Hartmann mask is a tool to help focusingtelescopes, mainly used by amateur astronomers. It is named after the German astronomer Johannes Franz Hartmann (1865–1936), who developed it around 1900.
Hartmann mask was invented as a tool to check the quality of large optical mirrors. It was especially useful for large non-spherical mirrors for telescopes.[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 In today's professional telescope making, it has been completely superseded by interferometric methods.
Theory and practice
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.[2]) The mask covers the aperture of the telescope.
When the apparatus is out of focus, multiple images can be seen if the telescope is pointed towards a bright light source (Moon, bright star). Adjusting the focuser, the images can be made to overlap, forming a single bright, clear picture. The mask may also be used to check the figure of a mirror, as all the holes in the mask should each produce the same image.
