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 - reflectors, rafractors, Dobsonians, Mak-Cas and SCTs. They are made from precision laser cut enforced plexiglass, which 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 you valued telescope surface. This prevents slippage, whilst allowing the mask to remaining perfectly central. Because of this 3-post mounting, the mask remains centered, whilst fitting a wider range of telescopes. | They are made from 2mm thick laser cut plexiglass and the securing screws are capped with soft plastic so that they do not damage your telescope.These high quality Bahtinov masks are adjustable to fit a wide range of optical tubes, unlike masks which are only able to fit one telescope. They have adjustment latitude of about 290-340mm, depending on model, in aperture making them very versatile. | Please be assured that these focusing masks will work excellently for both reflectors and refractors with whatever focal length. All you need to do is to measure your telescope tube size and choose the right size of the mask (contact us if you are not sure where to measure).