Sky-Watcher 90° 1.25″ Amici Erecting Prism for Terrestrial Observation
Anyone who has tried to observe terrestrial objects through an astronomical telescope has encountered the same problem: the image in the eyepiece is inverted or mirror-reversed. What should be on the left appears on the right, and what should be at the top appears at the bottom. For astronomy this does not matter - stars and nebulae have no "correct" side up. But when watching landscapes, birds, ships or any ground-level subject, a disorienting image is a genuine obstacle. The Sky-Watcher Amici prism solves this problem once and for all.
What Is an Amici Prism and How Does It Work?
The Amici prism (named after Italian astronomer Giovanni Battista Amici) is a specialised optical element that combines the function of a star diagonal with image orientation correction. While a standard star diagonal or mirror redirects the light path to a convenient 90° viewing angle, the image remains mirror-reversed - as if seen in a mirror. The Amici prism eliminates this by means of a special geometry of the optical surfaces inside the prism, which ensures that the resulting image is correctly oriented both laterally and vertically - exactly as you would see the subject with the naked eye.
From an optical standpoint, the Amici prism is a more complex element than a simple star diagonal. It incorporates a special roof-shaped surface (Dachkant prism) that introduces an additional flip of the image along one axis, compensating for the mirror reversal inherent in other types of optical diagonals. The result is an image that corresponds to natural perception - without any disorientation whatsoever.
Why Correct Image Orientation Matters
In astronomical observing, image orientation rarely matters. A planet, star cluster or galaxy looks the same regardless of whether it is rotated in the eyepiece. In terrestrial observing the situation is completely different. When watching birds in flight, ships moving along the horizon, activity on a mountain ridge or a landscape during a hiking trip, a mirror-reversed image makes it significantly harder to orient yourself and locate objects. The brain has to constantly translate what it sees into reality - and when tracking fast-moving subjects this becomes genuinely frustrating.
The Amici prism eliminates this problem entirely. A telescope equipped with a standard Amici prism behaves like a powerful spotting scope with a natural, undistorted image - essentially a large, high-magnification binocular on a tripod.
Which Telescopes Is the Prism Suitable For?
The Sky-Watcher Amici prism is suitable for refractors (lens-based telescopes) and catadioptric systems such as Schmidt-Cassegrain (SCT) and Maksutov telescopes. In these types of telescopes the light beam travels straight along the optical axis of the instrument, and the Amici prism redirects it by 90° without any difficulty while preserving the correct image orientation.
The prism is not suitable for Newtonian reflectors. A Newton has the eyepiece positioned at the side of the tube, and the light beam has already been reflected once by the secondary mirror. Adding another reflecting optical element would produce incorrect image orientation and optical complications.
Both the input and output of the prism use the standard 1.25″ (31.75 mm) format, which is the most widely used standard in amateur astronomy. The prism therefore connects directly to any telescope with a 1.25″ focuser without adapters, and accepts any standard 1.25″ eyepiece.
Practical Applications
The practical uses of the Amici prism are very wide-ranging. It is an indispensable accessory for anyone who wants to use their astronomical telescope during the day for terrestrial observation. A refractor or Maksutov on a stable tripod with an Amici prism becomes a capable spotting scope for birdwatching, watching distant sporting events, observing ships at sea or scanning mountain panoramas. Thanks to the high magnification that astronomical telescopes offer and the quality optics of refractors and Maksutovs, the resulting image is considerably more detailed than with standard binoculars or terrestrial spotting scopes of comparable size.
The Amici prism is also a practical aid for beginning astronomers who are still getting to know their telescope and want to use it not only at night but also during the day - for daytime orientation, finding planets at dusk or simply enjoying the surrounding landscape in extraordinary detail.
Build Quality
The Sky-Watcher prism is manufactured from high-quality optical glass with multi-layer anti-reflection coatings on all air-to-glass surfaces, minimising light loss and internal reflections. The metal body is precision-machined and ensures firm, accurate eyepiece retention without play. The reflective surfaces inside the prism carry a highly efficient reflective coating for maximum light transmission.
Technical Specifications
| Type |
Amici erecting prism |
| Deflection angle |
90° |
| Input / output diameter |
1.25″ (31.75 mm) |
| Image orientation |
Correct laterally and vertically (natural) |
| Suitable telescopes |
Refractors, Schmidt-Cassegrain, Maksutov |
| Not suitable for |
Newtonian reflectors |
| Optical glass |
Multi-layer anti-reflection coated |
| Manufacturer |
Sky-Watcher |
