Definitions for "Focuser"
The device that holds the eyepiece, and allows a viewer to move it back and forth in order to bring the scope into focus.
The part on a visual telescope that allows the user to focus an image into the eyepiece.
The part of the telescope which varies the optical distance between the objective lens or primary mirror and the eyepiece. This is usually achieved by moving the eyepiece in a drawtube, but in some catadioptric telescopes it is the primary mirror that is moved.