Tele Vue Nagler T7 9mm 82° Eyepiece - 1.25"/2"

The 82° Nagler "Spacewalk" Field

The original 13mm Nagler was the first eyepiece to give well-corrected wide "spacewalk" views with an 82° apparent field. Plössls give 50°; at the same magnification an 82° field covers about 2.7 times the area of a 50° field.

19mm Eye Relief: Designed for Eyeglass Wearers

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. The Type 7 is also ideal for DIOPTRX, which locks onto the top of the eyepiece; the chart marks it Dioptrx ready (Y).

To ensure perfect eye positioning for observers with or without glasses, the Nagler T7 9mm includes a locking, adjustable-height eyeguard. 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. It keeps you from getting too close to the eyelens and blocks contrast-robbing stray light. The 19mm of eye relief also minimizes eye lens fogging in cold weather.

Dual 1.25"/2" Barrel and System Parfocality

The 9mm has a removable 2" adapter, so it fits 2" or 1¼" focusers and diagonals. It is designed as a 1¼" eyepiece, so there is no optical benefit to the 2" barrel; the adapter is there for convenience and is threaded for 48mm filters (M48x0.75). Tighten the 2" barrel only until snug: over-tightening loosens the 1¼" barrel. 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 9mm is parfocal with the other Type 7s and most Tele Vue 1¼" eyepieces.

Optimizing Fast Newtonians with the 1.15x 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, and scopes faster than f/5.0 see the greatest results. Its 1.15x magnification factor makes the 9mm act like a 7.8mm.

Calculating True Field and Exit Pupil for Your Telescope

The true field of view (TFoV) you see is determined by the eyepiece's 13mm field stop and your telescope's focal length. For an 8" f/6 Dobsonian (1200mm focal length), the true field is approximately 0.62°, room for the Hercules Cluster (M13) or the small Ring Nebula (M57). The formula is: TFoV = (13mm / Telescope Focal Length in mm) × 57.3.

The exit pupil, which determines image brightness, is calculated by dividing the 9mm focal length by your telescope's f-ratio. Tele Vue's chart gives 2.0, 1.7, 1.3, 0.9 and 0.6mm at f/4.5, f/5.2, f/7, f/10 and f/14:

• Fast Dobsonian (f/4.5): 2.0mm exit pupil, for galaxies and nebulae.
• Apochromatic Refractor (f/7): 1.3mm exit pupil, for globular clusters and planetary nebulae.
• Schmidt-Cassegrain (f/10): 0.9mm exit pupil, for Jupiter and Saturn.

Nagler T7 9mm vs. T6 9mm: Choosing Your Eyepiece Set

Comparisons from Tele Vue's chart:

• Nagler Type 7 (19mm eye relief): 13.0mm field stop, Dioptrx ready (Y), 547 g; suits eyeglass wearers.
• Nagler Type 6 (12mm eye relief): 12.4mm field stop, Dioptrx with adapter (Y*), 190 g.

Choose low and medium power eyepieces in ratios of field stop diameter, such as 1.4 or 2.0. The 9mm's 13.0mm sits between the T7 14mm (20.5mm) and the T7 5.5mm (8.1mm).