Star diagonal

A star diagonal is an attachment, usually a mirror or a prism inside a small housing, inserted between a telescope's focuser and the eyepiece to bend the light path, most commonly by 90 degrees. Without one, viewing an object high overhead through a straight-through refractor or Maksutov-Cassegrain would mean crouching or lying awkwardly beneath the tube to line up an eye with the eyepiece, which becomes uncomfortable quickly during a long session.

By redirecting the light path upward or to the side, a star diagonal lets an observer stand or sit comfortably while looking through an eyepiece at a much more natural angle, regardless of where in the sky the telescope is pointed. Diagonals are a standard part of refractor and Maksutov-Cassegrain setups, and they are largely why those designs remain practical for high-power observing near the zenith.

Diagonal quality affects image sharpness and contrast, since the reflecting or refracting surface inside sits directly in the light path to the eyepiece, so a low-quality diagonal can noticeably soften an otherwise sharp telescope's view. When buying, check that the diagonal's barrel size matches the focuser and eyepieces being used, since a mismatched size either will not fit or will vignette, darkening the edges of the view.