The 100° apparent field of the Astro-Tech 20mm waterproof 2" eyepiece provides superb maximum field/medium power observing in all scopes, from small apo refractors to the largest Dobsonians and catadioptrics .
The 20mm Astro-Tech eyepiece, has high contrast, comfortable eye relief, and full field sharpness - with well-controlled astigmatism, field curvature, and lateral color. Its nine element optical system is designed for maximum field/medium power deep space use with telescopes having focal ratios down as low as f/4. It is exceptionally effective on nebulas with fast f/ratio reflectors, such as Dobsonians, as it gives both an immense field and a bright exit pupil (almost 4.5mm with the typical f/4.5 Dob). Its very wide field also makes it convenient for moderate power lunar and globular cluster 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 than you do with a 60° or 70° (or even 82°) field eyepiece. In addition to Dobs, the new 20mm Astro-Tech is marvelous for observing open clusters, nebulas, galaxy clusters, and globular clusters with any telescope capable of using a 2" eyepiece.
The 100° apparent field of this new 20mm 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 20mm Astro-Tech has the benefits of the higher power and darker sky backgrounds that are inherent in its shorter focal length.
The 20mm Astro-Tech 100° field 2" eyepiece is O-ring sealed and to be waterproof. While waterproofing might seem important only for observing underwater, waterproofing serves several useful purposes for the astronomical observer as well.
Internally sealing the eyepiece helps prevent internal fogging in the eyepiece when observing at sub-freezing temperatures. For observers in dusty climates, sealing keeps wind-blown dust and fine particulates from getting into the eyepiece body. For observers in very humid climates, sealing prevents the growth of fungus on the internal lens elements. For all observers, complete sealing maximizes the life of the internal optical coatings and makes the eyepiece 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 your eye from ambient light (from a neighbor's security light, for example) and improve the image contrast. With an eye relief of 15mm, the 20mm Astro-Tech would normally somewhat vignette the field of view for 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 20mm Astro-Tech fits 2" focusers only. A safety groove is machined into the 2" chrome barrel to engage your focuser thumbscrew. This helps prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.
