A focuser is used to hold an eyepiece. The image can be focussed using the focus wheels on the side.
The position of the focus, measured from the plug connection, with the eyepiece fully retracted.
Connection ( to eyepiece)
Focusers are available in 1.25" or 2" formats. Many 2" focusers also have a 1.25" adapter.
Type of build
There are basically two different construction types: the rack and pinion focuser and the Crayford focuser.