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A flat mirror or prism that reflects a light beam through 90°, for example the secondary mirror in a Newtonian reflector. A star diagonal, consisting of a short tube into which the eyepiece fits, is used in refractors to give a more comfortable viewing position, while a Sun diagonal is similar but has a non-silvered mirror to cut down the light intensity. Both of these have the drawback that they give a laterally inverted mirror image.

Subjects: Astronomy and Astrophysics.

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