Secondary mirror

The secondary mirror or secondary mirror is an optical component in reflecting telescopes, the reflected rays deflected by the main mirror ( primary mirror ) from the telescope tube out to the eyepiece tubes (observer, camera) or to the connected measuring instruments.

Different types of mirrors

Depending on the nature of the telescope secondary mirror have different cut:

  • When Newton telescope plane mirror are used, the direct the light just before the primary focus at 90 ° to the side where the eyepiece tubes outside sitting on the tube. Because the secondary mirror is mounted at 45 °, it has mostly elliptical shape (axis ratio of 1: 0.707 ). Such mirrors are also required for the Nasmyth and Coudé focus.
  • For the other types, it is concave or convex mirror. The latter can be found in several types - both observatories as well as reflecting telescopes for amateur astronomers:
  • The convex mirror can be used, for example, the Cassegrain telescope. They reflect the light through a hole in the primary mirror, behind which the eyepiece is. Due to their convex shape they extend the predetermined by the primary mirror focal length similar to a Barlow lens. This, and the two-time folding of the beam path is possible a very compact, temperature- resistant construction. In order to introduce no spherical aberration, the secondary mirror, however, must be ground while hyperbolic Cassegrain telescope. The primary focus must be the same lying on the concave side focal point of the hyperbola, then there is the secondary focus on the place of the other Hyperbelbrennpunkts.
  • With catadioptric types such as Maksutov, Schmidt- Cassegrain or Schmidt- Newtonian telescopes for amateur astronomers of the auxiliary mirror almost always sitting on the corrector plate which closes the tube forward. In Maksutov - Cassegrains he is ( according to Gregory ) is often simply a mirrored area around the center of Meniskenlinsen back. The other beam path then extends to other Cassegrains through the hole in the primary mirror.
  • Concave auxiliary mirrors are used, inter alia, to shorten the focal length - such as when a mirror telescope will also be useful as strong light Astro Graf. However, the barely built Gregory telescope a burning contempt for far longer, concave secondary mirror, which must be elliptically deformed to produce no spherical aberration.

Disadvantages of secondary mirror designs

Depending on its size reduced a secondary mirror, the resolving power ( the selectivity ) of the telescope mirror, because the mirror edge and the struts, with which it is mounted in the telescope tube cause additional light inflections.

This is the reason why an equal refracting telescope binary stars or the fine details can resolve better on planets. For example, a primary mirror of 15-20 cm aperture is designed for a resolution of 1 "is required, whereas in the refracting telescope, a 12 -cm lens does the same job. The larger reflector telescope but can be cheaper to produce than the smaller refracting telescope.

To avoid these additional inflections (which also cause the intersecting silhouettes with bright stars ), has been developed, inter alia, the Schiefspiegler (System A. Kutter, Brachyt etc. )

  • Optical Telescope Technology
  • Optical mirror
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