Bahtinov masks take the guesswork out of focusing and even get over the seeing conditions as it is a mathematical type approach to the task; the best focus for your telescope quickly and easily Suitable for use on all types of telescope; they are made from precision laser cut enforced plexiglass that is stiff and long lasting; they will never be brittle in the cold; unlike some other masks which are made from normal plastics The masks grip the telescope tube gently but firmly with three metal posts which have non-scratching silicon sleeves to protect valued telescope surface; mask remains centered since 3-post mounting; while fitting a wider range of telescopes Made from 2mm thick laser cut plexiglass and securing screws are capped with soft plastic so that do not damage telescope; They have adjustment latitude of about 150-200mm that adjustable to fit optical tubes unlike masks which only fit one telescope Please be assured these focusing masks will work excellently for both reflectors and refractors with whatever focal length; all you need to do is to measure telescope tube size and choose right size of mask contact us if not sure where to measure