{{short description|Telescope focusing device}} [[File:Hartmann Screen.jpg|thumbnail|right|Hartmann screen used for the {{convert|200|in|m|adj=on}} Hale Telescope]] thumb|Using a Hartmann mask to adjust focus '''Hartmann mask''' is a tool to help focusing telescopes, mainly used by amateur astronomers. It is named after the German astronomer Johannes Franz Hartmann (1865–1936), who developed it around 1900.

== History == 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.<ref>Micro, B. (2015, October 26). Hartmann test. Il Grattavettro. Retrieved September 14, 2023, from https://web.archive.org/web/20230724130837/https://www.grattavetro.it/hartmann-test/?lang=en</ref> 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 In today's professional telescope making, it has been completely superseded by interferometric methods.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.<ref>{{cite web | url=http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM#HM | title=Focusing Methods }}</ref>) 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.

==See also== * Bahtinov mask * Carey mask * Shack–Hartmann wavefront sensor

==Notes== {{reflist}}

==References== * [http://www.cloudynights.com/item.php?item_id=518 How-to Make a Hartmann Mask] * [https://web.archive.org/web/20090711210541/http://www.dabsonastronomy.com/index_files/Page356.htm Hartman Mask Project]

Category:Optical devices Category:Telescopes