Designed by Russian amateur telescope maker and astronomer Pavel Bahtinov, the Farpoint Bahtinov Focus Mask can be used to quickly and easily focus your telescope for all types of astrophotography with DSLRs or imaging systems. Made of durable ABS plastic, the mask is designed to fit telescopes with front end or dew shield diameters from 8.5 to 10.5", and simply fits over the lens for use. It takes the guesswork out of focusing on astronomical objects and ensures tack-sharp stars, planets, and just about any other celestial subject you want to photograph or image.