A right angle that saves the neck and flips the view

The best star diagonals for 2026

A star diagonal bends the light path through a right angle so that the eyepiece points upward instead of straight back, which is what makes a refractor or a folded telescope usable on anything higher than the horizon without kneeling. It is a small accessory with a large effect on whether an evening is comfortable. The first division is mirror or prism. A dielectric-coated mirror reflects a very high and very even percentage of the light across the spectrum, and the best figures on this shelf belong to mirrors. A prism uses total internal reflection, can be slightly better on a long-focal-ratio refractor where it also corrects a little spherical aberration, and is more sensitive to the focal ratio it is used at. Both produce an image that is the right way up and reversed left to right, which is the orientation that makes printed star charts confusing and is simply learned. Read the reflectivity figure the maker publishes, and treat an absent figure as an answer. Read the barrel size, since a two-inch diagonal passes the wide fields that a two-inch eyepiece exists to deliver, and many come with an adapter for smaller eyepieces. The clamping method matters more than expected: a compression ring grips a barrel without scoring it, a bare thumbscrew marks every eyepiece it holds. Optical path length belongs in any camera calculation. We compare mirror or prism construction, reflectivity, barrel size, clamping method, optical path length and build.

The parts here, column by column

1. 

   Celestron 1.25 in star diagonal (94115-A)

   A 90-degree mirror diagonal for 1.25 in focusers, giving an image that is the right way up and reversed left to right.

   Mirror or prism construction

   90-degree mirror star diagonal

   Reflectivity

   The listing does not carry it

   Barrel size

   1.25 in focuser and visual back

   Clamping method

   Standard 1.25 in eyepieces, Barlow lenses and accessories

   Optical path length

   Upright, with left and right reversed, as the maker states it

   Build

   Not published by the maker.

   Amazon reference

   B00009X3UO (same reference on 6 Amazon marketplaces)

   EAN

   0050234941150

   • The maker states the image orientation precisely instead of calling it “correct image”
   • Accepts everything with a 1.25 in barrel, including Barlows
   • No coating specification: dielectric, aluminium and enhanced aluminium all differ in reflectivity

   Best for: A refractor or a Maksutov used visually, where looking up is otherwise uncomfortable

   ≈ $42 / €47

   Check price on Amazon
2. 

   Seben AP3 90° star diagonal 1.25 in

   A 90-degree star diagonal with a 1.25 in fitting at both ends, explicitly excluded by the maker from use on Newtonian telescopes.

   Mirror or prism construction

   90-degree star diagonal · Not described — neither mirror nor prism is specified

   Reflectivity

   The listing does not carry it

   Barrel size

   31.7 mm (1.25 in) on both sides

   Clamping method

   Not published by the maker.

   Optical path length

   Not published by the maker.

   Build

   Refractors, Schmidt-Cassegrains and Maksutovs, as the maker lists them · Newtonian telescopes, stated outright by the maker

   Amazon reference

   B07NPC3SBN (same reference on 5 Amazon marketplaces)

   EAN

   4049467616306

   • The maker names both what it suits and what it does not, which is rare and useful
   • The least expensive diagonal on this shelf by a considerable margin
   • Neither the optical element nor its coating is described

   Best for: Replacing a missing or broken diagonal on a small refractor at minimum cost

   ≈ £6 / €6

   Check price on Amazon