Gosky Bahtinov Focusing Mask for Telescopes Dew Shield Diameter From 75mm -114mm (2.95inch - 4.5nch) - For 80mm, 100mm, 102mm ED Skywatcher and Other Refractors - For More Precise and Faster Focusing Bahtinov masks make focusing for astrophotography an easy task. Watch the two slanted areas create an "X" over the star. The vertical slits create a moving bar that is centered on the X when you are at focus. Suitable for telescopes Dew Shield or Front Outer Diameter Diameter From 75mm -114mm (2.95inch - 4.5nch) - For 80mm, 100mm, 102mm ED Skywatcher and Other Refractors The mask attaches securely to your scope using brass screws with soft transparent nylon cover to ensure the finish of your telescope is protected. Light weight so you wont upset the balance on your scope. It will take less time to achieve perfect focus with your new mask which mean you spend more time imaging photons rather than focusing them