TeleVue Ethos 8mm 100° Eyepiece - 1.25"/2"

The 100° Ethos Spacewalk Experience

The Ethos line has a 100° apparent field; only the 110° Ethos-SX is wider in Tele Vue's range (Plössl 50°, Panoptic 68°, Delos 72°, Nagler 82°). The 8mm brings that field to high power, so you can examine objects in detail with plenty of sky around them.

This combination of high magnification and an ultra-wide field helps on non-driven mounts like Dobsonians, as it allows targets to drift for a longer time before needing repositioning. For observers under light-polluted skies, the small exit pupil produced by the 8mm focal length darkens the sky background, increasing the contrast on faint deep-sky objects.

Calculating True Field from the 13.9mm Field Stop

While the 100° apparent field describes the immersive feel, the true field of view (how much sky the eyepiece actually shows) is determined by its 13.9mm field stop diameter and your telescope's focal length. You can calculate the true field using Tele Vue's formula:

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

For example, in a Schmidt-Cassegrain telescope with a 2032mm focal length, the Ethos 8mm yields a true field of 0.39 degrees. This is wide enough to frame the entire Ring Nebula (M57) or the core of the Hercules Cluster (M13) at a powerful 254x magnification, showing the object in its immediate context.

Magnification and Exit Pupil from f/4.5 to f/14

In a fast f/4.5 Dobsonian, the 8mm produces a 1.8mm exit pupil, suited to globular clusters and planetary nebulae. In a long f/14 Maksutov-Cassegrain, it delivers a 0.6mm exit pupil for splitting tight double stars and examining fine planetary detail.

The exit pupil, calculated by dividing the eyepiece focal length by the telescope's f-ratio, determines image brightness and how well the eyepiece matches your eye. Tele Vue's chart gives these exit pupils for the Ethos 8mm:

• f/4.5 Telescope: 1.8mm Exit Pupil
• f/5.2 Telescope: 1.5mm Exit Pupil
• f/7.0 Telescope: 1.1mm Exit Pupil
• f/10.0 Telescope: 0.8mm Exit Pupil
• f/14.0 Telescope: 0.6mm Exit Pupil

15mm Eye Relief and Dioptrx Compatibility

The Ethos 8mm features 15mm of eye relief, the same as every Ethos and Ethos-SX in Tele Vue's chart. Al Nagler's guidance: if you do not need glasses for astigmatism, observe without them; with small exit pupils of about 1mm or less you probably will not need eyeglasses at all.

For those who require astigmatism correction, the chart marks the Ethos 8mm Dioptrx ready: you can add a Dioptrx astigmatism corrector to the Ethos instead of using eyeglasses, to get the sharpest view possible.

The Dual 1¼"/2" Barrel

The Ethos 8mm has a dual barrel, allowing it to be used in both 1.25" and 2" focusers and diagonals. It is designed as a 1¼" eyepiece, and the 1¼" barrel is threaded for 1¼" filters. The 2" barrel section lets it to be used in larger focusers without an adapter.

• No Optical Benefit in 2" Mode: The 2" barrel is for convenience only and does not provide a wider true field of view.
• Secure Mounting: In a 1¼" holder, rotate the eyepiece so the holder's lock screw slips into the slot in the 2" section and clamps the 1¼" barrel; the eyepiece's own lock screw can clamp the outside of the holder. The eyepiece is heavy and can fall out of slip-fit holders unless the lock screw is used.
• Filter Use: While it accepts 1.25" filters directly, you can use 2" (48mm) filters by attaching the optional Tele Vue EBX-2120 2" extension tube.
• In-Travel Caveat: The design may have insufficient in-travel to reach focus in some refractors when used with a Tele Vue 60° Everbrite diagonal, which has a long optical path.

Parfocal Design with the 6mm Ethos

The TeleVue Ethos 8mm eyepiece is designed to be parfocal with the 6mm Ethos. This means you can swap between these two high-power eyepieces with little to no refocusing required.

