Sky-Watcher 90° 1.25″ Diagonal Mirror

Sky-Watcher 90° 1.25″ Diagonal Mirror

A diagonal mirror is a fundamental piece of equipment for every refractor and catadioptric telescope user. Without a star diagonal, observing objects high above the horizon becomes physically demanding - the observer must bend or crane their neck into an unnatural position that quickly strains the cervical spine and makes focused, sustained observation impossible. The Sky-Watcher 90° diagonal mirror with a 1.25″ barrel is a compact, affordable and reliable solution that transforms any refractor or SCT into a comfortable and ergonomic observing instrument.

How a Diagonal Mirror Works and Why It Helps

The diagonal mirror redirects the light beam passing through the telescope tube by precisely 90°. This brings the eyepiece into a comfortable position for the observer - approximately at eye level, regardless of how high in the sky the target object is located. When looking at stars or planets near the zenith, the observer can simply stand upright and look horizontally into the eyepiece, rather than contorting themselves beneath the telescope in an awkward and tiring position.

Compared to a star diagonal prism, the mirror has one specific advantage: it introduces no additional glass element into the optical path. Light is simply reflected from a highly polished reflective surface, with zero absorption in glass. The result is maximum transmission of brightness and contrast - an advantage you will appreciate when observing faint stars or subtle surface details on planets.

Why the 1.25″ Version Is the Right Choice for Many Observers

The 1.25″ (31.75 mm) diameter is the most widely used standard in amateur astronomy. The vast majority of eyepieces, especially in the more accessible price ranges, are produced in this size. The 1.25″ diagonal therefore perfectly suits beginning astronomers with a set of standard eyepieces, as well as more advanced observers who want to keep their kit compact and easily portable.

The 1.25″ version is also the natural choice for smaller refractors and travel telescopes whose focuser works exclusively with the 1.25″ format. Compared to a 2″ diagonal, it is significantly more compact and lighter, placing less strain on the focuser and the overall balance of the setup on the mount. For travel or grab-and-go configurations, this is a meaningful practical advantage.

Image Orientation - What the Diagonal Changes and What It Does Not

A standard diagonal mirror delivers an image that is correctly oriented vertically - what is at the top of the sky remains at the top in the eyepiece. Along the horizontal axis, however, the image is mirror-reversed: left and right are transposed compared to what you see with the naked eye or in the finder. For astronomical observation this presents no practical problem whatsoever - the night sky and astronomical objects are symmetric, and their lateral orientation has no effect on either observation or identification. When comparing with a star chart it is simply worth keeping this reversal in mind.

If you need an image correctly oriented in all axes - for example for daytime terrestrial observation or nature watching - we recommend using an Amici erecting prism rather than a standard diagonal mirror.

Construction and Compatibility

The diagonal connects to any standard 1.25″ focuser on a refractor or catadioptric telescope and accepts any standard 1.25″ eyepiece. The metal body is robust and durable, holding the eyepiece firmly without play. The reflective surface inside the diagonal is precisely applied and protected against dust and moisture. The diagonal is compact and easily carried as part of a standard telescope kit.

The diagonal mirror is not intended for Newtonian reflectors. In a Newton the eyepiece is positioned at the side of the tube and the light beam has already been reflected once by the secondary mirror - adding another reflecting element would produce incorrect image orientation and prevent proper focusing.

Technical Specifications