Four millimeters is the extreme end of useful magnification in a 1.25" eyepiece — the one you reach for when the atmosphere is dead calm and the planet is begging for more power. In a 1250mm Dob, that's 312x. In a 714mm refractor, 179x. In a 2032mm SCT, 508x. The 82° apparent field keeps the view comfortable and gives you real drift time even at these magnifications, where a narrow-field eyepiece would have you chasing the planet across a keyhole. With plenty of five-star reviews and a price that makes extreme magnification accessible, this is the eyepiece that lets you push your telescope to its atmospheric limit without pushing your budget.
The Optics
Seven elements, fully multi-coated with blackened lens edges. At 4mm, optical quality is non-negotiable — the exit pupil is tiny, the magnification is extreme, and every flaw in the glass shows up immediately. The 4mm UWA delivers a sharp, contrasty image across the field. Alan Dyer's review in Cloudy Nights' Astro Gear Today tested the 4mm UWA in three different telescopes — a 94mm f/5.5 refractor, a 105mm f/6 Astro-Physics Traveler, and a 200mm f/6 Dobsonian — and reported that it performs superbly. No ghost images or flares were visible from bright objects in or just outside the field. The 82° field is wide enough to keep objects in view without constant nudging. The one optical note: the field stop is more softly defined than in premium models like the Nagler Type 6 — a small trade-off for the enormous cost saving.
1.25" Barrel
Fits any 1.25" focuser, diagonal, or Barlow. Safety groove engages your focuser's thumbscrew. At just 164 grams (measured by Alan Dyer), the 4mm UWA is 60 grams lighter than the 5mm Nagler Type 6 and 77 grams lighter than the 3.5mm Type 6 — it won't present balance issues on any telescope. At 4mm, this is already extreme magnification in most telescopes — a Barlow would push it beyond what the atmosphere typically supports, but in smaller scopes (4–6 inches), a 2x Barlow turns the 4mm into an effectively 2mm eyepiece for those rare nights of exceptional seeing.
Waterproof
O-ring sealed and waterproof. Prevents internal fogging during cold planetary sessions — and at 4mm, you'll be using this eyepiece on the nights when the temperature has dropped and the seeing has stabilized, exactly when fogging is most likely. Keeps dust and grit out of the optics and protects the coatings from humidity and fungus.
What's Included
- Astro-Tech 4mm 82° UWA 1.25" eyepiece
- Lens caps (top and bottom)
Features
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82° apparent field of view — At extreme magnification, the wider field is the difference between comfortable observing and constant nudging. More drift time on untracked telescopes, more context around the target.
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7-element fully multi-coated optics — High contrast with controlled scatter. Owners choose it over premium alternatives at several times the price.
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4mm focal length — Extreme power. 312x in a 1250mm Dob, 179x in a 714mm refractor, 508x in a 2032mm SCT. The eyepiece for the steadiest nights.
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12mm eye relief — Exceptional for a 4mm eyepiece. Most 4mm eyepieces force you to press your eye against the lens. This one doesn't.
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Waterproof, O-ring sealed — Cold-weather performance and long-term protection.
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1.25" barrel with safety groove — Universal compatibility.
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Lightweight — No balance concerns.
Under the Night Sky
In a 10-inch f/5 Dob at 312x, Jupiter on a steady night shows the two equatorial belts with festoons, barges, and scalloping in real detail. The Great Red Spot shows internal structure — not just color and shape, but the wake turbulence trailing behind it. On the best nights, four or five belts are visible with hints of detail in each. The Galilean moons show disks, not points. The 82° field holds the planet for about 20 seconds of drift — enough time to study a feature before nudging the scope.
Saturn at 312x shows the ring system in fine detail. Cassini's Division is a broad dark gap, not a thin line. The Encke minimum is visible on the best nights. The shadow of the globe on the rings shows crisp geometry. Multiple moons are visible around the planet. The crepe ring is distinct against the globe.
