Explore Scientific 3-inch 99% Dielectric Star Diagonal with 2-inch Adapter

Product Overview

The Explore Scientific 3-inch Dielectric-Coated Diagonal Mirror turns the light path 90 degrees for comfortable viewing on a refractor, and does so with a first-surface coated mirror that reflects 99% of the light reaching it. It ships with a 2-inch adapter and dust covers, and weighs 2.0 lb.

Who It's For

This is the diagonal for an observer running a large refractor with a 3-inch focuser who does not want the diagonal to be the narrowest point in the optical path. A 2-inch diagonal on a 3-inch focuser throws away field at the edges; this one passes the full aperture. It suits anyone using wide-field 2-inch eyepieces on a 120 mm or larger refractor.

Key Features & Design

• 99% reflective dielectric coating on a first-surface mirror, preserving image brightness
• Full 3-inch clear path with no restriction ahead of the eyepiece
• Double-taper insert barrel for smooth insertion and withdrawal from the focuser
• Wide section of the taper prevents the diagonal dropping out if the focuser set screw loosens
• Compression ring lock on the 2-inch eyepiece holder, gripping without marking the barrel
• 2-inch adapter and dust covers included
• Solid but light construction at 2.0 lb

Optical and Mechanical Design

A first-surface mirror carries its reflective coating on the front of the glass, so light never passes through the substrate — that removes the faint secondary reflection a rear-coated mirror produces around bright objects. The dielectric coating is a multi-layer stack tuned for reflectivity across the visual band rather than a metal film, which is what allows a 99% figure and why it does not tarnish with age. The double-taper barrel is a mechanical safety feature as much as a convenience: a heavy diagonal and eyepiece cannot slide free of a loosened set screw.

Recommended Uses

• Visual observing with a large refractor at a comfortable eyepiece angle
• Wide-field views with 2-inch eyepieces where full aperture matters at the field edge
• Lunar, planetary and double star work where every percent of light and contrast counts
• Any 3-inch focuser setup where a 2-inch diagonal would be the limiting aperture

Compatibility and Accessory Notes

The diagonal is built for a 3-inch focuser such as the Explore Scientific ES-FOC3, and the supplied 2-inch adapter lets you use your existing 2-inch eyepieces straight away. A 1.25-inch eyepiece holder is available separately if you also want to use smaller barrels; it has its own compression ring lock. Both holders secure eyepieces with compression rings rather than a bare set screw, so eyepiece barrels stay unmarked. We stock 2-inch eyepieces across the Explore Scientific 68°, 82° and 92° series if you are building out a set.

Good to Know Before You Order

This is a 3-inch diagonal — it needs a 3-inch focuser, and will not fit a 2-inch one. The 1.25-inch eyepiece holder is a separate purchase, so if most of your eyepieces are 1.25-inch it is worth adding. At 2.0 lb plus an eyepiece it is a real load on the focuser, which is part of why it pairs naturally with a heavy-duty 3-inch focuser rated for the weight. Not sure whether your focuser takes it? Send us the model and we will confirm.

Frequently Asked Questions

Is it difficult to set up? No. It slides into a 3-inch focuser and locks with the focuser's own mechanism; the eyepiece drops into the compression ring holder and tightens by hand.

Will my 2-inch eyepieces work? Yes — the 2-inch adapter is included in the box.

What about 1.25-inch eyepieces? A 1.25-inch holder is available separately and adds a second compression ring lock.

Why choose a 3-inch diagonal over a 2-inch? On a 3-inch focuser a 2-inch diagonal becomes the narrowest point in the light path and clips the outer field. A 3-inch diagonal keeps the full aperture through to the eyepiece.

Bottom Line

A 99% dielectric first-surface mirror in a full 3-inch body, with the 2-inch adapter and dust covers included and a barrel designed not to let go. The right diagonal for a large refractor running a 3-inch focuser. Our Bolton team can confirm fit and suggest eyepieces to match.