This dual purpose viewing/imaging Celestron Ultima Duo 21mm 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 21mm is is a good low to medium power (depending on the focal length of your telescope) eyepiece for observing galaxy clusters, framing emission nebulas with the black of space, observing the nebulosity around the Pleiades, etc. It provides a big 4.66mm exit pupil with an f/4.5 Dobsonian reflector for rich field scanning of the Milky Way star clouds in Sagittarius from a dark sky site.
The 21mm Ultima Duo is an excellent choice to upgrade the optics of the inexpensive and narrower field 25mm or 26mm eyepiece that comes with many moderately-priced scopes, and to complement the 40mm eyepiece provided with some go-to Schmidt-Cassegrains.
With short focal length refractors, the 21mm Ultima Duo is a low power, very wide field eyepiece. With the 430mm focal length Astro-Tech AT72ED, for example, it gives you 20.5x and a huge 3.3° field that is nearly seven times as wide as the full Moon. This is well suited for observing open clusters such as the Beehive cluster and large nebulas such as the Lagoon. Even the 2° x 1.6° North American Nebula will fit nicely into its field when used with the Astro-Tech AT72ED.
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 21mm Celestron Ultima Duo eyepiece with your particular telescope.
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