Drop this eyepiece into a 12-inch Dobsonian and you'll understand what 100° does to a deep-sky view. The Orion Nebula doesn't just sit in the center of the field — the nebula, the Trapezium, and the surrounding star field all hang in space together, edge to edge, with room to spare. You stop thinking about the eyepiece and start thinking about the object. That's what the Astro-Tech 20mm 100° XWA is designed to do: give you an immersive, walk-around field of view at moderate power, with enough exit pupil to keep the background dark and the nebulosity bright.
Where It Works Best
The 20mm XWA is a wide-field, moderate-power eyepiece. In a typical f/5 Newtonian (1250mm focal length), it delivers 63x with a 1.6° true field — wide enough to frame the entire Veil Nebula complex. In an f/4.5 Dobsonian (1800mm focal length on a 16-incher), it gives 90x and still holds a degree of true field, enough to take in large globular clusters like M13 with the full halo resolved. In a short-focal-length APO refractor — say, the AT86EDQ at 500mm — it runs 25x with a massive 4° true field: useful for scanning the Milky Way or framing large open clusters like the Double Cluster in their full extent.
The nine-element optical design is corrected for telescopes as fast as f/4. In an independent Cloudy Nights review, astronomer and author Alan Dyer tested the 20mm XWA head-to-head against a TeleVue 17mm Ethos in an 8-inch f/6 Newtonian, a 94mm f/5.5 APO, and a 105mm f/6 Astro-Physics refractor. His finding: stars remained pinpoint across the central 75% of the field, softening only slightly in the outer 25% — only marginally behind the Ethos. No ghost images, minimal pincushion distortion, and no kidney-bean blackout. At f/5 and slower, the field is sharp essentially to the edge. The 20mm also weighed in lighter than all competing 20mm 100° eyepieces — a meaningful advantage on Dobsonians where balance matters.
2" Barrel Only
The 20mm XWA fits 2" focusers only. There is no 1.25" adapter option. The field stop diameter on a 20mm, 100° eyepiece is physically larger than a 1.25" barrel can accommodate — this is why the smaller XWAs (13mm, 9mm, 7mm, 4.8mm) come with a removable 2" collar for dual-barrel use, but the 20mm cannot. If your scope only accepts 1.25" eyepieces, this eyepiece won't work. A safety groove is machined into the chrome barrel to engage your focuser's thumbscrew, preventing the eyepiece from falling if the thumbscrew loosens.
Eye Relief and Eyeglass Use
Eye relief is 15mm — enough for comfortable viewing without eyeglasses. With glasses on, you'll lose some of the outer field, but that matters less than it sounds: the 100° apparent field is so wide that you can't take it all in at once anyway. You naturally move your eye around to explore the edges, and the soft rolldown eyecup lets you position comfortably whether you wear glasses or not.
Waterproof
The eyepiece is O-ring sealed and waterproof. The practical benefit for astronomy isn't about rain — it's about preventing internal fogging when you bring a cold eyepiece indoors, and keeping dust and fine grit out of the optical assembly in windy or dusty conditions. It also means the optics stay clean longer, because the sealed barrel keeps contaminants from reaching the interior elements.
What's Included
- Astro-Tech 20mm 100° XWA 2" eyepiece
- Lens caps (top and bottom)
Features
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100° apparent field of view — Nearly 50% more area than an 82° eyepiece at the same focal length. The wider field gives you a panoramic, immersive view of deep-sky objects without sacrificing magnification.
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9-element fully multi-coated optics — Optimized for high contrast and edge correction at focal ratios down to f/4. Blackened lens edges reduce internal scatter.
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20mm focal length — A versatile moderate-power, wide-field focal length. Pairs well with Dobsonians (60–100x), refractors (25–50x), and SCTs (100–150x) depending on focal length.
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15mm eye relief — Comfortable for extended observing sessions. Usable with eyeglasses, though the outer field may vignette slightly with glasses on.
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Waterproof, O-ring sealed — Prevents internal fogging at cold temperatures and keeps dust out of the optical assembly. Sealed construction means fewer cleanings over the life of the eyepiece.
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Soft rolldown eyecup — Shields against ambient light from nearby security lights or neighbors. Rolls down flat for eyeglass wearers.
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2" chrome barrel with safety groove — The safety groove engages your focuser's thumbscrew to prevent the eyepiece from falling if the thumbscrew loosens during observing.
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24 ounces — This is a substantial eyepiece. It will shift the balance point on lighter scopes — check your balance before letting go of the tube.
