Hartmann mask (sometimes called a Hartmann hat) is a tool to help focusing telescopes, mainly used by amateur astronomers. Every part of a mirror or lens produces the same image as the whole optical element. The light is focused in the focal point. The light rays, however, go through different points of a plane before or behind the focus. This phenomenon can be used when focusing a telescope. The Hartmann mask is a simple opaque mask containing two or three holes. (This device is called a Hartmann mask if it has multiple holes, or a Scheiner disk if it has two holes.) The mask covers the aperture of the telescope. Read more
Focusing my telescope using a Bahtinov mask. This is a very sensitive focusing system. You can tell perfect focus when the middle diffraction line is perfectly centred. It is about 1/8th turn of the focus knob from one extreme to the other demonstrating how sensitive this method is.
A Bahtinov mask is a telescope focusing aid for astrophotography. The device consists of a disk made of opaque material with slots cut out in a specific pattern that is fitted into the light path of a telescope. The focusing mask uses the optics diffraction spike pattern to aid in focusing. The mask was invented by Russian astronomer Pavel Bahtinov.