Bahtinov Mask Focusing Mask for Telescopes - FM180mm for a Tube Outer Diameter (OD) from 195mm-240mm - can be Mounted with the Brass Screws Inside or Outside the drawtube

The Bahtinov mask has become the preferred method of focusing a telescope, either visually or when imaging with a CCD camera, DSLR or webcam.They are an inexpensive tool which will allow you to achieve pin-sharp levels of focus, saving you time and improving your work. Simply place the mask over the front of your 'scope, align the scope towards a star. The mask displays a unique diffraction profile; one part of the diffraction pattern moves in response to changes in focus and displays error as a displacement from a clearly defined point.Whether you're doing planetary visual observing, planetary imaging,or deep sky imaging, the Bahtinov Mask is a great accessory to get the best focus for your telescope quickly and easily.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.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.There is a myth about different Bahtinov grating pattern is required for different focal length telescopes. There is no real practical reason for the use of special patterns for different telescope types and focal lengths. In fact, the frequency and angle of Bahtinov mask patterns aren't really that important. The diffraction effect is so sensitive to changes in focus, that the results are excellent regardless. 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.