The TeleVue 12mm Nagler Type 4 eyepiece is a premium-quality eyepiece that further refines the "spacewalk" observing experience of the original Nagler and Nagler Type 2 eyepieces that were the optical standard of the industry for years. The 12mm Type 4 replaces the 12mm Type 2, the original 13mm Nagler, and the original 11mm Nagler that is now considered a collector's item.
The 12mm Nagler Type 4 has longer eye relief, higher contrast, and reduced pincushion when compared to the original 12mm Nagler Type 2. This makes it exceptional for achieving a wide field of view at a medium power with Dobsonian reflectors.
The 12mm has concentric 1.25" and 2" barrels that allows it be used in both 1.25" and 2" focusers and star diagonals without the need to change diagonal or eyepiece adapter. In fact, be very sure not to unscrew the central 1.25" barrel when using this eyepiece, as the lenses will fall out, making a lovely –- but very expensive – tinkling sound as they hit the ground and shatter. All the light enters through the 1.25" barrel, even when used in a 2" focuser or star diagonal, so do not unscrew the 1.25" barrel!
The 12mm has TeleVue's unique "Instadjust" click-stop adjustable-height eyeguard, with a soft fold-down rubber eyecup. With the eyeguard retracted all the way, all of the generous 17mm eye relief is available for eyeglass use. The eyeguard can be raised in small and repeatable increments to position the eye of the non-eyeglass wearer the precise distance from the eyepiece to see the full field of view with maximum suppression of ambient stray light. A special doughnut-shaped removable plastic insert fits into the eyeguard to center the eye for those who have trouble positioning their eye precisely on a small exit pupil.
It is primarily designed for no-holds-barred medium 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 82 degree field also make 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 weight of this eyepiece if you have a Dobsonian telescope. 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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