Prisms and Star Diagonals for Telescopes
A star diagonal or zenith prism redirects the light path by about 90 degrees, making viewing more comfortable with many refractors, Schmidt-Cassegrain and Maksutov telescopes. Amici prisms provide an upright, correctly oriented image and are especially useful when a telescope is also used for nature observation. Choose the design, connection size and image orientation that suit your instrument.
This category includes 1.25-inch and 2-inch models, as well as T-2 and SC-thread connections. Before ordering, check that the telescope-side fitting matches your focuser and that the eyepiece connection suits your accessories.
Star diagonal, zenith prism or Amici prism?
A star diagonal or zenith prism fits between the telescope and eyepiece. Its approximately 90-degree light-path bend makes observing objects high in the sky more comfortable. These diagonals are commonly used with refractors, Schmidt-Cassegrain and Maksutov telescopes. A standard star diagonal is generally not intended for Newtonian telescopes; always check the instrument design and the manufacturer's compatibility notes.
With many astronomical telescopes, a conventional star diagonal gives an upright but laterally reversed view. This is usually not a concern for observing the Moon and planets. An Amici prism produces an upright, correctly oriented image, making it a practical option for nature and landscape viewing when the telescope, prism and magnification are suitable. A 45-degree model can provide a comfortable viewing angle for objects nearer the horizon; 90-degree models have an eyepiece angle similar to astronomical diagonals.
Choose the correct connection and accessory size
- 1.25 inch (31.7 mm): a common barrel size for many telescopes and eyepieces. Check both the telescope-side barrel and the eyepiece-side holder.
- 2 inch (50.8 mm): accommodates 2-inch eyepieces and wider apparent fields when the telescope and focuser illuminate them. A suitable reducer often allows 1.25-inch eyepieces to be used as well.
- SC thread and T-2: threaded connections differ from barrel fittings. Check the thread type, internal or external thread, included adapters and required optical distance.
- Filters: filter threads vary by product. A filter thread on one side does not mean every filter will fit every connection.
Image quality, clamping and practical details
Specifications vary between products and may include different coatings, reflectivity and surface accuracy. Figures such as 99% reflectivity or 1/12 lambda apply only to products explicitly listed with those specifications; they are not universal properties of all star diagonals. Also consider a solid metal body, a secure compression-ring or twist-lock clamp, and enough clearance for a camera, filters and heavy eyepieces. Accessories should be held securely without damaging the barrel or eyepiece.
Flip mirrors provide multiple optical paths, for example to switch between a centring eyepiece and a camera. For imaging, the camera connection, adapters and working distance must all match; follow the connections stated on the individual product page.
Quick checks before ordering
- What telescope design and focuser connection do you have?
- Do you need a 1.25-inch or 2-inch system, or a threaded adapter?
- Do you need a comfortable viewing angle for astronomy, or an upright, correctly oriented image for terrestrial use?
- Are the clamp, clearance and optical path suitable for your eyepiece or camera?
Browse more telescope accessories in the parent category.