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Hartmann mask is a tool to help
How-to Make a Hartmann Mask
Optical devices Telescopes
Hartmann mask is a tool to help focus
Focus (: foci or focuses) may refer to:
Arts
* Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film
*Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel
*Focus (2015 ...
ing telescope
A telescope is a device used to observe distant objects by their emission, Absorption (electromagnetic radiation), absorption, or Reflection (physics), reflection of electromagnetic radiation. Originally, it was an optical instrument using len ...
s, 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. 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 In today's professional telescope making, it has been completely superseded by interferometric methods.Theory and practice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 amirror
A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
or lens
A lens is a transmissive optical device that 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, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
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 all the holes in the mask should each produce the same image.
See also
* Bahtinov mask * Carey mask * Shack–Hartmann wavefront sensorNotes
{{reflistReferences
How-to Make a Hartmann Mask
Optical devices Telescopes