The Takahashi collimating scope is a 6 x 18mm telescope designed for use on refractor, Ritchey-Chretien and classical Cassegrain telescopes and also used on the Takahashi CN-212, Mewlons and refractors of 820mm or more in focal length. Cannot be used on a Cassegrain instrument that employs a relay lens as part of it optical system. At the top of the scope, a focusing eyepiece is moved in or out manually to focus on the internal components. At the front of the Takahashi collimating telescope is a mirror threaded ahead of the objective lens. This provides a peep hole in the center the scope looks through. The reflective side of the mirror faces the scope you are collimating at a 45° angle to the scope's light path and is illuminated by an opaque opening in the side of the Takahashi collimating telescope. The Takahashi collimating telescope threads into the focuser and gives a round, white target to align the optics. Because it is threaded into the telescope being collimated, it's always precisely aligned. The Takahashi collimating telescope naturally provides a four-page instruction manual with multiple illustrations for use.