Tele Vue Nagler T6 3.5mm 82° Eyepiece - 1.25"

The 3.5mm Nagler T6: An 82° High-Magnification Instrument

The original 13mm Nagler was the first eyepiece to give well-corrected wide "spacewalk" views with an 82° apparent field. The 3.5mm Type 6 brings that field to very high power. It has 7 elements in 4 groups; no further details of its correction are published.

At high power a wide apparent field gives a drifting planet more time in view, which helps with a Dobsonian or other telescope without a drive. The eyepiece weighs 0.53 lbs (241 g) and stands 81.3mm (3.2 in) tall.

Field of View Geometry: The 4.8mm Field Stop

While the 82° apparent field of view defines the immersive quality of the view, the true field of view, the actual patch of sky you see, is determined by the eyepiece's 4.8mm field stop and your telescope's focal length. You can calculate the true field in degrees using Tele Vue's formula: True Field = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3.

For example, in a telescope with a 2000mm focal length, such as an 8" f/10 Schmidt-Cassegrain, the Nagler T6 3.5mm yields a true field of 0.14°. That holds Jupiter with any moons close to it, or the centre of the Hercules Cluster (M13); the outer moons often lie beyond it.

Magnification and Exit Pupil: Matching the 3.5mm to Your Telescope

The 3.5mm focal length produces very high magnification, making it dependent on excellent atmospheric seeing. In that same 2000mm focal length telescope it delivers 571x, more than most nights support. It suits shorter telescopes better: in a 480mm TV-76 it gives 137x, and in a 600mm TV-85 171x.

The chart gives 0.8, 0.7, 0.5, 0.4 and 0.3mm at f/4.5, f/5.2, f/7, f/10 and f/14. Avoid exit pupils smaller than 0.5mm in reflectors.

• At f/4.5: 0.8mm exit pupil, for globular clusters and planets in fast Dobsonians.
• At f/7: 0.5mm exit pupil, for planets and double stars in refractors.
• At f/10: 0.4mm exit pupil, smaller than suggested for reflectors such as SCTs.

Ergonomics in a 1.25" Barrel: 12mm Eye Relief and Dioptrx

The Nagler T6 3.5mm provides 12mm of eye relief. Observers who wear glasses to correct astigmatism should look for 15mm to 20mm, but with exit pupils of 1mm or less you probably will not need eyeglasses, which covers this eyepiece in most telescopes.

It needs a Tele Vue DEA / DNA adapter for DIOPTRX (Y* in the chart); the adapter converts its small eyeguard to a larger one. These eyepieces have shorter eye relief than others recommended for DIOPTRX, so they are best used with it for deep-sky viewing, where your eye pupil is large.

Paracorr, Barlows and Powermates

Newtonians show coma toward the edge of the field, which no eyepiece removes. The Tele Vue Paracorr Type-2 eliminates it; scopes faster than f/5.0 see the greatest results. It has a 1.15x magnification factor, so the 3.5mm acts like a 3.0mm.

A Barlow or Powermate raises the power further, but in most telescopes the 3.5mm alone already reaches what the air supports.

Nagler T6 3.5mm vs. Delos 3.5mm: A Tele Vue Comparison

Comparisons from Tele Vue's chart:

• Nagler T6 3.5mm: 82° apparent field, 4.8mm field stop, 12mm of eye relief, Dioptrx with adapter (Y*).
• Delos 3.5mm: 72° apparent field, 4.4mm field stop, 20mm of eye relief, Dioptrx ready (Y).

The Nagler shows a little more sky; the Delos leaves room for glasses.