The Moon at 312x is where extreme magnification pays off most, because the Moon is bright enough to tolerate high power even on average nights. Crater floors show fine detail — rilles, secondary impacts, slumped walls. The Straight Wall shows as a true shadow line. Hadley Rille, the Apollo 15 landing site valley, is resolvable. You're looking at the Moon the way spacecraft see it.
Mars during opposition at 312x shows the dark surface markings — Syrtis Major, Hellas Basin, and the polar caps — with enough resolution to track changes from night to night. This is the eyepiece you wait all opposition season to use, saving it for the one or two nights when the seeing is truly exceptional.
For deep-sky, 312x is specialized territory: planetary nebulae like NGC 7662 (Blue Snowball) and NGC 6826 (Blinking Planetary) show disk structure. Tight double stars split cleanly — Dawes' limit territory in a 10-inch scope. Compact globulars resolve to the core.
Community Says
"Performs superbly." This reviewer tested the 4mm UWA in three telescopes — a 94mm f/5.5 refractor, a 105mm f/6 Astro-Physics Traveler, and a 200mm f/6 Dobsonian — and compared it directly to a 3.5mm TeleVue Delos at more than twice the price. The verdict: very similar views for sharpness, contrast, and neutral color. A Jovian moon transit was equally crisp in both eyepieces. The only trade-offs: a softly defined field stop compared to the Nagler Type 6, and some sensitivity to kidney-bean blackout that resolves with practice positioning the eye against the soft eyecup. At 164 grams, it's lighter than both the Nagler and Delos alternatives. — Cloudy Nights Astro Gear Today review.
Observing Tip
The 4mm UWA is a seeing-limited eyepiece. On most nights, the atmosphere won't support this magnification — the image will be soft and boiling. That's not the eyepiece; that's the air. Start your session with a longer focal length, work your way up, and reach for the 4mm only when the image at lower power is steady and sharp. When the seeing cooperates — and you'll know immediately — the jump in detail is dramatic. Patience with the atmosphere is the skill that makes this eyepiece worth owning.
FAQ
How does this compare to the 4.8mm 110° XWA?
The 4.8mm XWA has a wider field (110° vs. 82°), slightly longer focal length (4.8mm vs. 4mm), 15mm eye relief (vs. 12mm), and a dual 1.25"/2" barrel. The 4.8mm XWA is 8 elements versus 7, and weighs 19 oz versus the UWA's 164 grams (~5.8 oz). The 4mm UWA gives you slightly more magnification and costs significantly less. If you want the widest possible field and don't mind the weight and price, the XWA is the step up. If you want extreme magnification at a fraction of the cost, the UWA delivers.
Is 312x too much for my telescope?
The rule of thumb is 50x per inch of aperture as a practical ceiling — so 312x is reasonable for a 10-inch scope but pushing it for a 6-inch. In a 4-inch refractor at 179x (from a 714mm focal length), you're at a very comfortable power. The atmosphere, not the telescope, usually sets the limit. On most nights, 200–250x is the practical ceiling regardless of aperture. The 4mm UWA is the eyepiece for the nights that exceed that ceiling.
Is this parfocal with the other 82° UWAs?
Yes — confirmed by Cloudy Nights members. One owner tested all five 1.25" UWAs (4mm through 16mm) and found them parfocal, requiring only a small touch of the focuser when swapping between focal lengths.
Can I use this with an AT102ED?
Yes. One owner reports excellent results with the AT102ED. At 714mm focal length, the 4mm gives you 179x — well within the scope's capabilities and a very effective planetary magnification for a 4-inch ED doublet.
Final Thoughts
The 4mm UWA is the extreme-magnification specialist of the Astro-Tech 82° line — the eyepiece you buy knowing you won't use it every night, but knowing that when you do, nothing else in the case will show you what this one shows you. Alan Dyer's review for Cloudy Nights puts it plainly: it performs superbly. At 4mm, 82° field, 12mm eye relief, and 164 grams, it's an invitation to push your telescope to its limits on the nights that deserve it.
