Gosky Bahtinov Focusing Mask for Telescopes Dew Shield Diameter From 95mm -140mm (3.74inch - 5.50inch) - For 100mm,120mm Skywatcher Refractors, Celestron 4 SE Etc Suitable for telescopes Dew Shield or Front Outer Diameter From 95mm -140mm (3.74inch - 5.50inch) - For 100mm,120mm Skywatcher Refractors, Celestron 4 SE 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. 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