Bahtinov mask

The Bahtinov mask helps focus small astronomical telescopes with precision. Accurate focus is essential for astrophotography, and this mask creates a unique diffraction pattern to assist. It was invented in 2005 by Russian amateur astrophotographer Pavel Bahtinov. To use it, point the telescope at a bright star and place the mask in front of the objective or aperture. The mask has three separate grids, which produce three angled diffraction spikes at the focal plane for each bright image element. As focus changes, the central spike appears to move from one side of the star to the other—in reality, all three spikes shift, but the middle one moves opposite to the two that form an "X." Optimal focus occurs when the middle spike is centered between the other two. Small focus errors are easily visible, and in digital astrophotography, software can analyze spike alignment to sub-pixel resolution. The direction of the spike displacement shows which way to adjust focus. Rotating the mask 180° reverses spike movement, so consistent orientation makes it easier to use. The mask must be removed after achieving focus. It works by replacing the circular aperture stop with an asymmetric, periodic stop. Viewing a point source like a star produces a Fraunhofer diffraction pattern at the focal plane—normally an Airy disk from a circular aperture, but with the mask, asymmetric spikes appear that reflect the mask pattern's spatial frequency and orientation. A very bright star and very dark sky are needed for highly contrasted, clearly visible spikes. This diffraction effect is similar to sunstar patterns in landscape photography, where a camera lens's iris is adjusted to a small polygonal shape with sharp corners. In the example below, the central pattern shows good focus, while the central spike is noticeably displaced in the left and right images.