[WIDE RANGE OF APPLICATIONS] Works with all types of telescopes, including mirrors, refractors, Dobsonians, Mak Cas, and SCT. Choose the appropriate mask size by measuring your telescope tube. [BAHTINOV MASKS FUNCTION] Takes the guesswork out of focusing, provides quick and easy focus, and overcomes conditions for the best telescope focus. [SECURE 3 METAL POSTS] Mask grips the telescope tube gently and firmly with scratch-resistant silicone covers on three metal posts, preventing slippage and ensuring perfect centering. [LIGHTWEIGHT DESIGN] 3 post mount keeps the mask centered while fitting a wide range of telescopes without upsetting the scope's balance. [APPLICABLE SIZE] Supports telescope objective lens sizes in the 60-90mm range, with a lightweight and user-friendly design for optimal astrophotography.