Astro-Tech 21mm UWA 82 2″ Eyepiece

Description

Same optical family, different jobs. The 28mm is your widest-field, lowest-power eyepiece maximum true field, maximum exit pupil, the big-sweep view. The 21mm steps up the magnification and darkens the sky background for higher-contrast deep sky views. It’s almost ten ounces lighter than the 28mm and produces a flatter field edge-to-edge, likely because the optical design was refined after the rest of the UWA series. Think of the 28mm as your finder-chart view and the 21mm as your “now let’s look closer” view.

Is this eyepiece sharp at the edges in fast scopes?
In f/7 and slower scopes, the 21mm UWA is exceptionally sharp to the edge multiple Cloudy Nights reviewers have noted it’s the best edge performer in the AT UWA series. In f/5 to f/6 scopes, you may see some field curvature or slight softening at the extreme edge, which is normal for an 82 eyepiece at those speeds. In a fast Dob (f/4 to f/5), a coma corrector like a Paracorr helps, and users report results comparable to a Nagler T5 in that configuration.

What’s the true field of view in my scope?
Divide your telescope’s focal length into 21mm to get the magnification, then divide 82 by that magnification to get an approximate true field. For example: 1000mm focal length 48x 1.7 true field. A 1200mm SCT 57x 1.4. A 714mm f/7 refractor 34x 2.4. The manufacturer’s spec has been independently confirmed by drift tests.

Is this eyepiece parfocal with the other AT UWAs?
Yes. It’s designed to be parfocal across the AT UWA 82 series (28mm, 21mm, 16mm, 13mm, 10mm, 7mm, 4mm). You can swap eyepieces without refocusing, which keeps your target centered and speeds up your observing.

Accessories

• Astro-Tech 28mm UWA 82 2″ Eyepiece: The wide-field companion. Use the 28mm for maximum true field and the 21mm when you need more magnification and contrast. Together they cover the low-to-medium power range in any 2″ focuser.
• Astro-Tech 16mm UWA 82 1.25″ Eyepiece: Step up from the 21mm when you need tighter framing on smaller objects. Parfocal with the 21mm, so you swap without refocusing.
• Celestron LensPen Optics Cleaning Tool: The best way to clean an eyepiece lens in the field. Retractable brush on one end, carbon cleaning pad on the other.
• 2″ UHC or OIII Narrowband Filter: Thread onto the 21mm’s 2″ barrel for emission nebula work. The wide field and high contrast of the 21mm UWA are ideal partners for narrowband filtering.

Final Thoughts

The AT UWA 82 series has been one of the quiet success stories in affordable wide-field eyepieces solid performers that don’t pretend to be Naglers but deliver a lot more than their price would suggest. The 21mm is the newest addition to the line, and it may be the best one yet. The flat-field performance that Cloudy Nights reviewers keep pointing to sharp stars to the edge in an f/7 refractor, competitive with a Nagler T5 in a fast Dob suggests the optical design genuinely improved between the original series and this release. If you’re building an AT UWA set for deep sky work, this is the eyepiece that fills the gap between the 28mm’s big-sweep view and the 16mm’s tighter framing. If you just want one wide-field 2″ eyepiece that balances magnification, contrast, true field, and weight, the 21mm is a very strong place to start.

Tech Details: