Tele Vue Nagler T6 7mm 82° Eyepiece - 1.25"

The Nagler Type 6 7mm

The Nagler T6 7mm has 7 elements in 4 groups. The original 13mm Nagler was the first eyepiece to give well-corrected wide "spacewalk" views with an 82° apparent field.

Tele Vue does not publish glass types, coatings or correction details for this model.

An 82° "Spacewalk" Apparent Field of View

Al Nagler's guide gives Naglers an 82° apparent field of view (AFOV), against 50° for Plössls, 68° for Panoptics and 72° for the Delos.

At high power, this wide field allows you to study an object like Jupiter or Saturn for longer as it drifts through the field before you need to nudge the telescope. In a 1200mm telescope the 0.46° true field usually holds Jupiter and all four Galilean moons.

Calculating True Field with the 9.7mm Field Stop

While the AFOV describes the immersive experience, the true field of view (TFOV) tells you how much sky the eyepiece actually covers. This is determined by the eyepiece's 9.7mm field stop and your telescope's focal length. You can calculate the TFOV using the formula: TFOV = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3.

• Example with a 1200mm f/5.9 Dobsonian: (9.7mm / 1200mm) x 57.3 = 0.46° of true field. That holds the small Ring Nebula (M57) or the centre of the Hercules Cluster (M13).
• Example with a 600mm f/7.5 Refractor: (9.7mm / 600mm) x 57.3 = 0.92° of true field. That is 86x with nearly a degree of sky, about twice the Moon's width.

High-Power Performance: Exit Pupils from f/4.5 to f/14

The exit pupil, the beam of light leaving the eyepiece, sets image brightness. The exit pupil is calculated by dividing the eyepiece focal length (7mm) by the telescope's focal ratio.

Tele Vue's chart gives 1.6, 1.3, 1.0, 0.7 and 0.5mm at f/4.5, f/5.2, f/7, f/10 and f/14:

• f/4.5 (Fast Dobsonian): 1.6mm exit pupil, for brighter deep-sky objects.
• f/7 (Apochromatic Refractor): 1.0mm exit pupil, for the Moon and planets.
• f/10 (Schmidt-Cassegrain): 0.7mm exit pupil, for double stars and planets on steady nights.
• f/14 (Maksutov-Cassegrain): 0.5mm exit pupil, the smallest suggested in reflectors; it needs excellent seeing.

Ergonomics and Eyeglasses: The 12mm Eye Relief

The Nagler T6 7mm has 12mm of eye relief. This is the distance you must place your eye from the top lens to see the full field. Observers who wear glasses to correct astigmatism should look for 15mm to 20mm, so with glasses you may lose some of the field. With exit pupils of 1mm or less you probably will not need eyeglasses.

For observers with astigmatism, Tele Vue offers the Dioptrx corrector. The Nagler T6 7mm needs a DEA / DNA adapter for Dioptrx (Y* in the chart); the adapter converts its small eyeguard to a larger one so Dioptrx and FoneMate can be installed. 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.

Nagler T6 7mm Correction in Fast Newtonians

Newtonian and Dobsonian telescopes have coma, which makes stars toward the edge look like small comets. No eyepiece removes it.

Tele Vue's Paracorr Type-2 eliminates it; scopes faster than f/5.0 see the greatest results. It goes into your 2" focuser like a Barlow and has a 1.15x magnification factor, so the 7mm acts like a 6.1mm.

Reaching Higher Magnification with the Nagler T6 7mm

The 7mm focal length already provides high power in most telescopes, but you can increase it further using Tele Vue's image amplifiers. These suit nights of very steady seeing.

• With a 2x Barlow: The Nagler T6 7mm effectively becomes a 3.5mm eyepiece, doubling the magnification.
• With a 3x Barlow or Powermate: It acts like a 2.3mm eyepiece, a very high power that few nights support.

Nagler T6 7mm vs. Sibling Eyepieces

Comparisons from Tele Vue's chart:

• vs. Nagler T6 5mm: The 5mm Nagler gives 1.4x the power with a 7.0mm field stop and a 1.1mm exit pupil at f/4.5. The 7mm gives a 1.6mm exit pupil at f/4.5, a brighter image.
• vs. Nagler T6 9mm: The 9mm Nagler offers a wider true field of view thanks to its larger 12.4mm field stop. The 7mm gives 1.3x the magnification of the 9mm.
• vs. Delos 6mm: The Delos 6mm has a 72° apparent field, a 7.6mm field stop and 20mm of eye relief, and is Dioptrx ready (Y), which suits eyeglass wearers. The Nagler has the wider 82° field and the larger 9.7mm field stop.