These are my home-made Bahtinov masks. On the left is the one for my LX200, which is built on a 12-inch embroidery ring. On the right is one for my ETX 125.
When focused on a star, the mask produces diffraction spikes as shown below. As the system moves through focus, the two outer spikes move one way and the central spike moves the other way. So when the central spike is centred within the outer two the system is in focus.
