| This very short focal length (but physically long) Celestron X-Cel ED (extra-low dispersion) glass six-element eyepiece is best suited to high magnification viewing with short focal length refractors and fast focal ratio/short focal length reflectors. Its five-layer full multicoatings provide high contrast that makes it a good choice for splitting close binary stars and for seeing faint low contrast lunar and planetary details. It is generally too short a focal length 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 870x (109x per inch of aperture) and a 0.23mm exit pupil (about 1/100th 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 excellent seeing conditions, even for experienced observers. For eyeglass wearers, its long eye relief makes this parfocal eyepiece an obvious choice for comfortable very high power observing. A soft rubber eyecup shields the eye from ambient light and rolls down for more comfortable eyeglass use. Two knurled rubber rings on the barrel provides a secure grip when changing eyepieces in cold weather. At a price only about 30% the cost of other premium ED glass eyepieces of similar focal lengths, it's a bargain indeed. |
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