The Astro-Tech 9mm o-ring sealed 1.25"eyepiece combines a high power capability with a 100° apparent field to provide a jaw-dropping observing experience in your telescope at a price that is unreal.

The 9mm 100° field o-ring sealed Astro-Tech eyepiece is designed to provide a spectacular maximum field/medium to medium high power image with telescopes having focal ratios as fast as f/4. It is exceptionally effective on nebulas with fast f/ratio reflectors, such as Dobsonians, giving both an immense field and a good 2mm exit pupil with the typical f/4.5 Dob that is very effective at darkening the sky background so that faint stars and diffuse objects pop out more clearly.

Its very wide field also makes it convenient for moderate power lunar and planetary observing with Dobsonians, as you have much longer to observe before the planet drifts out of the field and the scope has to be repositioned. That said, it is worth noting that the 9mm Astro-Tech is marvelous for open clusters, nebulas, galaxy clusters, and globular clusters with any telescope.

The 100° apparent field of this 9mm Astro-Tech eyepiece is nearly 50% larger in area than an 82° field eyepiece. You could get a similar true field with a longer focal length (but narrower apparent field) conventional eyepiece, but the 9mm Astro-Tech has the benefits of the higher power and darker sky backgrounds that are inherent in its shorter focal length.  The eyepiece consists of nine elements in six groups.  The lens elements are fully multi-coated and have blackened lens edges to improve contrast and eliminate light scatter.

The 9mm Astro-Tech 100° field eyepiece is O-ring sealed against water. While waterproofing might seem important only for observing underwater, waterproofing serves five useful purposes for the astronomical observer as well.

One, internally sealing the eyepiece helps prevent internal fogging in the eyepiece when observing at very low temperatures. Two, for observers in dusty climates, it halts the intrusion of wind-blown dust and fine particulates into the eyepiece body. Three, for observers in very humid climates, it prevents the growth of fungus on the internal lens elements. Four, for all observers, it maximizes the life of the internal optical coatings. And finally, five, the waterproof 9mm Astro-Tech eyepiece is easier to clean, as there is no risk of cleaning solution migrating from the top of the lens and seeping around edges to be trapped in between elements. 

There is a soft rolldown eyecup to shield the eye from ambient light (from a neighbor’s security light, for example) and improve the image contrast. At an eye relief of 13mm, the 9mm Astro-Tech might vignette some eyeglass wearers. However, since the 100° field is so wide that you can't see the full field without having to move your head from side to side to take it all in anyway, vignetting in the conventional sense is not a problem.

The 9mm Astro-Tech has high contrast, comfortable eye relief, and full field sharpness – with well-controlled astigmatism, field curvature, and lateral color. The 9mm Astro-Tech fits 1.25" focusers or diagonals.  A safety groove is machined into the barrels to engage your focuser thumbscrew. This helps prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.