The Nagler eyepiece was revolutionary as the first eyepiece to deliver breath taking wide fields of view while still providing sharp images from edge to edge.
The eyepiece is an extremely important, but often over looked element in the image your telescope delivers.
In contrast to the objective of your telescope which rarely covers an image angle of more than 2°, the eyepiece has to remain sharp over an angle of more than 50°...or a staggering 82° in the case of the Nagler. For this reason extreme care must be taken to correct for aberrations - especially away from the optical axis.
Astigmatism and field curvature are 2 factors that most influence the off axis sharpness especially in "fast" objectives (below f/6). Eyepiece designers had tried for many years to eliminate these aberrations but it was not until the 1980s that a breakthrough occurred - the Nagler eyepiece.
Since then Al Nagler has refined this series of eyepieces taking advantage of different exotic glasses and improved coatings to deliver quite simply the best performance you can get out of an eyepiece. While other manufacturers may boast impressive field sizes, Televue's philosophy of "showing no field unless it is fine" stresses full field image quality as the ingredient for impressive views.
Review
The TeleVue 22mm Nagler Type 4 is another of the newer improved Naglers.
The 19mm of eye relief as well as TeleVue's "Instadjust" click-stop adjustable-height eyeguard with a soft fold-down rubber eyecup allows for eyeglass wearers to experience a full 82° apparent field of view when fully retracted. The click stop increments of "Instadjust" allows non eyeglass wearer to find the exact height for full field of view images. A small plastic insert that fits in the eyeguard allows non eyeglass wearers to find the correct position of the exit pupil.
In big, fast Dobs this is an amazing medium-low power eyepiece great for open clusters, galaxy clusters and emission nebulae. Eta Carinae is totally awesome in this eyepiece.
In SCT's it offers 2.2mm exit pupil making images very bright and this is ideal for open and globular clusters. Have a look at the Tarantula Nebulae to be convinced how good this eyepiece is.
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.