Gosky Bahtinov Focusing Mask for for Telescopes Dew Shield Front Diameter From 55-90mm (2.2inch - 3.6inch) - For Small Camera Lenses orl Refractors Like Ts 50mm Apo, or Other 60mm Refractors Etc 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 From 55-90mm (2.2inch - 3.6inch) - For Small Camera Lenses or Small Refractors Like Ts 50mm Apo, or Other 60mm Refractors Etc. 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