Hartmann mask

Hartmann mask is a tool to help focusing telescopes, mainly used by amateur astronomers.

Hartmann mask was invented as a tool to check the quality of large optical mirrors.

[1] For example, the illustration on the right shows the Hartmann mask used to test the Hale Telescope five-meter primary mirror.

In today's professional telescope making, it has been completely superseded by interferometric methods.

Adjusting the focuser, the images can be made to overlap, forming a single bright, clear picture.

Hartmann screen used for the 200-inch (5.1 m) Hale Telescope
Using a Hartmann mask to adjust focus