When used in a 2" holder, the 8mm Ethos will typically require you to move the focuser "out" by about 3/4" compared to a 26mm Nagler Type 5. When used in a 1.25" holder with Tele Vue's "high-hat" adapter, it is parfocal with Tele Vue Plössls, Type 6 Naglers, and 1.25" Panoptics.

Pairing the Ethos 8mm with Paracorr Type-2

For owners of fast Newtonian and Dobsonian telescopes (f/5.5 or faster), mirror coma makes off-centre stars look like little comets across the Ethos's 100° view. The Tele Vue Paracorr Type-2 coma corrector is recommended by Tele Vue to eliminate it, for the same kind of full-field sharpness its NP-Series refractors show.

The Paracorr inserts into your 2" focuser and has a 1.15x magnification factor, like a mild 2" Barlow. When using the Ethos 8mm with a Paracorr, be cautious if you have a 1.25" filter threaded into the eyepiece, as it could contact the Paracorr's upper lens if the tunable top is set to its lowest setting. The recommended solution is to thread a 2" filter onto the front of the Paracorr instead.

Using the Ethos 8mm with Barlows & Powermates

To achieve even higher magnifications while retaining the immersive 100° apparent field, Tele Vue Barlows and Powermates are the recommended choice. A 2x unit doubles the magnification for planets and close double stars on steady nights.

When pairing the Ethos 8mm with a 2" Powermate, you should use its native 2" barrel for a secure fit. As with the Paracorr, a 1.25" filter threaded into the eyepiece may contact the top lens of a 2x Powermate. Tele Vue's advice is to thread 2" filters onto the front of the Powermate itself.

The 8mm Ethos vs. Delos and Nagler Eyepieces

Choosing a high-power eyepiece involves balancing apparent field, eye relief, and physical size. Comparisons from Tele Vue's chart:

• vs. 8mm Delos: The 8mm Delos offers a more forgiving 20mm of eye relief, making it a better choice for eyeglass wearers. However, its apparent field is smaller at 72°, and its 9.9mm field stop delivers a narrower true field of view.
• vs. 9mm Nagler Type 6: The 9mm Nagler offers an 82° apparent field with 12mm of eye relief. Its field stop is 12.4mm against the Ethos's 13.9mm, and it is a compact 1¼" eyepiece; the Ethos has the wider 100° field and more eye relief.
• Building a Set: A good way to build an eyepiece set is to choose models with decreasing field stop diameters. With its 13.9mm field stop, the 8mm Ethos fits logically between the 10mm Ethos (17.7mm stop) and the 6mm Ethos (10.4mm stop).

Expanding Capability with FoneMate and BandMate

The Ethos manual lists these Tele Vue accessories:

• FoneMate Adapter: Tele Vue's smartphone adapter clamps to compatible Tele Vue eyepieces for still, video or timelapse images through the eyepiece.
• Bandmate Filters: The eyepiece's 1.25" barrel is threaded to accept Tele Vue's Bandmate nebula filters. The Nebustar is a general-purpose nebula filter that, unlike typical UHC designs, blocks red wavelengths, and the OIII enhances planetary nebulae and supernova remnants.
• EBX-2120 Extender: This 2" barrel extender threads onto the eyepiece's 2" barrel, providing a place to mount 2" (48mm) filters.

The Ethos 8mm's 15.2 oz Build and Dimensions

The Ethos 8mm is a substantial eyepiece, weighing 15.2 oz (430 g). Balance your telescope accordingly, especially on smaller mounts. The eyepiece measures 5.0" (127 mm) in length with a maximum diameter of 2.1" (53.3 mm).

Do not unscrew the chrome barrel on the 10mm, 13mm or 17mm Ethos, as the lenses will fall out and void the warranty; leave every Ethos assembled. Always use the included dust caps when the eyepiece is not in use.