Star diagonal

A star diagonal is an angled mirror or prism used in telescopes that allows viewing of objects that are at a 90° angle from the direction the telescope is pointing. It allows convenient and comfortable viewing when the telescope is pointed at, or near the zenith (i.e. directly overhead). The resulting image is right side up, but is reversed from left to right. Star diagonals are available in 1.25" and 2" diameters. The 2" diagonals allow for a greater field of view and improved image quality.

Types of Diagonals

Mirror (Reflective) Diagonals:These diagonals (often called Star diagonals) use a mirror set at a 45° angle inside the diagonal (so that you view the scope’s image at a 90° angle to the rear cell). Mirror diagonals produce an image in your eyepiece that is correctly oriented vertically, but is reversed left-to-right horizontally. Until one becomes accustomed to this reversal, it causes some confusion when comparing what one sees in the eyepiece versus a star chart. On the other hand, the major advantage to mirror diagonals is that they produce very little light loss in the optical path as long as the mirror is manufactured to be very highly reflective.

Prism Diagonals:A prism diagonal uses an Amici roof prism rather than a mirror to bend the light path, as do the prisms used in binoculars or the pentaprism used at the top of Single Lens Reflex cameras. The advantage to a prism diagonal is that if it is designed to be an “image erecting” or “correct image” prism it provides a view in the eyepiece that is correctly oriented, both horizontally and vertically. This means that what is seen in the eyepiece is the same as what is seen when looking at the sky, or a star chart or lunar map. The disadvantage to prism diagonals can be that because the light path bounces around through a piece of glass, inexpensive prism diagonals reduce the total amount of light transmitted. So unless one purchases a very good one, they can lose 10% or more (depending on the prism design) of the light the scope collects. Also, prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this isn’t a problem with most Schmidt-Cassegrain telescopes, which have long focal lengths. Because of their light loss, inexpensive prism diagonals are commonly used when using a telescope for terrestrial viewing (as a “spotting scope” for bird watching or similar applications) because in typical terrestrial situations there is plenty of light available and a 10% light loss isn’t a problem.