Crayford focuser - AbsoluteAstronomy.com

The Crayford focuser is a simplified focusing

Focus (optics)

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

mechanism for amateur astronomical

Amateur astronomy

Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...

telescope

Optical telescope

An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....

s. The design replaces the complicated rack and pinion

Rack and pinion

A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion. A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby...

drive normally found in this type of device with a smooth spring-loaded shaft which holds the focus tube against four opposing bearing

Bearing (mechanical)

A bearing is a device to allow constrained relative motion between two or more parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can...

surfaces. It is named after the Crayford Manor House Astronomical Society

Crayford Manor House Astronomical Society

-History:There have been classes on astronomy at Crayford since the 1950s, the advanced astronomy course was originally run by Percy Wilkins and the society was subsequently estabilshed in 1961, by Gordon Taylor who travelled from Herstmonceux to lecture on Astronomy at Crayford Manor House, he...

, Crayford

Crayford

Crayford is a town and electoral ward in the London Borough of Bexley that was an important bridging point in Roman times across the River Cray, a tributary of the River Darent, which is itself a tributary of the River Thames.-History:...

, London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

, England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

where it was invented by John Wall

John Wall (inventor)

John Wall is a British design engineer, amateur astronomer, amateur telescope maker and member of the British Astronomical Association, now living Coventry England.-Biography:...

, a member of the astronomical society which meets there. The original Crayford Focuser is on display there.

The Crayford focuser was initially demonstrated to the Crayford Manor House Astronomical Society, and then 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.

Crayfords are popular among amateur telescope makers

Amateur telescope making

Amateur telescope making is the activity of building telescopes as a hobby, as opposed to being a paid professional. Amateur telescope makers build their instruments for personal enjoyment of a technical challenge, as a way to obtain an inexpensive or personally customized telescope, or as a...

as they are easy to make without any high precision machining, yet provide precise focusing with no gear slop or backlash

Backlash (engineering)

In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...

. They are also made in a variety of designs by companies specializing in amateur telescope supplies.

The concept

The Crayford focuser design is reminiscent of the Dobsonian design for telescopes, since instead of precision machining, it relies on geometry to preclude any wobble or backlash.

The Crayford is similar in appearance to a Rack and pinion

Rack and pinion

focuser, but has no teeth on either the rack or the pinion. Instead, a round axle is pressed (for example by a spring-loaded or thumbscrew-tightened piece of PTFE plastic) against a flat on the side of the focuser drawtube, relying only on friction to move the drawtube as the axle is turned. This also presses the drawtube against a set of four ball bearing

Ball bearing

A ball bearing is a type of rolling-element bearing that uses balls to maintain the separation between the bearing races.The purpose of a ball bearing is to reduce rotational friction and support radial and axial loads. It achieves this by using at least two races to contain the balls and transmit...

s against which it moves smoothly with minimal friction. The pressure exerted on the axle can often be adjusted for smoothest operation, and the drawtube may be locked in position to support heavy eyepiece

Eyepiece

An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

s or camera

Camera

A camera is a device that records and stores images. These images may be still photographs or moving images such as videos or movies. The term camera comes from the camera obscura , an early mechanism for projecting images...

s.

Subsequently, variations on the Crayford focuser concept have emerged, e.g. a helical Crayford where the drawtube is pressed against four tilted ball bearings, and focusing action is accomplished by rotating the drawtube.

Links

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