Feature: 1. Universal Compatibility: Suitable for all types of telescopes including reflectors, refractors, for Dobsonians, for Mak Cas, and for SCT, fitting telescopes with an outer diameter of 63 to 89mm. The focusing mask can be easily installed on any telescope with 3 bolts. 2. Precise Focusing: The Bahtinov mask eliminates guesswork during focusing, overcoming visual conditions to quickly and easily achieve optimal focus for your telescope. 3. Secure Grip: Three metal columns of the mask clamp the telescope to prevent slipping while keeping the mask perfectly centered. The 3 column design allows for central alignment and compatibility with a wider range of telescopes. 4. Diffraction Patterns: The focus fishbone acts as a set of artificial "trusses." As light passes through the fishbone plate, it diffracts in 3 different directions, creating 3 pairs of "rays." With focus adjustments, the ray patterns move until the 3 sets of rays align, indicating proper focus. 5. Lightweight and Durable Aluminum Alloy: The telescope focusing mask features a thin and sturdy design made of 1mm thick aluminum alloy material, lightweight and durable to maintain balance without damaging the telescope. Specification: Item Type: Telescope Focusing Mask Material: Brass, Aluminum Alloy, Rubber Weight: Approx. 47g/1.7oz . Package List: 1 x Telescope Focusing Mask