Post Installation and Lightweight Design: Thanks to this 3 post mount, the mask remains centered while fitting a wider range of telescopes. The bahtinov focus mask for telescopes is lightweight so you won't upset the balance of the scope. Wide Range of Applications: Works with all types of telescopes, mirrors, refractors, for Dobsonians, for Mak Cas and for SCT, all you need to do is measure your telescope tube size and choose the appropriate mask size. Applicable Size: The outer diameter of the bahtinov mask supports the telescope objective lens in the range of 60‑90mm (there is a 1‑2mm error in manual measurement). Bahtinov Masks Function: The mask takes the guesswork out of focusing and even overcomes conditions to give your telescope the best focus quickly and easily. 3 Metal Posts: The mask grips the telescope tube gently and firmly with three metal posts. These metal posts come with scratch resistant silicone covers to protect your telescope . This prevents slippage while keeping the mask perfectly centered.