TeleVue 17mm Nagler Type 4

Applying the experience of the Radian development to the 82° Nagler series, yielded more contrast, reduced pincushion, more true field, longer eye-relief, and fewer elements. With added eye-relief, the click-stop Instadjust eyeguard helps maintain proper eye distance and centration.
  

For deep sky viewing, the comfort, clarity and contrast gets us closer to our "spacewalk" viewing goal.
  

Why Are There 3 Types of Naglers?
  

To achieve the full magnification range over 82° apparent field of view requires two types: Nagler Type-5 with focal lengths of 31, 26, 20, 16mm and Nagler Type-6 with focal lengths of 13, 11, 9, 7, 5, 3.5, 2.5mm.
  

The Type-5 permits the largest true field possible (31mm has 42mm field stop), while the shorter focal length Type-6 models are parfocal, with similiar size and weight and all with a comfortable 12mm eye relief.
  

The Type-4 models feature about 18mm of eye relief and with the "instajust" eyeguard are best for eyeglass wearers or Dioptrx users in their 22, 17, 12mm focal length range.