Caption: Star diagonals.

Features:

1. A star diagonal is an optical device that effectively bends the optical axis (and accompaning light ray paths) of a small telescope used for visual astronomy by 90°.

2. The purpose of bending is to allow easier viewing through the eyepiece which is attached at the end of the star diagonal, and so is perpendicular to the main optical axis of the telescope. By having the eyepiece perpendicular to the main optical axis, the observer is saved from krinking his/her neck.

3. The bending is typically done by an angled plane mirror, a Porro prism, a pentaprism, or an Amici roof prism.

4. The plane mirror and Porro prism give an axis reflection about a plane perpendicular to the symmetry plane of the star diagonal.

   The pentaprism gives NO inversion.

   The Amici roof prism gives a point inversion which can be used to correct for the point inversion of most small astronomical telescope designs (e.g., the Keplerian telescope).

5. The Celestron C8 telescope star diagonal uses a Porro prism which adds an axis reflection to the point inversion given by the C8 itself.

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