What is a star diagonal

A star diagonal is a prism or mirror set to direct light at a 90-degree angle to the telescope’s optical axis. It’s an ergonomic accessory to make viewing of the sky more convenient and comfortable for objects nearly overhead or for a less neck-craning angle for objects at other altitudes.

Newtonian-type reflecting telescopes don’t need a star diagonal, since their design already incorporates a right-angle reflection to the side of the tube and a comfortable eyepiece position.

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