Under the Night Sky
The 20mm XWA is a deep-sky eyepiece, full stop. In a 10-inch f/5 Dob, it gives you 64x and a 1.6° true field — enough to frame the entire Orion Nebula complex including the Running Man (NGC 1977) in the same view. The Pleiades sit inside a single field with room around the edges. The Double Cluster in Perseus shows both clusters simultaneously, fully resolved, with a dark lane running between them.
Step up to a 12-inch f/5, and you're at 76x with the same generous true field. M42 shows the wings extending well beyond the Trapezium, with the nebulosity rolling off into the background. Large globular clusters — M13, M22, Omega Centauri from southern latitudes — start resolving individual stars across the face while the full halo stays within the field.
In a short-focal-length refractor like the AT66EDQ (390mm), the 20mm runs just 20x with a 5° true field — essentially a rich-field scanning eyepiece. In his CN review, Dyer tested it in a 94mm f/5.5 refractor at 26x, where it delivered a 3.9° true field that he called "great for sweeping along the Milky Way." Point it at the Cygnus Milky Way and pan slowly. The North America Nebula, the Pelican, the Veil — they drift through the field one after another.
Where the 100° field really earns its keep is on untracked Dobsonians. At 60–80x, objects stay in the field roughly 50% longer than they would in an 82° eyepiece at the same magnification. That's 50% more time observing before you have to nudge the scope. For sketchers and patient observers, that extra dwell time adds up over a session.
Community Says
"I can highly recommend the 20mm XWA." This reviewer tested the 20mm head-to-head against a TeleVue 17mm Ethos in three telescopes — an 8-inch f/6 Newtonian, a 94mm f/5.5 APO, and a 105mm f/6 Astro-Physics refractor. Stars remained pinpoint across the central 75% of the field, softening only slightly in the outer 25%. No ghost images, minimal pincushion distortion, and no kidney-bean blackout. The 20mm also weighed in lighter than all competing 20mm 100° eyepieces. — Cloudy Nights Astro Gear Today review.
This owner runs both the 28mm 82° UWA and the 20mm XWA and recommends having each for different situations — the 28mm for maximum true field and lowest power, the 20mm for immersive wide-angle views with better contrast against faint nebulosity. — Cloudy Nights AT 20mm XWA discussion.
Observing Tip
At 24 ounces, this eyepiece weighs more than some finder scopes. If you're using a lightweight refractor on an alt-az mount — or a tabletop Dob — the scope will try to nose-dive when you drop the 20mm XWA in. Before you let go of the tube, slide the scope slightly back in the saddle or add a counterweight to the rear. On a Dobsonian, check your altitude tension. Once you've balanced for the 20mm, mark the position — you'll be swapping it in and out all night.
FAQ
Will this fit a 1.25" focuser?
No. The 20mm XWA is 2" barrel only. The field stop is too large to fit inside a 1.25" barrel. If your scope only accepts 1.25" eyepieces, look at the shorter-focal-length XWAs (13mm, 9mm, 7mm, 4.8mm), which come with a removable 2" collar and fit both barrel sizes.
How does this compare to the Astro-Tech 28mm 82° UWA?
The 28mm 82° UWA gives a wider true field (longer focal length means lower power and more sky), but the 20mm 100° XWA has a wider apparent field — the view feels more immersive. The 20mm also gives you higher magnification and a smaller exit pupil, which darkens the sky background and improves contrast on nebulae. Experienced CN members who own both recommend having each for different situations: the 28mm for maximum true field and the 20mm for the immersive wide-angle experience with better contrast. They serve different roles in the same eyepiece case.
Is the eye relief enough for eyeglasses?
At 15mm, you can use it with glasses, but you'll lose some of the outer field. Since the 100° field is too wide to see all at once anyway — you naturally look around to explore the edges — the vignetting is less of a practical issue than it would be with a narrower eyepiece. Most eyeglass wearers find it comfortable with the eyecup rolled down.
Can I use a filter with this eyepiece?
Yes. Standard 2" filters thread onto the barrel. This is a common pairing — an O-III or UHC filter on the 20mm XWA makes a formidable combination for large emission nebulae like the Veil, the North America, and the Rosette.
Final Thoughts
The 20mm XWA fills a specific role: it's the wide-field, moderate-power eyepiece you reach for when the target is big, faint, or both. In a Dobsonian, it's the eyepiece that turns the scope into a deep-sky panorama machine — the Veil complex in one field, the Double Cluster in one glance, the Orion Nebula with room to breathe. It's not light, it's not small, and it only fits 2" focusers. But if you have a scope that can handle it, the 100° field changes how you observe extended objects. You stop chasing the edge of the field and start just looking.
