Explore Scientific 99% Reflective 3" Diagonal

A 3-inch dielectric-coated star diagonal that reflects 99% of incoming light through a full 3-inch optical path — designed for telescopes with 3-inch focusers where a standard 2-inch diagonal would vignette the light cone and clip the edges of wide-field views.

The first-surface dielectric coating reflects virtually all incoming light, delivering the brightest and highest-contrast image your telescope can produce. This is particularly important at the 3-inch aperture class, where the telescopes paired with this diagonal are typically fast, large-aperture refractors — instruments where every fraction of a percent of light transmission matters for deep-sky performance.

The double-taper insert barrel slides smoothly into 3-inch focusers and the wider taper section prevents the diagonal from accidentally falling free. A compression ring lock secures eyepieces without set screw marks. Includes a 2-inch adapter so you can use your existing 2-inch eyepiece collection, and dust covers to keep optics clean in storage.

Key Features

• 99% first-surface dielectric coating for maximum brightness
• Full 3" optical path — eliminates vignetting on large-format focusers
• Double-taper insert barrel for secure, smooth fit
• Compression ring eyepiece lock — no set screw marks
• 2" adapter and dust covers included
• Weight: 2.0 lbs (0.90 kg)

Ideal For

• Large refractor owners — the ES ED152, FPL53-series, and other 3"-focuser telescopes need a 3" diagonal to avoid vignetting
• Deep-sky visual observers — the full 3" light path paired with 99% dielectric reflectivity delivers the brightest possible wide-field views
• 2" eyepiece users — included adapter means your existing eyepieces work immediately

Good to Know

• 1.25" eyepiece adapter is sold separately
• Designed for telescopes with a 3" focuser — will not fit standard 2" focuser drawtube