These are superb and only available here at Surplus Shed! Glass, air-spaced achromatic optics, multi-coated! 127mm clear diameter with a focal length of 700mm making this an f/5.5. The aluminum barrel has a diameter of 145mm (OD), is 46mm thick, and ID is 135mm. The lenses can be easily removed (if needed) by unscrewing the retaining ring. An incredible price that may not last long!! NEW! Edmunds sells a similar one for over $1000. This is the objective you need for a quality refractor scope or an awesome binocular telescope. Wollensak brand. Quality optics since 1899.