Bahtinov focus masks provide an ingenious way to easily focus your DSLR camera, webcam or CCD camera for astrophotography. The method is simple, very intuitive, and the device has universally been named after the inventor, Pavel Bahtinov.
The mask works by creating a group of three diffraction spikes. Two of these generate a cross pattern, and the 3rd moves with your focus. You know you have achieved perfect focus when the 3rd diffraction spike is exactly centered with where the other two diffraction spikes cross.
Select the size for your telescope.