| This medium focal length Astro-Tech 12.5mm Long Eye Relief eyepiece is usable with any telescope type, under virtually any seeing conditions – from fair to superb. (Rain, snow, or fog might limit its usefulness, however.) It has a long 20mm eye relief that provides comfortable full field viewing for all observers, including eyeglass wearers. Its very flat field keeps small details sharply in focus from one edge of its 55° field of view to the other, making it an appropriate choice for resolving star clusters (both globular clusters and the more compact open clusters) and for scanning the lunar surface.
It is also exceptional for low to medium power viewing with short focal length refractors and fast focal ratio Dobsonian reflectors. With short focal length refractors, such as those from Astro-Tech, it’s into the low power range. With the 72mm aperture 430mm focal length AT72ED, for example, it provides 34x and a field over three times as wide as the Moon. This is well suited for observing open clusters such as the Pleiades and large nebulas such as the Lagoon. Both will fit nicely into the AT72ED’s 1.58° field, with a framework of black sky around them to set them off.
The 12.5mm Astro-Tech Long Eye Relief is also excellent for medium high magnification observing with long focal length refractors, reflectors, or catadioptric scopes. On an 8" f/10 catadioptric, for example, it yields 160x. This is an excellent magnification for planetary and close-up lunar observing. Its performance makes it a much better choice as a second eyepiece than merely doubling the power of most scopes’ standard 25-26mm eyepiece with a Barlow.
The Astro-Tech’s very high contrast makes it an exceptional choice for resolving faint low contrast lunar details and for seeing subtle detail in planetary nebulas, while its sharpness across the field makes globular clusters vivid and nearly three-dimensional.
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