The 17mm TeleVue Ethos 100° field eyepiece is designed for no-holds-barred medium power deep space use with telescopes having focal ratios down to f/4. It is exceptionally effective on nebulas with fast f/ratio reflectors, such as Dobsonians, giving both an immense field and a bright exit pupil (nearly 4mm with the typical f/4.5 Dob). 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 new 17mm TeleVue Ethos is marvelous for open clusters, nebulas, galaxy clusters, and globular clusters with any telescope.
The 100° apparent field of this new 17mm Ethos eyepiece is nearly 50% larger in area than the 82° field of a "space-walking" 17mm Nagler. You could get a similar true field with a longer focal length (but narrower apparent field) eyepiece, but the 17mm TeleVue Ethos has the benefits of the higher power and darker sky backgrounds that are inherent in its shorter focal length.
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 a usable eye relief of 15mm, the 17mm Ethos 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 17mm Ethos has all the virtues that professional and amateur astronomers have come to expect from a TeleVue eyepiece: high contrast, comfortable eye relief, and full field sharpness - with well-controlled astigmatism, field curvature, lateral color, angular magnification distortion correction, and low pupil sensitivity (kidney bean effect) for daytime use. While sharpness is inherent to the optical design itself, the contrast is maximized through the intelligent use of flat finished baffles and ultra low reflectance, high efficiency coatings that are tuned to the glass composition of each lens element.
The 17mm Ethos fits 2" focusers only. A safety groove is machined into the 2" chrome barrel to engage your focuser thumbscrew. This will help prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.
The 17mm TeleVue Ethos eyepiece will accept TeleVue Dioptrx eyesight astigmatism correcting lenses.
Please note the weight of this eyepiece if you have a Dobsonian telescope. Its use may require some temporary rebalancing of your scope, or the use of a TeleVue #TVEQC equalizing adapter with your 1.25" eyepieces to avoid balance problems.
