A star diagonal is an angled mirror or prism used in telescopes that allows viewing from a direction that is perpendicular to the usual eyepiece axis.
It makes observing the sky far more convenient and comfortable for objects that are almost overhead or for a less neck straining angle for objects at various other altitudes.
A star diagonal is designed for comfortable viewing with astronomical telescopes. The 90 degree angle allows the telescope to be pointed at high angles whilst the eyepiece remains easily accessible.
A 1.25" diagonal simply pushes into the standard focuser of any refractor or catadioptric telescope that accepts 1.25" accessories.
The resulting image is right way up, but is reversed from left to right. Star diagonals are available for the majority of telescopes in 1.25" and 2" diameters.
The Astro-Tech 1.25" star diagonal is a good example and has received some glowing comments.
The 2" diagonals allow larger longer focal length low power 2 inch barrel eyepieces for a greater field of view.
Newtonian-type reflecting telescopes don’t require a star diagonal, because their design already incorporates a right angle reflection to the side of the tube and a comfortable eyepiece position.
The diagonal is absolutely essential if you intend to do any terrestrial viewing.
Celestron Star Diagonals This high quality multi-coated mirror diagonal allows you to take advantage of the wider field of view provided by 2" eyepieces.
Lumicon Star Diagonals Its single 1/8 wave precision-polished surface is superior to the three surfaces of prism diagonals.
William Optics Star Diagonal Offering top class performance and brightness, polished to 1/10th wave accuracy.
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