level. Today, companies that supply amateur telescope parts produce them in various styles, including dual-speed and motorized or robotic focusing options.
The design relies on geometry rather than tight machining to prevent wobble. It looks like a rack and pinion focuser but has no teeth. A round axle, pressed by a spring or thumbscrew against a flat PTFE pad, uses friction to move the drawtube. This pressure also holds the tube against four ball bearings, allowing smooth, low-friction travel. The pressure on the axle is often adjustable, and the drawtube can be locked to support heavy eyepieces or cameras. Later variations include a helical Crayford, where the drawtube rotates against four tilted bearings to achieve focus.
- Inventor
- John Wall
- Place of origin
- Crayford Manor House Astronomical Society, Crayford, London, England
- First demonstration
- Crayford Manor House Astronomical Society
- Publication dates
- Journal of the British Astronomical Association (February 1971), Model Engineer magazine (May 1972), Sky & Telescope magazine (September 1972)
- Patent status
- Not patented; donated to the amateur astronomical community
Lore & Background
The Crayford focuser was invented by John Wall, a member of the Crayford Manor House Astronomical Society in Crayford, London, England. The original focuser is on display at the society. It was initially demonstrated to the society, and descriptions were published in the Journal of the British Astronomical Association (February 1971), Model Engineer magazine (May 1972), and Sky & Telescope magazine (September 1972). John Wall decided not to patent the idea, effectively donating it to the amateur astronomical community.
The design is reminiscent of the Dobsonian telescope design, relying on geometry rather than precision machining to prevent wobble or backlash. It is similar in appearance to a rack and pinion focuser but has no teeth. Instead, a round axle is pressed (e.g., by a spring-loaded or thumbscrew-tightened piece of PTFE plastic) against a flat on the side of the focuser drawtube, relying on friction to move the drawtube. This also presses the drawtube against a set of four ball bearings for smooth movement. The pressure on the axle can often be adjusted, and the drawtube may be locked to support heavy eyepieces or cameras.
Subsequently, variations have emerged, such as a helical Crayford where the drawtube is pressed against four tilted ball bearings and focusing is accomplished by rotating the drawtube.
Reader's Guide
The Crayford focuser became popular among amateur telescope makers for being easy to make without high precision machining, yet providing precise focusing with no gear slop or backlash. This trend accelerated as commercial Crayfords, while relatively inexpensive, proved superior to most rack-and-pinion devices. They became the default focusing device on many amateur telescopes just above entry level and better. They are made in a variety of designs by companies specializing in amateur telescope supplies, featuring options such as dual speed focusing and interfaces for motorized or robotic focusing. The design's reliance on geometry and friction, rather than precision machining, made it accessible to hobbyists and contributed to its widespread adoption in the amateur astronomy community.
Did You Know?
- The original Crayford focuser is on display at the Crayford Manor House Astronomical Society.
- John Wall decided not to patent the focuser, effectively donating it to the amateur astronomical community.
- The design relies on a spring-loaded shaft and four ball bearings, with no rack and pinion teeth.
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