The very short 3.5mm focal length of this Astro-Tech 70° Field AF Series dual-barrel eyepiece is designed for superb high magnification wide field observing with short focal length refractors and fast focal ratio/short focal length reflectors. Its high contrast makes it an exceptional choice for splitting close binary stars and for seeing faint low contrast lunar and planetary details. With its dual barrel design, it can be used in both 2” and 1.25” focusers and star diagonals with no adapters needed.
While it can also be used with longer focal ratio Schmidt-Cassegrains and Maksutov-Cassegrains, the exit pupil of the 3.5mm Astro-Tech 70° Field AF is very small and the images very dim with these scopes. For example, using it with a 2000mm 8” f/10 catadioptric will result in 571x (71x per inch of aperture) and a 0.35mm exit pupil (about 1/90th of an inch across). Both of these figures exceed the maximum rule of thumb values for usable every-night observing of 50-60x per inch of aperture magnification and a 0.5mm exit pupil. The eyepiece would require exceptional seeing conditions to be usable with such scopes, even for an experienced observer.
The 17mm eye relief of this 3.5mm Astro-Tech is ideal for those who must wear eyeglasses while observing due to astigmatism. Vignetting will be minimal for eyeglass wearers, although the very wide field may require some moving of the head from side to side to see the entire field, even for those who don’t wear glasses.
This Astro-Tech comes with both 1.25” and 2” barrels, dust covers for both ends of the eyepiece, and a plastic bolt-type eyepiece container.
The optical performance of this 3.5mm Astro-Tech is exceptional at any price, and truly outstanding at its very sensible price – well corrected, contrasty, flat and sharp across its entire 70° field. We feel that this Astro-Tech 70° Field AF Series eyepiece equals or surpasses the performance of exotic designs twice its very reasonable cost and would be a welcome addition to your observing kit.
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