Tele Vue Delos 8mm 72° Eyepiece - 1.25"

A 72° Field with 20mm of Constant Eye Relief

The Delos 8mm is defined by two key specifications: a 72° apparent field of view and 20mm of eye relief. Al Nagler's guide puts Plössls at 50°, so the 72° Delos shows a wider view at the same magnification.

Each Delos in the 3.5mm to 17.3mm instruction sheet has 20mm of eye relief (the 24mm has 22mm). The Delos is designed for optimum viewing whether or not you wear eyeglasses, and the eyeguard housing in the down position gives eyeglass wearers the full eye relief.

Framing the Sky with the 9.9mm Field Stop

An eyepiece's true field of view, how much sky it actually shows, is determined by its field stop diameter and the focal length of the telescope. The Delos 8mm has a 9.9mm field stop. You can calculate the true field in degrees with a simple formula: True Field = (Eyepiece Field Stop ÷ Telescope Focal Length) × 57.3.

For example, in a telescope with a 1000mm focal length, the Delos 8mm yields a true field of (9.9 / 1000) × 57.3 = 0.57°. That holds the small Ring Nebula (M57) with room around it, or the bright centre of the Hercules Cluster (M13). Tele Vue's chart gives exit pupils of 1.8, 1.5, 1.1, 0.8 and 0.6mm at f/4.5, f/5.2, f/7, f/10 and f/14.

The Adjustable Locking Eyeguard

To make full use of its long 20mm eye relief, the Delos 8mm has an adjustable, locking eyeguard housing in two pieces. An eye too far in or too far out causes edge darkening and field loss. Twisting the lower grip ring to the left loosens the housing so it slides; twisting it to the right locks it.

Without glasses, extend the eyeguard fully, then slide it down while looking through the eyepiece until you just see the full field with your peripheral vision. Green indicator lines on the inner barrel record your height. The system also minimizes extraneous outside light, for the highest contrast on faint objects. Eyeglass wearers use it in the down position.

Sharpness and Linearity at Any f-Ratio

Tele Vue's Ethos design experience made it possible to keep Delos size and weight down while achieving sharpness, linearity (freedom from pincushion distortion) and ghost suppression over the entire field for any speed (f/#) telescope.

A fast Newtonian also has coma of its own, which no eyepiece removes. The Paracorr Type-2 eliminates it, with the greatest results in scopes faster than f/5.0.

Higher Powers with Barlows, Powermates, and Paracorr

With a Tele Vue 2x Barlow or 2x Powermate, the Delos 8mm acts like a 4mm eyepiece, for nights of steady seeing on Jupiter or Saturn.

In a Newtonian or Dobsonian, the Paracorr Type-2 also has a 1.15x magnification factor, so the 8mm Delos acts like a 7mm eyepiece.

Delos 8mm vs. Ethos & Nagler: Building an Eyepiece Set

Comparisons from Tele Vue's chart:

• vs. Ethos 8mm: The Ethos has a 100° apparent field and a 13.9mm field stop, with 15mm of eye relief. The Delos 8mm has 20mm of eye relief and a 9.9mm field stop.
• vs. Nagler 7mm: The 7mm Nagler Type 6 has an 82° apparent field, a 9.7mm field stop and 12mm of eye relief, and needs an adapter for Dioptrx (Y*). The Delos 8mm has 20mm of eye relief and is Dioptrx ready (Y).

Al Nagler's guide notes that eyepieces with the same field stop show the same true field, so a set should step through different field stops. The Delos 8mm (9.9mm) sits between the 17.3mm Delos (21.2mm) and the 4.5mm Delos (5.6mm).