An 82° Eyepiece for Just $100?: Astro-Tech 4mm UWA Review

Selfie Testing Astro-Tech UWA with Eyepiece. Credit: Alan Dyer

This Astro-Tech eyepiece delivers a wide 82° field for only $100. But is it sharp? 

Astro-Tech UWA eyepieces come in plain boxes, with rubber end caps, but no niceties such as spec sheets or lens cloths as with competing models. Credit: Alan Dyer

Forty years ago, the US company Tele Vue revolutionized the eyepiece market by introducing models with the unheard of (at the time) apparent field of 82°, far surpassing the 65° fields of previous wide-field favorites, the old Erfle and König designs. The new models were named for their designer and company owner Al Nagler.

Today, after undergoing several design changes, the Nagler series still sets the standard of performance for ultra-wide-field (82°) eyepieces. But even the lowest-cost Type 6 models sell for about $350 each.

As with the premium 100° XWA eyepieces sold by Astronomics under their Astro-Tech house brand (I reviewed the Astro-Tech 20mm XWA here at AGT), I was intrigued by their affordable seven-element 82° Ultra-Wide-Angle (UWA) series. Models sell for $120 to $240, the latter price reserved for the big 2-inch-barrel 28mm UWA. The other UWAs from 16mm to 4mm, all with 1.25-inch barrels, list for $119.95 each.

The Astro-Tech 4mm UWA is just 95mm (3.7 inches) high, making it easy to store with any collection of eyepieces. Credit: Alan Dyer

At the time I purchased my UWA, it was “on sale” for $99.95, which I suspect is the normal price, thus I call these “$100” eyepieces. At the time of my purchase only the 4mm and 28mm UWAs were in stock.

Having little need – nor room in my eyepiece box! – for another big low-power eyepiece, I chose the 4mm UWA to try out. Its high power is suitable for planetary and double star viewing with the short focal length refractors I like to use. The question was, would it be sharp, or would it exhibit some major flaw at its low price?

The UWA optics are fully multi-coated and edge blackened. No ghost images or flares were visible from bright objects in or just outside the field. Credit: Alan Dyer

UWA Optics

I can happily report that the 4mm UWA performs superbly. I tested it with a 94mm f/5.5 SharpStar refractor, a 105mm f/6 Astro-Physics Traveler refractor, and a 200mm f/6 Sky-Watcher Dobsonian reflector. In all scopes, stars remained sharp across the entire field of view, elongating only slightly with some lateral chromatic aberration at the very edge of the field. This flaw added short blue tails to star images, but I see this same effect in premium models.

I saw no image softness on-axis, or any sign of longitudinal chromatic aberration adding false color. Switching between the UWA and a top-of-the-line 3.5mm Tele Vue Delos yielded very similar views for sharpness, contrast, and neutral color. The view of a Jovian moon transit was equally crisp and superb in both eyepieces.

Without a 3.5mm or 5mm Nagler Type 6 on hand, I could not make the direct comparison with the obvious competitors. But comparing the UWA to a 9mm Nagler Type 6 revealed the UWA’s most noticeable shortcoming – its edge-of-field stop looked soft and not as crisply defined as in the Nagler. This is a small deficiency for the huge cost saving of the UWA.

The eyecup rolls down easily. The painted-on lettering indicating the focal length could be larger, for ease of identifying the eyepiece in the dark. Credit: Alan Dyer

I measured the eye relief of the UWA from the eye lens to be 12mm, just as specified. For a 4mm-focal-length eyepiece, this is good, though certainly not as long as the 20mm offered by long-eye-relief models such as Tele Vue’s Delos or DeLite series, or the 16mm eye relief of Astro-Tech’s own $55 Premium Flat Field (PF) models.

However, 12mm of eye relief is typical of this class of eyepiece, such as the Nagler Type 6s, and proved sufficient for a comfortable view without eyeglasses. The eyecup placed my eye at just the right spot to see the entire field. Moving my eye slightly off-axis did result in some kidney-bean blackout squirming around the outer part of the field. So, the view is sensitive to eye position. However, the soft eyecup aids in maintaining best eye placement. 

In all, despite the UWA’s high power and only modest eye relief, I found it a comfortable eyepiece to use.

The UWA’s 1.25-inch lower barrel is tapered for added security when inserted into a telescope focuser or star diagonal. The bottom is threaded for filters. Credit: Alan Dyer

UWA Mechanics

While the rubber eyecup clips into a groove around the top of the eyepiece, it remained in place with use, and did not pop off easily to be lost. It is pliable and folds down easily if needed for eyeglass use.

Weighing just 164 grams, the 4mm UWA is 60 grams lighter than the published 224-gram weight of the 5mm Nagler Type 6, and 77 grams lighter than the 3.5mm Type 6. The UWA certainly won’t present balance issues on any telescope.

The construction is solid and well-finished, with a textured rubber grip ring that is somewhat loose, allowing it to slip around the barrel. Perhaps it could split and fall off, but I suspect it will stand up to normal use just fine. The UWAs don’t offer anything as luxurious as the waterproof and argon-purged interiors of some competing models, such as from Explore Scientific.

With a 4.1-inch f/6 refractor the actual field of view with the 4mm UWA is just wide enough to frame the entire Moon at 160x. (Simulation created with SkySafari.) Credit: Alan Dyer


Anyone looking for a high-power eyepiece with much better quality than the standard eyepieces supplied with beginner telescopes would do well to look at the UWA series, assuming the $100 price (as good as it is) is within the budget. The 4mm UWA is an excellent choice for telescopes with focal lengths from 400mm to 800mm, as it provides 100x to 200x, respectively. 

The wide 82° apparent field yields an actual field of view of 0.8° to 0.4° on such scopes, still generous for such high power. This sets a double star into context in a wider field, or allows all or most of the 0.5°-wide lunar disk to be framed, with the high power revealing small details. With non-tracking telescopes such as Dobsonians, the wide field keeps a target in view longer before the telescope has to be nudged to follow the object. 

The UWA’s small size made it easy for me to find a place for it in my eyepiece box. And the low price was attractive, too! 



Excellent image sharpness across the field
Comfortable eye relief
Low price for an 82° eyepiece



Softly defined field stop
Sensitive to kidney-bean blackout
Labeling hard to read in the dark


MSRP: $119.95



About Alan Dyer

Alan Dyer is an astrophotographer and astronomy author based in Alberta, Canada. His website at has galleries of his images, plus links to his product review blog posts, video tutorials, and ebooks on astrophotography.

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