The diffraction pattern of the Bahtinov mask is used in telescopes to focus accurately. I made this simulation to show how it works.


The diffraction pattern of the Bahtinov mask is used in telescopes to focus precisely. I made this simulation to show how it works.

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  1. The light coming from a star can be considered spatially coherent because of how far they are. In other words, the optical signal arriving at earth is “very nearly” a plane wave with the same phase over a large extent.

    This property allows us to use it to do interferometry and produce a sharp diffraction pattern when the light passes through the Bahtinov mask.

    We can identify if the diffraction pattern is in the focus plane because the diffraction pattern is exactly the Fourier Transform of the Bahtinov Mask, which is a pattern with three spikes that intersect at the same point.

    Small deviations from the perfect focus are easily visible, making a Bahtinov mask a very simple tool to focus an amateur telescope.

    There are also Tri-Bahtinov masks. In addition to focusing, the tri-Bahtinov so also indicates if is it’s well collimated!

    For making this simulation I used the angular spectrum method, which is a technique that solves the wave equation exactly, and I uploaded the source code of the simulation on GitHub, so it can be replicated with other parameters:

    [https://github.com/rafael-fuente/Diffraction-Simulations–Angular-Spectrum-Method/blob/main/Simulations%20with%20lenses.md](https://github.com/rafael-fuente/Diffraction-Simulations–Angular-Spectrum-Method/blob/main/Simulations%20with%20lenses.md)

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