Tele Vue Nagler T7 5.5mm 82° Eyepiece - 1.25"/2"

The 82° Spacewalk View with 19mm of Eye Relief

If you have been waiting for a Nagler series for eyeglass wearers, the Type 7 is it, combining the 82° field with 19mm of eyeglass-friendly eye relief and a large eyelens. Eyeglass wearers should look for 15mm to 20mm of eye relief.

The original 13mm Nagler was the first eyepiece to give well-corrected wide "spacewalk" views with an 82° apparent field. At high power a wide field also gives a drifting planet more time in view.

High-Magnification Performance: Calculating True Field and Exit Pupil

The 5.5mm focal length is squarely in high-power territory for most telescopes, for planets and close double stars. The actual magnification and true field of view (TFoV) depend on your telescope's focal length.

You can calculate the true field of view using Tele Vue's formula: TFoV = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3. For a telescope with a 2000mm focal length, the Nagler T7 5.5mm yields a true field of (8.1mm ÷ 2000mm) x 57.3 = 0.23 degrees. The exit pupil, or the beam of light exiting the eyepiece, is calculated as Eyepiece Focal Length ÷ Telescope f-ratio. Tele Vue's chart gives 1.2, 1.1, 0.8, 0.6 and 0.4mm at f/4.5, f/5.2, f/7, f/10 and f/14.

• Exit Pupil at f/4.5: 1.2mm
• Exit Pupil at f/7: 0.8mm
• Exit Pupil at f/10: 0.6mm
• Exit Pupil at f/14: 0.4mm

Ergonomics for Eyeglass Wearers: DIOPTRX and the Adjustable Eyeguard

The Type 7 is designed for optimum viewing whether or not you wear eyeglasses. The adjustable, locking eyeguard is for observers who don't.

Twist the lower grip ring to the left to loosen it, pull it fully out, then slide it down while looking through the eyepiece until you just see the full field with your peripheral vision; the green indicator lines record the height, and twisting to the right locks it. It keeps you from getting too close to the eyelens and blocks contrast-robbing stray light. The Type 7 is ideal for DIOPTRX, which locks onto the top of the eyepiece; the chart marks the 5.5mm Dioptrx ready (Y).

Mechanical Integration: Dual 1.25"/2" Barrels and Parfocality

The Nagler T7 5.5mm features a removable 2" adapter over its native 1.25" barrel, so it fits 1¼" or 2" focusers and diagonals. The 2" adapter is threaded for 48mm filters (M48x0.75). Tighten it only until snug: over-tightening loosens the 1¼" barrel. There is no optical benefit to using the 2" barrel; the eyepiece was designed as a 1¼" eyepiece and the adapter is included for convenience.

The Type 7s are parfocal with each other in 2" mode. In 1¼" mode, in the Tele Vue Hi-Hat 1¼" to 2" adapter (ASF-8125), the 5.5mm is parfocal with the other Type 7s and most Tele Vue 1¼" eyepieces.

Fast Telescope Correction: Optimizing for Newtonians with Paracorr Type-2

Newtonian and Dobsonian telescopes show coma, which makes off-center stars look like little comets; no eyepiece removes it.

The Tele Vue Paracorr Type-2 eliminates it; scopes faster than f/5.0 see the greatest results, and it greatly reduces the need to nudge the scope to keep an object in the "sweet spot." Its 1.15x magnification factor makes the 5.5mm act like a 4.8mm.

Nagler T7 5.5mm vs. Nagler T6 and Delos: A Choice of Comfort

Comparisons from Tele Vue's chart:

• Nagler Type 7 (5.5mm): 82°, 8.1mm field stop, 19mm of eye relief, Dioptrx ready (Y).
• Nagler Type 6 (5mm): 82°, 7.0mm field stop, 12mm of eye relief, Dioptrx with adapter (Y*), 1¼" barrel.
• Delos (6mm): 72°, 7.6mm field stop, 20mm of eye relief, Dioptrx ready (Y), 1¼" barrel.

Amplification Options: Pairing with Tele Vue Barlows and Powermates

Instead of buying a full set of short focal length eyepieces, you can use an image amplifier to increase the magnification of the Nagler T7 5.5mm. Tele Vue Barlows and Powermates multiply the eyepiece's power.

A 2x Powermate makes the 5.5mm act like a 2.75mm eyepiece, a power only short telescopes on steady nights can use.

Accessory Ecosystem: Filters, FoneMate, and Building a Set

The 2" adapter takes 48mm filters, such as Tele Vue's BandMate Nebustar or O-III. The Type 7 table marks the 5.5mm Y for bino use, DIOPTRX and FoneMate, Tele Vue's smartphone adapter for still, video or timelapse images.

Choose higher power eyepieces in ratios of magnification. From the T7 9mm, the 5.5mm gives 1.6x the power.