Bathinov masks are a very useful aid for focusing in astrophotography.
They can be used for all optical systems, provided that the correct diameter is selected. The diameter in millimetres is specified in the product title. For a telescope with an aperture of 80 millimetres, for example, you should purchase the "Tri-Bathinov 80" mask, etc.
Each mask comes with three pins that can be adjusted as required. Simply attach the mask to the front of the tube and focus on a bright star. You will see a diffraction pattern of the star on your computer monitor or the display of your SLR camera. Focus is achieved when the middle rays are exactly in the middle of the other two rays.
The Tri-Bathinov is a modernised mask that allows you to achieve an even more precise focus with up to 18 lines.
