A focuser is used to hold an eyepiece. The image can be focussed using the focus wheels on the side.
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.