The very short 5mm focal length of this Astro-Tech 70° Field AF Series dual-barrel eyepiece is designed for unsurpassed 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.
It can also be used with longer focal ratio Schmidt-Cassegrains and Maksutov-Cassegrains. However, its exit pupil becomes small and the images dim with these scopes, so its use is generally limited to observing the Moon and planets and splitting close binary star pairs. However, it does provide about the maximum usable magnification attainable with a typical 8” f/10 catadioptric, where it provides a magnification of 400x (50x per inch of aperture) and a 0.5mm exit pupil. Both of these figures are right at the values generally accepted as the rule of thumb maximums for usable observing under good or better seeing conditions.
The 17mm eye relief of this 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 5mm Astro-Tech is exceptional at any price, but is truly outstanding at its very sensible price. It is well corrected, high in contrast, 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 costing twice its very reasonable price. It would be a welcome addition to any astronomer’s observing kit.
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