This very short focal length Astro-Tech 3mm Long Eye Relief eyepiece is superb for comfortable high magnification lunar and planetary observing with short focal length refractors and fast focal ratio/short focal length reflectors. It has a very flat field that keeps small details sharply in focus from one edge of its 55° field of view to the other. The Astro-Tech’s very high contrast makes it an exceptional choice for splitting close binary stars and globular clusters and for picking out faint low contrast deep space objects, such as the smaller planetary nebulas.
The 3mm focal length of this Astro-Tech Long Eye Relief eyepiece is very probably too short for use with long focal length refractors, reflectors, or catadioptric scopes under anything less than absolutely perfect (and accordingly very, very rare) seeing conditions. For example, using it with a 2000mm 8" f/10 Schmidt-Cassegrain will result in 667x (83x per inch of aperture) and a 0.3mm exit pupil (about 1/75th of an inch across). Both of these far exceed the rule of thumb maximum values of 50-60x per inch of aperture and a 0.5mm exit pupil for usable observing under good to very good seeing conditions, even for a reasonably experienced observer.
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