Star Diagonals

A star diagonal is an angled mirror or prism used with a telescopes to allows viewing from a direction that is perpendicular to the optical axis of the telescope. This permits a more comfortable viewing angle when the telescope is pointed up at the sky. A star diagonal uses either a prism or a mirror to redirect the light path. Almost all star diagonals available as accessories are of the mirror type, possibly because they will work better than the prism type on many of the fast focal ratio refractors that are available today.

Star diagonals are one of the first parts of a telescope that are typically upgraded from factory equipment. This is especially of Schmidt-Cassegrain telescopes that are normally supplied with prism diagonals in 1-1/4 inch format. Prism diagonals have can have an adverse impact on image quality, especially at higher magnifications or with a rapidly converging light cone of a fast focal ratio refractor. When upgrading the diagonal of a telescope, one should consider the benefits of a two-inch diagonal if the telescope will accommodate it. We recommend two-inch diagonals for Schmidt-Cassegrain telescopes of eight-inch or large aperture, and also for any refractor with a two-inch focuser. We also recommend di-electric coatings for their higher reflectivity and greater longevity.