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Hartmann mask is a tool to help
How-to Make a Hartmann Mask
Optical devices Telescopes {{astronomy-stub
Hartmann mask is a tool to help focus
Focus, or its plural form foci may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film
*''Focus'', a 1962 TV film starring James Whitmore
* ''Focus'' (2001 film), a 2001 film based ...
ing telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to obse ...
s, 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 amirror
A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...
or lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
produces the same image as the whole optical element. The light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 t ...
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.
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 holes in the mask should all produce the same image.
See also
* Bahtinov mask * Carey mask *Shack–Hartmann wavefront sensor
A Shack–Hartmann (or Hartmann–Shack) wavefront sensor (SHWFS) is an optical instrument used for characterizing an imaging system. It is a wavefront sensor commonly used in adaptive optics systems. It consists of an array of lenses (called ...
Notes
References
How-to Make a Hartmann Mask
Optical devices Telescopes {{astronomy-stub