Sky-Watcher 90° 2″/1.25″ Diagonal Mirror

Sky-Watcher 90° 2″/1.25″ Diagonal Mirror

A diagonal mirror is a practically indispensable accessory for users of refractors and catadioptric telescopes. Without one, observing objects above the horizon would require awkward, quickly tiring body positions - the cervical spine would bear a significant share of the strain throughout every session. The Sky-Watcher 90° diagonal mirror with a 2″ barrel elegantly eliminates this problem and, as a bonus, offers compatibility with eyepieces in both of the most widely used size standards.

Why Choose a Diagonal Mirror Rather Than a Prism?

There are two fundamental types of star diagonals - mirrors and prisms. The mirror has one practical advantage over the prism: it introduces no additional glass element into the optical path, thereby minimising potential light losses caused by absorption in glass. This is particularly beneficial with larger aperture telescopes or when observing faint objects where every captured photon counts. A mirror diagonal is also typically lighter than a prism of comparable size, which is appreciated on focusers with limited payload capacity.

The reflective coating of the Sky-Watcher diagonal mirror delivers high reflectivity for maximum light transmission with minimal loss at the reflection surface. The resulting image is bright and contrasty even at higher magnifications.

The 90° Angle - Comfortable Viewing of High-Altitude Objects

A 90° deflection angle is the classic and most widely used choice for both astronomical and terrestrial observation of objects situated high above the horizon. When looking at stars, planets or the Moon - objects towards which the telescope points steeply upward - the 90° diagonal allows the observer to stand upright or sit comfortably at the tripod and look into the eyepiece at approximately eye level. Compared to the 45° variant, the ergonomic advantage when observing objects at greater elevations is more pronounced - the head does not remain in a slight backward tilt but stays in a natural, straight position throughout.

For observing objects closer to the horizon - landscapes, birds or ships - the 45° version may be more comfortable. The 90° star diagonal is, however, the standard and most commonly used choice for night-time astronomy.

Image Orientation

It is important to understand what image orientation a star diagonal provides. Unlike the Amici prism, which corrects image orientation in all axes, a standard diagonal mirror delivers an image that is correctly oriented vertically - what is at the top remains at the top. Along the horizontal axis, however, the image remains mirror-reversed - left and right are transposed. For astronomical observation this is no disadvantage at all, since the night sky has no inherently "correct" left and right side. When using a star chart or navigating with a finder, the mirror reversal does need to be kept in mind, but experienced astronomers adapt to this very quickly.

For terrestrial observation where correct image orientation in all axes is important, we recommend using an Amici erecting prism rather than a standard diagonal mirror.

2″ Barrel with 1.25″ Reducer - Maximum Flexibility

The diagonal is primarily designed for focusers with a 2″ barrel, which is now used by the majority of quality refractors and mid-to-high-range catadioptric telescopes. The large 2″ diameter has a fundamental advantage: it allows the use of 2″ wide-angle eyepieces with larger apparent fields of view, fully exploiting the telescope's potential for panoramic observation. At the same time, the diagonal is fitted with a 1.25″ reducer in the eyepiece holder, so it accepts standard 1.25″ eyepieces and accessories without any difficulty. This combination provides maximum flexibility - a single diagonal covers your entire eyepiece collection regardless of whether you use 2″ or 1.25″ accessories.

On the telescope connection side, the diagonal features a standard 2″ filter thread, allowing 2″ astronomical filters to be screwed directly between the telescope and the diagonal - for example a lunar filter, neutral density filter or light pollution filter.

Which Telescopes Is the Diagonal Suitable For?

The Sky-Watcher 90° 2″ diagonal mirror is designed for refractors and catadioptric systems - Schmidt-Cassegrain (SCT), Maksutov-Cassegrain and similar designs where the light beam travels straight along the optical axis of the instrument. The diagonal is not suitable for Newtonian reflectors, where the addition of another reflecting element would produce incorrect image orientation and prevent proper focusing.

Technical Specifications