This is an optically superb and exceptionally short focal length ultra wide field eyepiece. This latest premium 1.25” TeleVue Nagler Type 6 uses seven optical elements for optimized performance across a very wide field of view. Several different extra low dispersion glass types and specialized coatings are employed for improved contrast and longer eye relief than previous Nagler designs, even those acclaimed as being among the finest wide field eyepieces available. At a usable eye relief of 12mm, the 2.5mm Nagler would normally vignette the field of view for eyeglass wearers. However, since the 82 degree field is so wide that you can’t see the full field without having to move your head from side to side to take it all in anyway, vignetting in the conventional sense is not a problem.
The 2.5mm Nagler is designed for very high magnification observing with short focal length refractors and fast focal ratio/short focal length reflectors. While it can also be used with longer focal ratio Schmidt-Cassegrains and Maksutov-Cassegrains, its exit pupil becomes prohibitively small and the images very dim with these scopes. For example, using it with a 2000mm 8” f/10 Schmidt-Cassegrain will result in 800x (100x per inch of aperture) and a 0.25mm exit pupil (about 1/100th of an inch across). Both of these figures 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 for a reasonably experienced observer.
The optical performance is typical TeleVue – well corrected and sharp from edge to edge. There is a slight loss of contrast when compared to an eyepiece with fewer elements, due to the additional lenses needed to achieve the unique combination of a very short focal length and very wide field. The loss is minimal though, on a par with the contrast loss in an eyepiece/Barlow combination with the same effective focal length. In the eyes of many observers, however, the “picture window on space” observing experience of its exceptionally wide field more than makes up for any slight contrast loss.
Supplied plastic caps seal both ends of the eyepiece to keep it dust free. A rubber grip ring gives a secure and comfortable grip in all temperature extremes. A groove machined into the chrome barrel of the eyepiece acts as a safety stop to keep the eyepiece secure should the focuser thumbscrew loosen slightly while using the eyepiece.
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