This dual purpose viewing/imaging Celestron Ultima Duo 17mm 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.
The Ultima Duo 17mm is usable with any scope type, under virtually any seeing conditions – from fair to superb. (Rain, snow, or fog might limit its usefulness, however.) An f/4.5 Dobsonian will yield a 3.8mm exit pupil, putting it into the rich field observing range.
With short focal length refractors, the 17mm Ultima Duo is a low power, very wide field eyepiece. With the 430mm focal length Astro-Tech AT72ED, for example, it gives you 25x and a 2.49° field that is five times as wide as the full 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 its field when used with the Astro-Tech AT72ED, with a framework of black sky around the objects to set them off.
This 17mm Celestron Duo is excellent for medium magnification observing with long focal length refractors, reflectors, or catadioptric scopes. On an 8" Schmidt-Cassegrain, for example, it yields 118x and a 0.57° field. This is an excellent magnification for lunar observing, where the Moon will nearly fill the eyepiece field at this magnification.
Its full multicoatings provide high contrast that makes it a good choice for resolving lunar detail and planetary nebulas, while its edge-to-edge sharpness makes it well-suited for lunar, planetary, and globular cluster observing with an SCT. Some of the larger globulars, such as M13, will fill half of its 0.57° field with such a scope, with a nice framework of black sky around the object.
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 17mm Celestron Ultima Duo eyepiece with your particular telescope.