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.
A telescope fitting that holds the eyepiece and lets you rack it in and out for sharp focus.
The focuser is a knob on a telescope that brings the image into focus. It works by moving the eyepiece in and out a slight amount.