GSO PR001 1.25 inch 90° Prism Diagonal with BaK-4 Prism and Fully Multi-Coated Optics

Ordinary star diagonals often compromise on optical quality, but the GSO PR001 Prismatic 90° 1.25" Right Angle Socket sets itself apart with a precision BaK-4 glass prism and fully multi-coated surfaces. This combination delivers bright, high-contrast views with minimal light loss, making it an excellent upgrade for refractors, Schmidt-Cassegrains, and Maksutov-Cassegrains. The 90° viewing angle provides comfortable observation, especially when aiming near the zenith, reducing strain on the neck and back during long sessions.

Highlights

• BaK-4 prism glass with fully multi-coated (FMC) surfaces for maximum light transmission and sharp, true-colour images.
• 90° viewing angle reduces neck strain during long sessions, especially when observing near the zenith.
• 1.25" (31.75 mm) telescope connection and eyepiece holder fit most refractors, SCTs, and Maksutov-Cassegrains.
• Brass compression ring holds eyepieces firmly without marring, and accepts 1.25" filters via internal threads.
• Anodized aluminium housing with blackened interior and anti-reflection baffling for enhanced contrast.
• Includes dust caps for both ends to protect optics when not in use.

In daily use

Installing the PR001 is straightforward: insert the 1.25" nosepiece into your telescope's focuser or visual back, tighten the thumbscrew, then slide your eyepiece into the holder and secure it with the compression ring. The diagonal's upright, left-right reversed image orientation is standard for astronomical use, and the internal filter threads allow you to add 1.25" filters for lunar or planetary observation. The blackened interior and baffling suppress stray light, ensuring that faint deep-sky objects remain visible against a dark background.

Full specifications

• Product type: Prismatic diagonal
• Diagonal angle: 90°
• Telescope connection: 1.25" (31.75 mm)
• Eyepiece holder: 1.25" (31.75 mm)
• Prism material: BaK-4 glass
• Optical coatings: Fully multi-coated (FMC)
• Housing material: Anodized aluminium
• Eyepiece clamping: Brass compression ring
• Filter threads: Yes, 1.25"
• Clear aperture: 25 mm
• Internal features: Blackened interior, anti-reflection baffling
• Compatibility: Refractor, SCT, Maksutov-Cassegrain (not for Newtonian)
• Dust caps: Included for both ends
• Colour: Black

Final note

The GSO PR001 is a reliable, high-performance diagonal that brings out the best in your telescope's optics, making every observing session more comfortable and rewarding.