Astro-Tech 3.5mm 110° field waterproof 1.25"/2" eyepiece

The 3.5mm 110° field o-ring sealed Astro-Tech eyepiece is designed to provide a spectacular maximum field/high power image with telescopes having focal ratios as fast as f/4. 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. 

The 110° apparent field of this 3.5mm 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 3.5mm Astro-Tech has the benefits of the higher power and darker sky backgrounds that are inherent in its shorter focal length.  The lens elements are fully multi-coated and have blackened lens edges to improve contrast and eliminate light scatter.

The new 3.5mm Astro-Tech 110° 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 3.5mm 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.

The 3.5mm Astro-Tech has high contrast, comfortable eye relief, and full field sharpness – with well-controlled astigmatism, field curvature, and lateral color. The 3.5mm Astro-Tech fits 1.25" or 2" 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. The eyepiece comes with a 2" collar that is removable exposing a 1.25" barrel.