Crayford focuser

A focuser ( OAZ ) is a device on an optical instrument ( telescope, microscope, etc.) with the help of a sharp image is produced.

Description

The focuser is attached to the instrument or is permanently installed and assumes its open end of the eyepiece or a photographic or photometric device such as a (nowadays id usually digital ) camera, a webcam, a spectrometer or similar on. Its position along the optical axis ( the long axis of the light path ) is shifted to the position of the sharp image is found on one or more thumbscrews on focuser. For a more precise setting some focusers also have a reduction. Depending on the telescopic type, there are various Okularauszugtypen:

• As the main extract, which travels the entire focussing range
• As auxiliary extract to enable a highly accurate focusing. For example, in a Schmidt- Cassegrain telescope, in which the main focus position is achieved by moving the main mirror.

Main statements are generally much longer ( in the picture above 8 cm) as auxiliary extracts; these are partly only 2.5 cm long ( upper panel).

Species

Rack extract

The rack pull is a simple method to move the focus. The problem here is the game that has the drive gear with the rack. It is often balanced by a layer of sticky fat, which dampens the movements of the parts.

Crayford style

The Crayford style was developed in 1971 by John Wall in Crayford. His drive is based on a smooth steel shaft, which is directly pressed on a rough metal surface on the extract or the extract. The small diameter of the shaft is a very accurate adjustment is possible. Additionally, a second rotary knob mounted at one side often which allows more than one planetary gear, a reduction ratio on 1:10 -fold and thus a more accurate adjustment. Crayford excerpts are available with 2-inch 2.5-inch and 3 -inch recordings and corresponding reducers down to 1.25 inches Okularsteckmaß.