| The 13mm focal length of this Astro-Tech 70° Field AF Series dual-barrel eyepiece is designed for unsurpassed wide field observing with any telescope type, under virtually any seeing conditions – from fair to superb. It is truly excellent for medium magnification lunar, planetary, and globular observing with long focal length refractors, reflectors, or catadioptric scopes. Contrast is very good, rivaling that of orthoscopic and Brandon eyepiece designs, but with a substantially wider field of view than either. Its high contrast also makes it a good choice for resolving detail in planetary and emission nebulas, while its sharpness across the field makes globular clusters vivid and nearly three-dimensional.
An f/4.5 Dobsonian will yield a 2.9mm exit pupil, for bright images of nebulas and galaxies. With short focal length refractors, such as the Astro-Techs or TMBs, it’s a low to medium power eyepiece. With the 603mm focal length Astro-Tech AT90EDT triplet apo, for example, it provides 46x, a 1.94mm exit pupil, and a 1.5° field. With the 506mm focal length TMB-92 Signature Series triplet apo, it provides 39x, a 2.36mm exit pupil, and a 1.8° field. On both scopes, the fields are three times as wide as the full Moon or more. These figures are all very usable for observing open clusters such as the Pleiades, large nebulas such as the Lagoon, and large globulars such as M-13. All will fit nicely into the field of view, with a framework of black velvet around them to set off their subtleties.
With its dual barrel design, the 13mm Astro-Tech 70° AF eyepiece can be used in both 2" and 1.25" focusers and star diagonals with no adapters needed.
The 17mm eye relief of this 13mm Astro-Tech is ideal for those who must wear eyeglasses when 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 13mm Astro-Tech would be exceptional at any price. At its actual and very sensible price, it is truly outstanding. Well corrected for all aberrations, high in contrast, flat and sharp across its entire 70° field, it is an extraordinary value. 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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