Bahtinov Mask for 85-120 mm outer diameter tubes

Bahtinov Mask

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. Credit goes to Russian amateur astronomer Pavel Bahtinov for making his idea available to the astronomy community.

Bahtinov masks are adjustable to fit a wide range of optical tubes, unlike masks which are only able to fit one telescope. They have an adjustment lattitude of about 50mm in aperture making them very versatile.

Bahtinov masks are made from precision laser cut ABS plastic, which is stiff enough to remain in position, yet has enough “give” to make them tough without being brittle in the cold, unlike acrylic masks which are made from thick, heavy, brittle and hard acrylic plastics.

Bahtinov masks grip the telescope tube gently but firmly with three metal posts covered in non-scratching silicon sleeves. This prevents slippage, whilst allowing the mask to remaining perfectly central. This is unlike other masks with two posts, which rely solely on gravity to hold them in position. Because of this 3-post mounting, the Starwave Bahtinov mask remains centred, whilst fitting a wider range of telescopes, saving you money if you decide to change optical tubes. So, all you need to do is measure the outer diameter of your telescope tube or deshield, and choose a model corresponding to that OD dimension.

The following range of sizes are available to fit just about any amateur telescope on the market, as well as DSLR camera lenses:

Starwave Bahtinov Mask Size Chart:

Q: How does a Bahtinov focusing mask actually work? When in place, the Starwave Bahtinov mask works by forming a diffraction pattern of “spikes” of light around a star or point light source, which is easily visible in your eyepiece or DSLR/CCD camera in live view mode. (See the thumbnail image above, or click on this Bahtinov Mask diffraction pattern example). When the pattern becomes perfectly symmetrical, the telescope is perfectly in focus. Focusing is quick, easy and it saves you time “hunting” back and forth to find the optimum focus, whilst trying to judge the size of a tiny point source of light. A Bahtinov mask useful for a quick focusing re-check if the temperature changes during your imaging session. The image below shows how star images appear during focusing. (Click to enlarge). The difference in focuser position is only a few tenths of a millimetre, however the diffraction lines shift considerably. This sensitivity means less time hunting for best focus. There's no more need to guess when you are focused during times of poor to moderate seeing. Simply put the mask, and the status of your focus will become immediately obvious.

Starwave Bahtinov Mask Focusing Pattern:	The focussed star-image as it appears in a camera / eyepiece:

Q: Is it true that a different Bahtinov grating pattern is required for different focal length telescopes? Although there has been endless discussion about this on internet forums, 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. We see no need to constrain the use of a mask to a particular telescope, and seeing they are adjustable in diameter, it’s better to make a Bahtinov mask which can used with a wide range of telescope models. This allows us to produce them on a large laser cutting machine, which means customers pay less per unit, whilst keeping quality high.

Q: Does it matter if the aperture of my telescope is slightly larger than the aperture of the Bahtinov mask? No, it doesn’t really matter. Provided about half the radius of the pattern on our masks is included in “clear aperture”, and the mask is reasonably central, you will still see the same old diffraction pattern every time. Isn’t physics great?

Q: Do Starwave Bahtinov Masks work with both reflectors and refractors? Yes they do. Even though reflectors have a central obstruction caused by a primary mirror, there is no difference whatsoever in the performance or these masks on a reflector or a refractor. Basically, the reflector secondary mirror just covers the central portion of the mask but the outer clear aperture still carries the pattern.