Bahtinov Mask to framing and focusing your target manually
If you’re like me and you haven’t quite mastered Sequence Generator Pro yet to help capture astro image target data with your equipment, you are probably framing and focusing your target manually. My main tool for this through the use of a Bahtinov mask. It’s a simple to use device and you need to get one to fit whatever aperture of optical tube you’re using. This device creates an optical diffraction of a bright star where there are straight lines in 3 separate grid patterns that are oriented at different angles. The slits in the mask create 3 strong diffraction spikes from the bright star; you manipulate the centermost diffraction spike with your focuser to align with the other 2 diffraction spikes until they all intersect at a point using your naked eye. When this alignment is made, the telescope will be properly focused.
Pavel Bahtinov, a Russian amateur astronomer, invented this device back around 2005. Below is a link to an interview he did in 2018 where he talks at some length about how he developed the mask. The interview is about 27 minutes long and in Russian but it’s subtitled. He also has some very interesting things to say about amateur astro imaging in general, including how he made the transition from shooting film cameras for deep sky imaging to digital cameras. For example, I did not know that the focusing methods you needed to use when using film cameras required you to actually remove the camera from the optical tube. I'd be grateful to anyone in the community who can describe this process in detail. However, if you want to just understand how he developed the focusing mask that bears his name, fast forward the video to around 12.5 minutes into the interview.
The basic story is this; he was looking at ways to improve a mask that was being used for focusing at the time called a Hartmann Mask. This is essentially a mask with a couple of triangular holes in it that is put over the aperture of the optical tube. The light rays coming from the triangle’s edges would provide a diffraction pattern that, while usable to get focus weren’t particularly strong and it wasn’t clear that your image train would be properly focused. He did an experiment using this kind of mask to achieve focus while imaging a planet using video at a specific frame rate and discovered that the intensity of the light at the edges of the triangular cutouts were very weak because of the small exposure times used. This type of mask didn’t seem very practical to use to get focus. He continued investigating and experimented further until he found that the solution to enhancing the intensity of the light rays from the mask to gain focus was to use multiple parallel lines to the original triangle line scheme cut out of the mask and that the mask had to cover the entire aperture of the optical tube. He created a prototype mask which became the basis of the mask design he invented and that bears his name.
Since he published his work in Russia and only in Russian, it was only known by Russian amateur astronomers. It remained this way for several years until another Russian amateur astronomer named Dennis Sakva, published it in english on the Cloudy Nights Forum referencing Bahtinov and his invention. The rest, as they say is history.
A final item to note is that Bahtinov deliberately did not patent this invention. He lists 2 reasons for this. First, he believes philosophically that copyright protection hinders development of science and technology because restricts the distribution of information and knowledge. He made an affirmative decision to allow this knowledge to be accessed by anyone. The second reason he gives is a more practical one: this device is pretty simple and easy to make and he didn’t think there would be much profit from this and he’d need to expend a lot of energy to collect anything from this.
Stay safe and get your neighbors to turn off their lights at night….
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