This dual purpose viewing/imaging Celestron Ultima Duo 5mm eyepiece uses a new eight element/five group fully-multicoated optical system to provide a wide 68° field of view for visual use. Integral 42mm T-threads under the removable roll-down rubber eyecup allow solar system imaging use with a SmartPhone, DSLR, or CCD camera.
It Ultima Duo 5mm gives you good medium to medium high magnification views with short focal length refractors and fast focal ratio/short focal length reflectors. It also provides about the maximum usable magnification for a typical 8” f/10 Schmidt-Cassegrain. Its full multicoatings provide high contrast that makes it a good choice for splitting close binary stars, for seeing faint low contrast lunar and planetary details, and for ferreting out small planetary nebulas.
It generally will be useful with long focal length refractors, reflectors, or catadioptric scopes only under very good or better seeing conditions. Average seeing conditions would limit its usefulness with these scopes. Using it with a 2000mm 8” f/10 Schmidt-Cassegrain, for example, will result in 400x (50x per inch of aperture) and a 0.5mm exit pupil (about 1/50th of an inch across). Both of these are right at the rule of thumb maximum values of magnification per inch of aperture and exit pupil size for usable observing under good to very good seeing conditions for a reasonably experienced observer.
Click on the "Telescope Eyepiece Formula" link in the "Formulas" tab above for a calculator that will show you the magnification and approximate actual field of view of this 5mm Celestron Ultima Duo eyepiece with your particular telescope.
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