TeleVue Ethos-SX 3.7mm 110° Eyepiece - 1.25"/2"

The 110° Simulator eXperience: A View From The Lunar Module

The "SX" in Ethos-SX stands for "Simulator eXperience," a reference to the 110° Infinity Display Projectors of the Apollo LEM simulator, which Al Nagler designed. The memory of the 110° view of the simulated lunar surface moving through the LEM window inspired him to develop an "Ultra Wide Angle" eyepiece, and with Ethos designer Paul Dellechiaie's help that dream is now fully realized.

The goal is an image as natural as one sees with the unaided eye, combining the wide field with contrast, colour rendition, distortion correction and centre-to-edge sharpness. The 110° field has 21% more apparent field area than 100°, and the Ethos-SX make superb planetary eyepieces.

Optical Geometry: The 7.0 mm Field Stop and 110° Apparent Field

The true field of view (TFOV) your telescope reveals is a function of the eyepiece's field stop and the telescope's focal length. The Ethos-SX 3.7mm has a 7.0 mm field stop, large for such a short focal length, which is what the 110° apparent field of view (AFOV) makes possible.

You can calculate the true field for your telescope using the formula: TFOV = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3. For example, in a Tele Vue NP101is refractor with a 540mm focal length, the Ethos-SX 3.7mm yields 145.9x and a true field of 0.75°, the figures Tele Vue prints for this pairing, enough to keep a planet and its brighter moons in view.

Performance at 3.7mm: Magnification, Exit Pupil, and Seeing

With a 3.7 mm focal length, this is a high-magnification eyepiece. The exit pupil, calculated as Eyepiece Focal Length ÷ Telescope f-ratio, becomes very small, which darkens the sky background and increases contrast on fine details. How much power you can use depends on the steadiness of the air.

• In f/10 Systems: An f/10 telescope gives a 0.4 mm exit pupil; Tele Vue's guide suggests avoiding exit pupils below about 0.5mm on reflectors, and the magnification is often too high for typical seeing.
• In f/7 Systems: An f/7 telescope yields a 0.5 mm exit pupil, a better balance for details within small planetary nebulae or globular cluster cores on steady nights.
• In f/4.5 Systems: A fast f/4.5 Dobsonian produces a 0.8 mm exit pupil, suited to lunar and planetary work in a short focal length scope.

The Removable 2" Adapter

The Ethos-SX 3.7mm is fundamentally a 1.25" eyepiece, but it includes a removable 2" adapter for convenience; there is no optical benefit in 2" mode. Each mode is parfocal with a different set of Tele Vue eyepieces. The mechanism is simple: unscrew the 2" adapter to reveal the native 1.25" barrel.

• 1.25" Mode: Without the adapter, the eyepiece is parfocal with Tele Vue's Plössls, Type 6 Naglers, Nagler Zooms, and 1.25" Panoptics.
• 2" Mode: Screw the adapter on until it stops (do not tighten) and the Ethos-SX 3.7mm becomes parfocal with the 10mm and 13mm Ethos eyepieces.
• Filter Threads: The design is threaded to accept both 1.25" and 2" filters, the 2" adapter accepting 48mm filters.

Fast Newtonians and the Paracorr

This eyepiece is very well corrected over the full field. Fast Newtonian and Dobsonian reflectors add mirror coma, which makes off-centre stars look like little comets; for Newtonians f/4 or faster, use the Paracorr coma corrector with the Ethos-SX.

The Paracorr installs into your 2" focuser and has a 1.15x magnification factor, turning an f/4.5 system into an f/5.2 system, and gives the same kind of full-field sharpness Tele Vue's NP-Series refractors show.

Ergonomics at 110°: 15 mm Eye Relief and Dioptrx Compatibility

The Ethos-SX 3.7mm has 15 mm of eye relief, the same as every Ethos in Tele Vue's chart. Al Nagler's guide notes that with exit pupils of about 1mm or less you probably will not need eyeglasses, and this eyepiece gives under 1mm in every telescope in Tele Vue's chart.

The chart marks it Dioptrx ready, but unless you have substantial astigmatism (such as 2 diopters) and a fast scope (such as f/4), you are not likely to need Dioptrx with this short focal length.

Ethos-SX 3.7mm vs. The Field: A 62% Power Jump from the 6mm Ethos

Tele Vue's own comparisons:

• vs. 6mm Ethos (100°): The 3.7mm gives a 62% power increase from the 6mm Ethos while retaining 68% of its true field.
• vs. 5mm Nagler Type-6 (82°): The 3.7mm actually has more true field than the 5mm Nagler Type 6 (field stops 7.04mm against 7.0mm), at higher power.
• vs. 8mm Plössl (50°): The 3.7mm has more true field than the Tele Vue 8mm Plössl (6.5 mm field stop), with more than twice the magnification.