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Glossary > Telescopes > Included accessories > Erecting systems

Glossary | Telescopes | Included accessories | Erecting systems

A telescope always displays an object upside down. This law of optics can only be compensated for by an erecting system. Erecting lenses, diagonal mirrors or Amici prisms provide you with an upright image. Diagonal mirrors provide an upright but back-to-front image and Amici prisms provide an upright and right-sided image.

When you look at the sky with a refractor or Schmidt-Cassegrain telescope, the focuser is pointing downwards. To avoid contorting yourself or kneeling on the floor while observing, you use a diagonal mirror or Amici prism. The result: a light path deflected upwards through 90° provides a comfortable viewing position.

Difference between a prism and a mirror

The prism has a triangular shape, like a house roof. This house roof is positioned at 45° in such a way that the surfaces stand in the light path. When a beam of light hits the surface, it is deflected by 90° and enters the eyepiece.

A diagonal mirror works in a similar way. It consists just of a mirror, which is set in the housing at exactly 45°. Thus, star diagonals and diagonal mirrors have the same effect.

Advantages and disadvantages at a glance:

  • · The light path in the prism is slightly longer
  • · Inferior prisms can tend to cause chromatic aberration
  • · It is important for diagonal mirrors that they are highly reflective