This is an optically superb short focal length ultra wide field eyepiece designed as an upgrade to the famed original 4.8mm Nagler. Due to the significant cost increase of this 5mm eyepiece over the original 4.8mm Nagler design, the original 4.8mm Nagler will continue to be available for those who wish a lower cost alternative to competitive designs that are based on the original Nagler optical formulation.
This premium 1.25" TeleVue eyepiece 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 5mm Nagler would normally vignette the field of view for eyeglass wearers. However, since the 82° 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 5mm Nagler is designed for high magnification observing with short focal length refractors and fast focal ratio/short focal length reflectors. It can also be used with longer focal ratio Schmidt-Cassegrains and Maksutov-Cassegrains, but its exit pupil becomes small and the images dim with these scopes, so its use is generally limited to observing the moon and planets and splitting close binary star pairs.
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 very short focal length and very wide field of this Nagler. 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.
