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What is a Bahtinov Mask?
A Bahtinov mask is a device that adds a geometric pattern to the front of any telescope. It aids in focusing the telescope and takes out the guess work.
The mask has 3 geometric grids. When focusing on a star, The defraction spike create a predictable pattern. As the focus is moved in and out, the central spike will also move. The objective is to center the central spike until an X appears.
When this is achieved, then you know that the focus is correct. This takes some of the guess work out of the whole procedure.
When you have your telescope in focus, lock the focus ring if available and then remove the mask.
Print Your Own
So as you probably know, I made my own Hadley Telescope. Since this product is so widely adopted, there is a handy Bahtinov Mask that you can print. I have not yet done if but I’ll do that as soon as the rain stops.
Why Would You Need One?
For casual observations a Bahtinov Mask is probably unnecessary. However if you are viewing the image through live view on your camera, then a focusing aide really helps out. Here you can verify that the telescope is focused the best that it can be without interpretation.