Tele Vue Nagler T5 31mm 82° Eyepiece - 2"

The Longest Nagler

Known affectionately among observers as "The Terminator," the 31mm Nagler Type 5 is the longest focal length Nagler in Tele Vue's chart, and its 42.0mm field stop is the largest of the 82° eyepieces. It is a low-power eyepiece for large objects and for using a telescope as a low-power finder; how much of a large object such as M31 or the Veil fits depends on your telescope's focal length (see below).

The eyepiece has 6 elements in 4 groups; the coatings are not published. For each field stop size, a shorter focal length with a larger apparent field shows more detail and fainter stars, which is the case for the 31mm Nagler against longer eyepieces with similar field stops.

The 42mm Field Stop and Maximizing True Field

An eyepiece's true field of view (TFoV), the actual patch of sky you see, is determined by its internal field stop diameter and the telescope's focal length. The 31mm Nagler features a massive 42mm field stop, close to the 46mm that Al Nagler's guide gives as the maximum for 2" eyepieces.

You can calculate the true field for your telescope using Tele Vue's formula: TFoV = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3. For example, in a telescope with a 2000mm focal length, the 31mm Nagler yields a true field of 1.2 degrees, more than twice the full Moon's width. The Pleiades (M45), about 2° across, need a shorter telescope: in a 1000mm telescope the field is 2.4°.

Exit Pupil from 6.9mm to 2.2mm: Performance by Telescope f/#

The exit pupil, calculated as Eyepiece Focal Length ÷ Telescope f/#, determines the brightness of the background sky and the visibility of faint details. Exit pupils are 6.9, 6.0, 4.4, 3.1 and 2.2mm at f/4.5, f/5.2, f/7, f/10 and f/14. Avoid exit pupils larger than 7mm in reflectors.

• At f/4.5 (Fast Dobsonians): The 6.9mm exit pupil is just inside that 7mm limit. If your eye's pupil does not open that wide, you lose some light and may see the secondary mirror's shadow.
• At f/7 (Apo Refractors): A 4.4mm exit pupil, for sweeping Milky Way star fields and large emission nebulae.
• At f/10 (SCTs/Maksutovs): A 3.1mm exit pupil, a darker sky background for galaxies and nebulae.

19mm of Eye Relief for Glasses and Dioptrx

The 31mm Nagler provides a generous 19mm of eye relief, the distance from the top lens to where your eye must be placed to see the full field. Eyeglass wearers with astigmatism do best with 15mm to 20mm of eye relief, to minimize field reduction.

The chart marks this eyepiece Dioptrx ready (Y). Dioptrx attaches and locks onto the top of the eyepiece and corrects astigmatism in place of your glasses; choose the model that matches your eyeglass prescription, from ¼ to 3½ diopter.

Managing the 2.2 lb Mass: Balance and Handling

At 2.2 lbs (1000 g), the 31mm Nagler can shift a telescope's balance. When switching from a lighter eyepiece, rebalance the tube with its counterweights or its position in the cradle.

Set your focuser's tension so the weight does not shift focus as the telescope moves.

Coma Correction in Fast Scopes with Paracorr Type-2

Newtonian and Dobsonian telescopes show coma, which makes off-center stars look like little comets, more so toward the edge of the field. No eyepiece removes it.

The Paracorr Type-2 eliminates it, with the greatest results in scopes faster than f/5.0. It goes into your 2" focuser like a 2" Barlow and has a 1.15x magnification factor, so the 31mm acts like a 27mm. Its tunable top positions each Tele Vue eyepiece for best performance.

Increasing Magnification with Barlows and Powermates

Al Nagler's guide suggests choosing a long focal length eyepiece with comfortable eye relief and using image amplifiers to increase power. Tele Vue makes Barlows and Powermates in 2x, 2.5x, 3x, 4x and 5x.

• With a 2x Powermate: It acts like a 15.5mm eyepiece, for open clusters and globular clusters such as M13.
• With a 3x Barlow: It acts like a 10.3mm eyepiece, for the Moon and Jupiter.

Nagler 31mm vs. Panoptic 41mm & Nagler 22mm T4

Comparisons from Tele Vue's chart:

• vs. 41mm Panoptic: The Panoptic's 46.0mm field stop is the 2" maximum, so it shows a little more sky, with 27mm of eye relief. The Nagler has the wider 82° apparent field and more magnification, which Al Nagler's guide says shows more detail and fainter stars.
• vs. 22mm Nagler Type 4: The 22mm Type 4 has the same 82° field and 19mm of eye relief and weighs 1.5 lbs. The 31mm's 42mm field stop (against 31.1mm) shows more sky, for use as a low-power finder eyepiece.

Build, Accessories, and Lifetime Warranty

The Nagler T5 31mm has 6 elements in 4 groups. Tele Vue's insert says the eyepiece accepts BandMate filters such as Nebustar, O-III and H-Beta, which pass select wavelengths and darken the sky so nebulae "pop out".

Tele Vue eyepieces carry a Lifetime Limited warranty. Store it with its dust caps in place, and do not unscrew the chrome barrel.