A Bahtinov mask is simple device used to accurately focus telescopes and cameras at night. It is placed on the front of the telescope or camera which is then pointed at a bright star. The pattern on the mask is designed in such a way that it produces three angled diffraction spikes. Two of these form an X, and accurate focus is achieved when the third spike is centred on the X.
This video shows what a focus sweep using a Bahtinov mask looks like. Starting off out of focus, accurate focus is achieved half way through the video, before dropping out of focus again.
Simulated with Maskulator 5.0
