Tele Vue Nagler Zoom 3-6mm 50° Eyepiece - 1.25"

The Nagler Zoom Concept: Four Focal Lengths from 3mm to 6mm

It is for planetary, lunar and double-star observing, where the useful power changes with the seeing. One eyepiece covers what would otherwise take separate 3, 4, 5 and 6mm eyepieces.

The spec table lists click stops at 3, 4, 5 and 6mm, which let you return to a known power.

A Constant 50° Apparent Field

Tele Vue's chart gives the Nagler Zoom a 50° apparent field across the range, the same as Al Nagler's guide gives for Plössls; the field stop shrinks from 5.1mm at 6mm to 2.6mm at 3mm.

Tele Vue's material does not say whether focus holds through the zoom range.

Calculating True Field: From a 5.1mm to 2.6mm Field Stop

The true field of view (TFoV) you see in the sky is determined by the eyepiece's internal field stop and your telescope's focal length. The Nagler Zoom's field stop varies from 5.1mm at the 6mm setting to 2.6mm at the 3mm setting. Tele Vue's formula:

True Field of View (°) = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3

• Example: In a telescope with a 2000mm focal length, the Nagler Zoom at 6mm (5.1mm field stop) yields a true field of (5.1mm / 2000mm) * 57.3 = 0.15°. At 3mm (2.6mm field stop), it delivers a true field of (2.6mm / 2000mm) * 57.3 = 0.07°.

Exit Pupil & Magnification: Performance from f/4.5 to f/14

The exit pupil is the small beam of light that exits the eyepiece, and its size is critical for high-power viewing. At these focal lengths the exit pupil is small, which darkens the sky background. The exit pupil is calculated as: Eyepiece Focal Length ÷ Telescope f/#.

Tele Vue's chart gives:

• f/4.5 Telescope: Exit pupil ranges from 1.3mm (at 6mm) down to 0.7mm (at 3mm).
• f/7 Telescope: Exit pupil ranges from 0.9mm down to 0.4mm.
• f/10 Telescope: Exit pupil ranges from 0.6mm down to 0.3mm.
• f/14 Telescope: Exit pupil ranges from 0.4mm down to 0.2mm.

Ergonomics in Practice: 10mm Eye Relief and Eyeglass Use

The Tele Vue Nagler Zoom has 10mm of eye relief throughout its range, which works for most observers without glasses.

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 most uses of this zoom. The chart marks it not Dioptrx ready (N).

Correcting Fast Telescopes with Paracorr Type-2

Newtonian and Dobsonian telescopes show coma, which makes off-center stars look like little comets. Tele Vue's Paracorr Type-2 eliminates it; scopes faster than f/5.0 see the greatest results.

The Paracorr Type-2 also acts as a 1.15x magnification amplifier, slightly increasing your telescope's effective focal length and f-ratio. For example, a 1000mm f/4.5 scope becomes an 1150mm f/5.2 scope when using the Paracorr. With it, the zoom acts like a 5.2 to 2.6mm.

Reaching Higher Magnifications with Barlows & Powermates

For nights of exceptional seeing or for use with shorter focal length telescopes, the Nagler Zoom's range can be extended with Tele Vue's image amplifiers. A Barlow or Powermate multiplies the magnification of the eyepiece, effectively shifting its focal range.

• With a 2x Barlow: The Nagler 3-6mm Zoom becomes a 1.5-3mm zoom, doubling the magnification at every setting.
• With a 2.5x Powermate: The eyepiece becomes a 1.2-2.4mm zoom, a power few telescopes or nights support.

Nagler Zoom vs. Fixed Focal Lengths: The High-Power Lineup

Comparisons from Tele Vue's chart:

• vs. Nagler Type 6 (e.g., 5mm, 3.5mm): 82° apparent field, 12mm of eye relief, Dioptrx with adapter (Y*), one focal length each.
• vs. Delos (e.g., 6mm, 4.5mm, 3.5mm): 72° apparent field, 20mm of eye relief, Dioptrx ready (Y), one focal length each.

The zoom trades apparent field for one eyepiece that covers 3 to 6mm.