In every telescope, the focuser is attached close to the focal point of the light-gathering optic. Eyepieces can be connected to the focuser. For astrophotography, a camera is attached to the focuser, and the focuser serves to focus the camera.
Eyepieces and cameras are usually equipped with connector sockets. Two diameters have been established for the connector socket, which are specified in inches: 1.25" = 31.8mm and 2" = 50.8mm.
The focuser consists of two interlocking and sliding tubes, so that the eyepiece can be moved into focus using a small wheel. Not only eyepieces can be attached to focusers, but cameras as well.
Precise focusing is especially important for astrophotography. Therefore, high-quality focusers with reduction adjustment should be used for this application. For astrophotography, many focusers also have a connector thread, so that the camera is screwed into place instead of being be plugged in.
Simple focusers are adjusted using a rack and pinion mechanism. In the higher quality versions, this rack and pinion is angled and made of brass. Instead of a rack and pinion, the adjustment can also be made by exerting pressure on a special surface. Such focusers are usually referred to as Crayford focusers.
Helical focusers work in a completely different way. Here, the adjustment is made by rotating the inner barrel using a longitudinal screw (helix). One advantage is that a rotary knob is not necessary, which can get in the way, for example, of a guidescope.
As a rule, focusers can also be motorised. This improves focusing accuracy and also allows the remote operation of the telescope.