Binorum Optima 90° 1.25″ Diagonal Mirror
Observing through a telescope without a diagonal mirror can quickly become uncomfortable - especially when viewing objects high above the horizon, where you have to twist your head into awkward positions. The Binorum Optima 90° diagonal mirror solves this problem neatly. It redirects the optical axis by 90°, so you look through the eyepiece from the side rather than straight back along the telescope tube. The result is a noticeably more comfortable observing experience, allowing you to focus on what matters - the view through the eyepiece. The diagonal fits the standard 1.25″ barrel size and works with both refractors and catadioptric telescopes.
How it works
The design uses a proven reflective surface set at a 45° angle, which redirects the light path by exactly 90°. This arrangement is the standard in astronomy and is used with the vast majority of refractors and Schmidt-Cassegrains. The 1.25″ barrel size makes it easy to combine with a wide range of eyepieces - from the Binorum range or from other manufacturers. Fitting the diagonal into the telescope focuser is quick and requires no tools.
When will you appreciate it most?
You will notice the difference most when observing objects at higher altitudes above the horizon - a full Moon, a planet at opposition, or star clusters near the zenith. Without a diagonal, you would need to hold an unnatural body position that quickly tires your neck and back. With the diagonal in place, you can stand or sit comfortably and look horizontally or slightly downward through the eyepiece. This becomes especially important during longer observing sessions, where physical fatigue can seriously affect your concentration and enjoyment.
Typical uses
- Observing the Moon and planets high in the sky
- Deep-sky objects near the zenith, such as star clusters or the Andromeda Galaxy
- Daytime terrestrial viewing - landscapes, nature, or distant subjects
- Astronomy outreach and group sessions where multiple people share the eyepiece
Image orientation
The diagonal mirror produces a mirror-reversed image - flipped left to right. For astronomical use, this is no problem at all and does not complicate orientation on a star chart. If you need a correctly oriented image for terrestrial observing, a dedicated prism diagonal is the right choice. For the night sky, a classic diagonal mirror is exactly what you need.
About Binorum
Binorum is a brand created by astronomers for astronomers. It is built on the belief that quality astronomical accessories should be accessible to everyone with a serious interest in observing the night sky - whether you are just starting out or have years of experience behind you. Binorum products go through careful selection and testing, with a focus on optical quality and long-term reliability. The range covers a wide variety of astronomical accessories - from eyepieces and filters to diagonal mirrors and other add-ons - so you can find everything you need to equip your telescope in one place.
Technical specifications
| Parameter |
Value |
| Brand |
Binorum |
| Model |
Optima |
| Type |
Diagonal mirror |
| Deflection angle |
90° |
| Barrel size |
1.25″ |
| Reflective surface angle |
45° |
| Compatible telescopes |
Refractors, Schmidt-Cassegrains, catadioptric telescopes |
| Tool-free fitting |
Yes |
| Image orientation |
Mirror-reversed (left-right) |
Compatible accessories
The Binorum Optima diagonal works with all standard 1.25″ eyepieces. For the best results, pair it with eyepieces from the Binorum Plössl range, which offer excellent optical quality and comfortable eye relief across a wide range of focal lengths.
