Used Astro-Tech 9mm 100° Field Waterproof 1.25"/2" Eyepiece

Nine millimeters is where the view starts to get serious. In a 10-inch f/5 Dobsonian, it gives you 139x — enough to split tight double stars, pull detail out of planetary nebulae, and show Jupiter's cloud belts with real structure. In an f/7 refractor, it runs 79x with a half-degree true field: tight enough for planets, wide enough for compact clusters. The challenge at this magnification is usually the eyepiece — most high-power eyepieces give you a narrow apparent field, so you're constantly bumping the Dob to keep the target in view. The 9mm XWA solves that with 100° of apparent field. Objects drift through slowly. You observe instead of chase.

The Optics

Nine elements, fully multi-coated with blackened lens edges. The design is corrected for telescopes as fast as f/4 — one Yotpo reviewer reports sharp, high-contrast views to the edge of the field in an f/4.9 Dobsonian. On Cloudy Nights, a detailed observing report from a 10-inch f/4.7 Dob user (same optical design, branded Sky Rover) describes the 9mm as delivering sharp images, excellent contrast, and saturated color rendition on everything from Venus to the Orion Nebula — calling M42 "simply stunning" and the Moon "my best ever view" through the eyepiece's 100° field. At f/5 and slower, the field is sharp essentially to the stop.

Eye Relief

Eye relief is 13mm — shorter than the 20mm and 13mm XWAs in this line, which have 15mm. For observers without glasses, 13mm is comfortable enough for extended sessions. For eyeglass wearers, it's tight. You can still use the eyepiece with glasses, but you'll lose more of the outer field than you would with the longer-focal-length XWAs. The soft rolldown eyecup helps — fold it down to get your glasses closer to the eye lens.

Dual 1.25"/2" Barrel

The eyepiece comes with a removable 2" collar. Take it off and you have a 1.25" barrel for smaller focusers, diagonals, and Barlows. Both barrels have a safety groove machined into the chrome to engage your focuser's thumbscrew. At 17 ounces, the eyepiece is lighter than the 20mm and 13mm XWAs — the shorter focal length means a smaller optical assembly — but it's still heavier than a typical Plossl or orthoscopic at this focal length.

Waterproof

O-ring sealed and waterproof. No internal fogging at freezing temperatures, no dust intrusion, no fungus on the interior elements. The sealed construction also makes exterior cleaning simpler — no risk of cleaning solution seeping past the lens edge into the interior.

What's Included

• Astro-Tech 9mm 100° XWA 1.25"/2" eyepiece
• Removable 2" collar (installed)
• Lens caps (top and bottom)

Features

• 100° apparent field of view — At high magnification, the wide field is a practical advantage, not just a luxury. Objects stay in view longer on untracked telescopes, and you see the target in context rather than isolated in a narrow tunnel.
• 9-element fully multi-coated optics — Blackened lens edges, designed for edge sharpness and high contrast at focal ratios down to f/4. Performs cleanly in fast Dobsonians.
• 9mm focal length — High-power territory. 139x in a 1250mm Dob, 111x in a 1000mm scope, 79x in a 714mm refractor. Useful for planets, planetary nebulae, globular cluster detail, double stars, and lunar close-ups.
• 13mm eye relief — Comfortable without glasses. Tight but usable with glasses — the rolldown eyecup helps.
• Dual 1.25"/2" barrel — Removable 2" collar. Safety groove on both barrels engages your focuser thumbscrew.
• Waterproof, O-ring sealed — Prevents internal fogging, dust intrusion, and fungus. Extends the life of the optics.
• 17 ounces — Lighter than the longer-focal-length XWAs in the line, but still a substantial eyepiece.

Under the Night Sky

In a 10-inch f/5 Dobsonian at 139x, Jupiter shows the two main equatorial belts with visible festoons and scalloping on good nights. The Great Red Spot shows color and shape when it transits. Saturn's rings are cleanly separated from the globe, Cassini's Division is a dark line on steady nights, and the shadow of the rings on the globe is visible. The 100° field means the planet drifts slowly across the view — you get a solid 45–60 seconds of observing before the planet reaches the edge, compared to maybe 30 seconds in an eyepiece with a 60° field.

Turn the scope to M57, the Ring Nebula, and the ring shape is obvious at this power. The central hole is clearly defined. With a 12-inch or larger scope, you can start to detect the slight color tint. M27, the Dumbbell Nebula, shows its apple-core shape with the fainter outer lobes becoming visible on transparent nights.

Globular clusters are where 139x really delivers. M13 breaks into individual stars well past the core. M3 shows its denser central condensation surrounded by resolved chains and arcs. In a 12-inch scope, the view is spectacular — hundreds of individual stars across the face of the cluster, with the 100° field holding the entire globular in context with the surrounding star field.

On the Moon, 139x is a comfortable power for touring the terminator. Craters show central peaks, terraced walls, and shadow detail. The major rilles — Vallis Schroteri, Hadley Rille — are visible on sharp nights. You can spend an entire session working along the terminator without the view feeling cramped.

Community Says

"Simply stunning." This owner tested the 9mm in a 10-inch f/4.7 Dobsonian at 133x and reported Venus sharp and bright, Betelgeuse showing saturated orange, and M42 filling the 100° field with nebulosity that he called his best Orion Nebula view. The Moon was "my best ever view" through this eyepiece. 

Observing Tip

The 9mm XWA makes an excellent Barlow companion. Drop it into a quality 2x Barlow and you effectively have a 4.5mm eyepiece with 100° apparent field — extreme magnification for planetary and lunar detail on nights of exceptional seeing. Use the 1.25" barrel (collar removed) in a 1.25" Barlow for the most secure connection. This gives you two high-power focal lengths from one eyepiece.

FAQ

Is 13mm eye relief enough for eyeglasses?
It's usable but tight. Most eyeglass wearers can see about 85–90% of the field with glasses on and the eyecup rolled down. If full-field eyeglass compatibility is critical, the 13mm or 20mm XWAs have 15mm eye relief and are more comfortable with glasses. If you observe without glasses (correcting astigmatism with the focuser), 13mm is not an issue.

How does this compare to the Astro-Tech 9mm 82° UWA?
The 82° UWA is lighter, less expensive, and has a narrower field. The 100° XWA has more glass (9 elements), a wider field, waterproof sealing, and better edge correction at fast focal ratios. If you observe with a fast Dob and want the widest possible field at high power, the XWA is the better choice. For a lighter, simpler eyepiece at a lower price, the 82° UWA is a solid alternative.

What magnification will I get?
Divide your telescope's focal length by 9. Examples: 1250mm Dob = 139x, 1000mm scope = 111x, 714mm refractor = 79x, 500mm short refractor = 56x. The 100° apparent field gives a true field of roughly 100° ÷ magnification.

Final Thoughts

The 9mm is the planetary and detail eyepiece in the XWA line. It delivers the magnification you need for Jupiter's cloud bands, Saturn's rings, and globular cluster resolution, wrapped in a 100° field that gives you real time to observe before the object drifts out. At 17 ounces it's lighter than the wider-field XWAs, and the dual barrel fits any focuser or diagonal in your collection. One owner called it a top-shelf performer in a fast Dob — sharp, contrasty, and easy to use. For the observer who wants high-power views without the narrow-field tunnel effect, this is the eyepiece that delivers.

Specifications