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Hartmann mask
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.
== 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.〔http://www.astropix.com/HTML/I_ASTROP/FOCUS/METHODS.HTM#HM〕) 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, the apparatus all need to produce the same image.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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