TeleVue Ethos 10mm 100° Eyepiece - 1.25"/2"

The 100° Spacewalk Experience

The Ethos series is defined by its 100° apparent field of view (AFOV), the widest in Tele Vue's range after the 110° Ethos-SX (Plössl 50°, Panoptic 68°, Delos 72°, Nagler 82°). The 10mm Ethos brings this experience to a critical medium-high power, where increased magnification darkens the sky background and boosts contrast on faint deep-sky objects.

For a given magnification, a wider apparent field means a wider true field, so on Dobsonians and other non-tracking mounts an object takes longer to drift across the view before you nudge the scope.

Framing the Sky with a 17.7mm Field Stop

While the apparent field describes the immersive experience, the true field of view (how much actual sky you see) is determined by the eyepiece's 17.7mm field stop and your telescope's focal length. The true field can be calculated with the formula: True Field = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3.

For example, in a telescope with a 2000mm focal length, the 10mm Ethos delivers a true field of about 0.51°, enough to frame the Dumbbell Nebula (M27) with sky around it. In a shorter 1200mm focal length instrument, the true field expands to 0.85° (17.7 / 1200 x 57.3), enough to show the Hercules Globular Cluster (M13) with surrounding sky.

Achieving a 1.0mm Exit Pupil at f/10

The exit pupil is the image of the objective formed by the eyepiece, where you place your eye. Its size, calculated by dividing the eyepiece focal length by the telescope's focal ratio, sets image brightness and sky darkness.

Tele Vue's chart gives 2.2, 1.9, 1.4, 1.0 and 0.7mm at f/4.5, f/5.2, f/7, f/10 and f/14:

• 1.0mm at f/10: In a typical Schmidt-Cassegrain, this suits resolving globular clusters, splitting tight double stars, and revealing detail in bright planetary nebulae.
• 1.4mm at f/7: Suits many refractors, giving high-contrast views of galaxies and nebulae while maintaining a dark sky background.
• 2.2mm at f/4.5: On a fast Dobsonian, this exit pupil is bright and well-suited for sweeping through star fields and observing larger nebulae, especially when paired with a Paracorr coma corrector.

Consistent 15mm Eye Relief and Dioptrx Correction

The TeleVue Ethos 10mm has 15mm of eye relief, the same as every Ethos and Ethos-SX in Tele Vue's chart, so your viewing position stays the same as you switch Ethos focal lengths.

For observers who wear glasses to correct astigmatism, the chart marks the 10mm Ethos Dioptrx ready. You can add a Dioptrx astigmatism corrector to the Ethos instead of using eyeglasses, to get the sharpest view possible, and you are more likely to see the full field of a 100° Ethos with Dioptrx than with eyeglasses.

The Dual 1¼"/2" Barrel

The 10mm Ethos has a dual barrel. It is designed as a 1¼" eyepiece with a 2" barrel section, allowing it to be used in either size focuser or diagonal without an adapter. The 2" barrel is included just for convenience; there is no optical benefit in 2" mode. Its in-travel may also prevent focusing in some refractors with the 60° Everbrite diagonal, which has an unusually long optical path.

The 1.25" barrel is threaded to accept standard 1.25" filters. For users who wish to use 2" (48mm) filters, the optional EBX-2120 barrel extender can be threaded onto the 2" barrel. A lock screw in the 2" barrel section can clamp to the outside of a 1¼" holder; the eyepiece is heavy and can fall out of slip-fit holders unless the lock screw is used. In 2" holders, remove the eyepiece lock screw and keep the holder screw out of the barrel slot.

Optimizing the View: Paracorr, Powermates, and Filters

Three accessories are available for the Ethos:

• Paracorr Type-2: For Newtonian/Dobsonian telescopes with fast focal ratios (f/5.5 or faster), the Paracorr eliminates the mirror coma, for the same kind of full-field sharpness Tele Vue's NP-Series refractors show. With a 1¼" filter in the Ethos, do not set the Paracorr's tunable top to its lowest setting.
• Powermates & Barlows: To achieve even higher magnifications for planetary and lunar observing while retaining the 100° field, Tele Vue Barlows and Powermates are the recommended match; use the Ethos's 2" barrel in 2" Powermates, and do not use a 1¼" filter with the 2x Powermate (it would touch the upper lens).
• Bandmate Filters: The eyepiece accepts 1.25" Bandmate filters, such as the Nebustar (Tele Vue's general-purpose nebula filter, which unlike typical UHC designs blocks red wavelengths) or the OIII, to increase contrast on nebulae.

Ethos 10mm vs. 13mm Ethos and 10mm Delos

Choosing an eyepiece is about finding the right balance of field, magnification, and comfort. Two Tele Vue comparisons, with chart figures:

• vs. 13mm Ethos: The 10mm's primary advantage is higher magnification, which results in a darker sky background and a smaller exit pupil. The 13mm has the larger field stop (22.3mm against 17.7mm). The two are parfocal, so you can switch between them with little refocusing.
• vs. 10mm Delos: The Delos offers a more generous 20mm of eye relief and a 72° apparent field. Its field stop is 12.7mm against the Ethos's 17.7mm. The Delos suits eyeglass wearers who want more eye relief; the Ethos gives the wider field.