The Bahtinov Mask for Focusing

One of the most critical processes preparing your equipment for imaging, is making sure you have excellent focus on your target.  Image capture programs such as Sequence Generator Pro or NINA have an auto focus feature, the easiest tool to use at this stage in your astrophotography journey will be the Bahtinov mask. This is a diffraction mask device that you can easily (and cheaply) get 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 each of the other 2 diffraction spikes until they all intersect at a point using your naked eye.  When this alignment is made, the telescope should 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 has some very interesting things to say about amateur astro imaging, 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.  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 would be completely focused.  He did an experiment using this kind of mask on trying 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 cutout were very weak because of the small exposure times.  This type of mask didn’t seem very practical to use to get focus.  He investigated and experimented further with the design and found that a 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 lines cut out of the mask and 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.

Because he published his idea in Russia, it was only known by Russian amateur astronomers and unknown elsewhere 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.