Finding razor-sharp focus when shooting astrophotography can be hard – stars appear as point light sources, and there are no detailed surfaces to aid the eye in achieving focus. The Bahtinov Mask is a widely used tool that cleverly uses diffraction as a focusing aid.
The Bahtinov Mask was invented by Russian amateur astrophotographer Pavel Bahtinov in 2005. It’s essentially an opaque disk with slots cut out in a specific pattern, and is fitted over your camera’s (or telescope’s) lens.
The slots are angled in such a way that they create a diffraction pattern with 3 spikes in an ‘X’ when aimed at a point light source. It’s best to find the brightest star in the sky in order to increase the visibility of the pattern. The central spike moves across the center of the ‘X’ as the focus is adjusted, and when the central spike is perfectly centered, focus is achieved.
The method is extremely sensitive, with the displacement of the diffraction spikes apparent with only the slightest focus shift. Once focus is achieved, the mask is removed and the photographer can shoot with confidence, knowing their images are tack sharp.
Here’s a simulation of the Bahtinov mask diffraction pattern by Niels Noordhoek:
Pavel Bahtinov has released his designs in to the public domain, so it’s possible to make your own mask using templates which can be found here or here. The easiest method is to print the mask and use it as a guide to cut the patterns in to something thin yet opaque, like a plastic binder.