These are superb and only available here at Surplus Shed! Glass, air-spaced achromatic optics, Multi-coated! 102mm clear diameter with a focal length of 900mm making this an f/8.8. The optics are mounted in a quality aluminum barrel. The lenses can be easily removed by unscrewing the retaining ring. An incredible price that may not last long!! if you get 2 you could make an awesome binocular telescope. NEW!