This Nagler Type Five eyepiece is an excellent low power, wide field eyepiece that’s particularly well-matched to big Dobsonian scopes. While the longer focal length 31mm Nagler provides a huge field of view with an f/4.5 scope, it also has a very large 6.9mm exit pupil, larger than most observers’ eyes can dilate. That effectively wastes some of the scope’s light gathering capacity, as some of the full light cone falls on the observer’s iris, rather than entering his or her eye. This new 26mm Nagler, on the other hand, has a 5.8mm exit pupil with an f/4.5 scope. That’s an exit pupil more in line with the maximum dilated pupil of most observers’ eyes. With this 26mm, you get the full light-gathering your scope is capable of, while still keeping nearly the same “picture window on space” wide field of view as the 31mm Nagler.
The six-lens design uses multiple rare earth glass elements. It is primarily designed for no-holds-barred low power deep space use with scopes having focal ratios down to f/4. It is marvelous for open clusters, nebulas, and globular clusters with any scope, but is particularly effective with fast f/ratio reflectors. That being said, it must also be noted that its very wide field also makes it convenient for moderate power lunar and planetary observing with Dobsonians, as you have much longer to observe before the planet drifts out of the field and the scope has to be repositioned.
Please note the 26 ounce weight of this eyepiece if you have a Dobsonian telescope. It may pose a balance problem with some Dobsonians when switching to the 26mm from a much lighter shorter focal length 1.25” eyepiece. Its use may require some temporary rebalancing of your scope, or the use of a TeleVue #TVEQC equalizing adapter with 1.25” eyepieces to avoid balance problems.
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