Celestron 1.25" Dielectric Mirror Diagonal, Twist-Lock

Celestron 1.25" Dielectric Star Diagonal with Twist-Lock Eyepiece Clamp

If you're still using a basic 1.25" diagonal that came bundled with your telescope, upgrading to this premium dielectric diagonal from Celestron is one of the easiest ways to boost your scope’s optical performance—especially when it comes to planetary detail, double stars, and faint deep sky targets. With its precision BK7 mirror substrate, ultra-high reflectivity dielectric coatings, and secure twist-lock eyepiece holder, this diagonal brings pro-level light throughput and mechanical quality to a very affordable price point.

Brighter Views, Sharper Edges

At the heart of this diagonal is a mirror coated with 58 layers of dielectric material, resulting in over 99% reflectivity across the visible spectrum. That means nearly every photon gathered by your telescope's optics reaches your eye—without the dimming or color shift that plagues older aluminum or silvered mirrors. Because the diagonal is diffraction-limited, it will not degrade the image quality provided by your telescope. It preserves the contrast, resolution, and pinpoint star sharpness that your optics were designed to deliver.

Twist-Lock: A Better Way to Hold Your Eyepiece

Instead of using a single thumbscrew to hold your eyepiece in place (which can tilt the optical axis and mar your eyepiece barrel), this diagonal uses a twist-lock collar. A gentle twist locks your eyepiece in place evenly and firmly, keeping it perfectly centered for optimal performance. The collar includes an internal compression ring to prevent scratches, and a built-in safety stop to protect the diagonal’s mirror from long eyepiece barrels or Barlows.

Durable Construction with Premium Touches

Celestron didn't cut corners on the housing. The diagonal body is made from machined aluminum with a black anodized finish for durability. The twist-lock rings are anodized Celestron orange for easy visibility at night. Inside, the barrel is glare-threaded and painted flat black to minimize stray reflections and preserve contrast—important when observing the Moon or bright targets like Venus or Jupiter.

Of course, the diagonal also includes 1.25" filter threads on the nosepiece so you can use your favorite planetary or nebula filters without juggling extra adapters.

Ideal for Refractors, SCTs, and Maks

This 90° mirror diagonal is ideal for refractor and catadioptric telescopes (like Schmidt-Cassegrains and Maksutovs) where it provides a comfortable viewing angle for objects high overhead. It accepts all standard 1.25" eyepieces, Barlows, and accessories, and installs easily into any 1.25" visual back or focuser.

Observing Tip:

If you’re observing from a location with any light pollution—or if you’re using high-end eyepieces or trying to tease out planetary detail—swapping your stock diagonal for a high-transmission dielectric one can make a surprising difference. Think of it like polishing a window: the better the diagonal, the less it gets between you and the view.

Final Thoughts

The Celestron 1.25" Dielectric Mirror Diagonal with Twist-Lock is one of those upgrades that simply makes sense. You’ll see brighter, sharper views right away, your eyepieces will stay securely in place without getting scratched, and the rugged design will hold up to years of field use. Whether you're fine-tuning a high-performance setup or just upgrading from a basic prism diagonal, this one checks all the boxes.

Tech Details