TeleVue Ethos-SX 4.7mm 110° Eyepiece - 1.25"/2"

The 110° Simulator Experience: A Spacewalk View at 4.7mm

The "SX" in Ethos-SX stands for "Simulator eXperience," a direct reference to the 110° field of view Al Nagler designed for NASA's Lunar Excursion Module (LEM) simulators. Al Nagler pushed Ethos designer Paul Dellechiaie to extend the Ethos-SX to 110°, the same field as those simulator optics, and the goal is an image as natural as one sees with the unaided eye. The 110° field has 21% more apparent field area than 100°.

The Ethos-SX are also superb planetary eyepieces. At high power, a wide field keeps objects in view longer. For observers with manual-tracking telescopes like Dobsonians, this means less time spent nudging the scope and more time focused on resolving fine detail on planetary surfaces or splitting tight double stars.

Framing the View: The 8.9mm Field Stop at High Magnification

The true field of view is set by the 8.9mm field stop and your telescope's focal length. The short 4.7mm focal length gives a small exit pupil (1.0mm at f/4.5, 0.5mm at f/10 in Tele Vue's chart), which darkens the sky background on small, bright targets such as planetary nebulae.

In a telescope with a 2000mm focal length, the 4.7mm Ethos-SX yields about 426x magnification. The true field can be calculated with Tele Vue's formula: (8.9mm Field Stop ÷ 2000mm Focal Length) x 57.3 = about 0.26° TFOV. That frames the Ring Nebula (M57) or the Trapezium region of the Orion Nebula (M42) at high power on steady nights.

Ergonomics and Design: 15mm Eye Relief and a Hybrid 1.25"/2" Barrel

The Ethos-SX 4.7mm has 15mm of eye relief. You can add a Dioptrx astigmatism corrector instead of eyeglasses, 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.

Designed as a 1¼" eyepiece, it includes a removable 2" adapter for convenience; there is no optical benefit in 2" mode. In each mode:

• 1.25" Mode: With the 2" adapter unscrewed, the eyepiece is parfocal with Tele Vue's Plössls, Type 6 Naglers, Nagler Zooms and 1¼" Panoptics.
• 2" Mode: With the adapter attached, screw it on until it stops (do not tighten); it becomes parfocal with the 10mm and 13mm Ethos. The adapter is also threaded to accept 2" (48mm) filters.

Optimizing the Field: The Ethos-SX 4.7mm with Paracorr Type-2

This eyepiece is very well corrected over the full field. Fast Newtonian and Dobsonian telescopes add their own aberration, coma, which makes off-centre stars look like little comets.

For Newtonians and Dobsonians f/4 or faster, use the Paracorr coma corrector with the Ethos-SX, for the same kind of full-field sharpness Tele Vue's NP-Series refractors show. The Paracorr goes in the 2" focuser like a Barlow and has a 1.15x magnification factor; the Type-2 works with scopes as fast as f/3.0.

Amplifying Detail: Using the Ethos-SX 4.7mm with Barlows and Powermates

On very steady nights the 4.7mm Ethos-SX can be paired with Tele Vue's Barlows or Powermates (the Powermate is an improvement on the Barlow-type design; it comes in 2x, 2.5x, 4x and 5x) for lunar, planetary and double-star work.

A 2x Powermate transforms the 4.7mm into an effective 2.35mm eyepiece, doubling the magnification. Al Nagler's guide notes that the atmosphere usually limits planetary observing to about 300x, and suggests no more than 60x per inch of aperture for scopes up to 6", so keep such powers for short focal length scopes or exceptional nights.

Ethos-SX 4.7mm vs. the 3.7mm SX and 6mm Ethos: Choosing Your Focal Length

The Ethos-SX 4.7mm rests midway between the 3.7mm and 6mm focal lengths.

• vs. 6mm Ethos (100°): The 4.7mm provides a 28% increase in magnification. Its main advantage is a smaller exit pupil for a darker sky background and higher contrast on planets, though it demands steadier seeing. The 6mm Ethos has a 10.4mm field stop and a 100° field.
• vs. 3.7mm Ethos-SX (110°): The 4.7mm gives about 21% lower magnification and 60% greater field area, which extends the SX-110° technology to more telescope applications. The 3.7mm has more true field than Tele Vue's 8mm Plössl and 5mm Nagler Type 6.