Omegon Pro Penta Prisma 90° 1.25″ pentagonal prism
Are you looking for a zenith prism that preserves the true orientation of the image - exactly as you would see it looking through the telescope without any accessories? That is precisely what the Omegon Pro Penta Prisma 90° pentagonal zenith prism offers. Unlike standard star diagonals or Amici prisms, this five-sided (pentagonal) prism delivers an image that is both laterally and vertically inverted - meaning the orientation matches exactly what you see with the naked eye or on a classic astronomical Moon map. This is especially appreciated by observers who work with a mirror-and-altitude-inverted lunar map and need what they see in the eyepiece to correspond precisely to the map.
Comfortable 90° viewing with natural image orientation
The pentagonal prism is primarily designed for refractors and SCT (Schmidt-Cassegrain) telescopes that use standard 1.25″ accessories. It is the right choice for any situation where you want comfortable viewing at a 90° angle without the drawbacks of mirror reversal. When observing the Moon with a detailed selenographic map in hand, you will appreciate that the orientation in the eyepiece matches the map exactly - the left edge is on the left, the right is on the right, north is at the top. No mental flipping of the image required.
BK-7 optical glass and fully dielectric coatings
The Pro series prism body is made from BK-7 optical glass, a proven choice in astronomical optics for its homogeneity and low bubble content. The clear aperture of 29 mm provides sufficient light throughput for eyepiece diameters commonly used with 1.25″ connections. The reflective surfaces of the prism are equipped with fully dielectric high-reflectivity coatings, which give the prism significantly higher light transmission than older alternatives with aluminium or silver coatings. The result is a bright, high-contrast image with accurate colour rendition.
The optical path length (backfocus) is 120 mm - an important parameter when focusing, particularly with telescopes that have a limited focuser travel range.
Practical design and secure eyepiece attachment
The eyepiece is secured with a brass compression ring (soft clamp), which protects the eyepiece barrel from scratches while providing a firm and safe hold. Key practical features include:
- Standard 1.25″ input and output connections
- Integrated filter thread - add a lunar, UHC, or other filter directly to the prism without additional adapters
- Brass compression ring eyepiece holder - no scratches on eyepiece barrels
- Weight of 260 g - robust yet acceptable load for most focusers
Technical specifications
| Parameter |
Value |
| Prism type |
Pentagonal (penta prism) |
| Deflection angle |
90° |
| Connection |
1.25″ |
| Clear aperture |
29 mm |
| Optical path length (backfocus) |
120 mm |
| Glass type |
BK-7 |
| Coatings |
Fully dielectric, high-reflectivity |
| Eyepiece holder |
Brass compression ring |
| Filter thread |
Yes (1.25″) |
| Weight |
260 g |
Who is this prism for?
The Omegon Pro Penta Prisma 90° 1.25″ is the ideal choice for anyone who wants comfortable right-angle viewing while retaining a natural image orientation. Whether you are an experienced selenographer mapping the lunar surface or an astronomer looking for an ergonomic solution for long observing sessions, this prism - with its dielectric coatings and premium Pro series build quality - will meet your demands for both optical performance and practical usability.
