A star diagonal, also known as a star prism or star mirror, is an accessory used in telescopes to divert the light path by 90 degrees. It is a device that allows observers to look into the side of the telescope tube at a more comfortable viewing angle, especially when the telescope is pointed at or near the zenith.
A star diagonal consists of a prism or a mirror mounted in a housing that fits into the focuser of a telescope. The prism or mirror reflects the light path by 90 degrees, allowing the observer to view the image without having to contort their neck or body.
Star diagonals are particularly useful for observers using refracting telescopes, where the eyepiece is typically located at the end of the telescope tube. When pointed at the zenith, the observer would have to crane their neck uncomfortably to look through the eyepiece. By using a star diagonal, the observer can sit or stand in a more comfortable position while observing.
There are different types of star diagonals available, including standard mirror and prism diagonals, dielectric diagonals, and enhanced aluminum coatings. Each type has its own advantages and disadvantages, depending on the observer’s needs and budget.