TeleVue Ethos 17mm 100° Eyepiece - 2"

The 100° Spacewalk: A 29.6mm Field Stop in a 17mm Eyepiece

The Ethos experience is defined by its 100° apparent field of view (AFOV), a very wide window that you scan by moving your eye around the field. In Tele Vue's range only the 110° Ethos-SX is wider (Plössl 50°, Panoptic 68°, Delos 72°, Nagler 82°). The 17mm focal length provides the magnification to resolve details in galaxies and nebulae while the 100° field still delivers a sweeping vista.

The true field of view (TFoV) you see is a function of the eyepiece's 29.6mm field stop and your telescope's focal length. Using Tele Vue's formula (TFoV = (Field Stop Diameter / Telescope Focal Length) x 57.3), this eyepiece produces a 3.14° true field at 31.8x in a Tele Vue NP101is refractor (540mm), the figures Tele Vue prints for this pairing. This combination of significant magnification and a wide true field lets you frame large objects such as the Pleiades (M45) with plenty of surrounding sky.

Fast Newtonians, Paracorr and Powermates

Tele Vue does not publish the 17mm's internal construction or element count.

Owners of fast Newtonians and Dobsonians should use the Paracorr Type-2 coma corrector. Use it in f/5.5 or faster scopes to eliminate the mirror's coma, giving the same kind of full-field sharpness seen in Tele Vue's NP-Series refractors; the Paracorr Type-2 lets an f/3 parabolic mirror perform like an f/12 without it. You can also amplify the Ethos 17mm for high-power planetary and double-star viewing using Tele Vue's Barlows and Powermates, recommended for higher magnifications with a 100° field.

The 17mm "Sweet Spot": A 3.8mm Exit Pupil in Fast Dobs

The exit pupil is the image of the objective formed by the eyepiece, where you place your eye; Tele Vue calculates it as eyepiece focal length divided by the telescope's f-number. A larger exit pupil delivers a brighter image, while a smaller one increases contrast and darkens the sky background.

The Ethos 17mm produces these exit pupils (Tele Vue chart: 3.8, 3.3, 2.4, 1.7 and 1.2mm at f/4.5, f/5.2, f/7, f/10 and f/14):

• Fast f/4.5 Telescopes: A 3.8mm exit pupil, good for brightness on faint nebulae and galaxies from a dark site.
• f/7 Apochromatic Refractors: A 2.4mm exit pupil, which darkens the sky background to enhance contrast on globular clusters and planetary nebulae.
• f/10 Schmidt-Cassegrains: A 1.7mm exit pupil, suited to fine detail in brighter objects like the Orion Nebula (M42) or teasing out structure in galaxies.

Observing Comfort: 15mm Eye Relief and Dioptrx Compatibility

An immersive view is only useful if it's comfortable to observe. The Ethos 17mm features 15mm of eye relief, which is the distance your eye must be from the top lens to see the full field. While observers who don't require glasses will find this ample, those with astigmatism can use a Dioptrx instead of eyeglasses at the telescope.

The Ethos 17mm is Dioptrx ready. You can add the Tele Vue 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 in 100° Ethos eyepieces with Dioptrx than with eyeglasses.

Handling a 1.55 lb Eyepiece: Design and Accessories

The Ethos 17mm is a substantial piece of optical equipment, weighing 24.8 oz (1.55 lb) and measuring 4.5" in length above the barrel shoulder. This requires a robust 2" focuser and may necessitate rebalancing your telescope when switching to or from smaller 1.25" eyepieces. Tele Vue's chart gives a 2.44" black barrel diameter, and the barrel is threaded for standard 2" filters.

To maintain optical alignment and prevent damage, do not unscrew any sections of the eyepiece, as doing so can cause lenses to fall out and will void the warranty. Also available is Tele Vue's FoneMate smartphone adapter, which clamps to compatible Tele Vue eyepieces for still, video or timelapse images.

Building a Tele Vue Set: The 17mm Ethos vs. the 31mm Nagler

The 17mm Ethos is an ideal complement to the 31mm Nagler Type 5. They are parfocal (both focus about 3/8" above the shoulder), so you can switch between them with little refocusing, and the 31mm Nagler has exactly twice the true field area, a natural low-power companion for finding and framing objects before moving to the 17mm for higher-magnification study.

Compared with other Tele Vue eyepieces (Tele Vue chart figures):

• vs. 21mm Ethos: The 21mm, with a 36.2mm field stop against 29.6mm, offers a wider true field of view for scanning, but the 17mm provides higher magnification that darkens the sky background and makes it easier to resolve detail in smaller deep-sky objects.
• vs. 17.3mm Delos: The Delos has 20mm of eye relief against 15mm and a 1¼" barrel, with a 72° apparent field and a 21.2mm field stop; the Ethos gives the wider 100° field and a larger field stop.