21mm 2" 100° field Ethos

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The new 21mm TeleVue Ethos 100° field eyepiece completes the logical set of 21mm, 13mm, and 8mm Ethos eyepieces. Each step to a shorter focal length eyepiece within the set provides an approximately 62-64% increase in magnification. Each step to a longer focal length within the set provides approximately a 2.6x gain in field area.

With a 21mm focal length,, the field of this new 100° apparent field Ethos eyepiece is nearly 64% larger in area than the 82° field of a “space-walking” 20mm Nagler. The TeleVue Ethos delivers the true field size of a longer focal length (but narrower apparent field) eyepiece, such as a 26mm Nagler, but has the benefits of the higher power and darker sky background that are inherent in a shorter focal length.

There is a soft rolldown eyecup to shield the eye from ambient light (from a neighbor’s security light, for example) and improve the image contrast. At a usable eye relief of 15mm, the 21mm Ethos would normally somewhat vignette the field of view for eyeglass wearers. However, since the 100° field is so wide that you can’t see the full field without having to move your head from side to side to take it all in anyway, vignetting in the conventional sense is not a problem.

The 21mm Ethos has all the virtues that professional and amateur astronomers have come to expect from a TeleVue eyepiece: high contrast, comfortable eye relief, and full field sharpness – with well-controlled astigmatism, field curvature, lateral color, angular magnification distortion correction, and low pupil sensitivity (kidney bean effect) for daytime use. While sharpness is inherent to the optical design itself, the contrast is maximized through the intelligent use of flat finished baffles and ultra low reflectance, high efficiency coatings that are tuned to the glass composition of each lens element.

The Ethos is designed for 2” focusers and star diagonals only. A safety groove is machined into the 2” chrome barrel to engage your focuser thumbscrew. This will help prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.

The 21mm TeleVue Ethos eyepiece will accept TeleVue Dioptrx eyesight astigmatism correcting lenses.

Please note the 36 ounce (2.25 lb.) weight of this eyepiece if you have a Dobsonian telescope. Its use may require some temporary rebalancing of your scope, or the use of a TeleVue #TVEQC equalizing adapter with lighter 1.25” eyepieces to avoid balance problems.

Field of view:
The field of view (FOV) is the amount of observable world one can see at any given moment.
100°
Eye Relief:
Eye relief is the distance from the last surface of the eye lens of an eyepiece to the plane behind the eyepiece where all the light rays of the exit pupil come to a focus and the circular image is formed, sometimes called the “Ramsden Disk.” This is where your eye should be positioned to see the full field of view of the eyepiece. If you must wear glasses because of astigmatism, you’ll usually need at least 15mm of eye relief or longer if you want to see the full field of view with your glasses on.

A note on our eye relief figures: Quite often, our eye relief figures will differ from those of the manufacturer. This is because we measure the “usable” eye relief, while the manufacturers specify their usually-longer (but technically correct) “designed” eye relief.

The eye lens of the eyepiece is normally recessed below the rubber eyeguard or rubber rim of the eyepiece to keep the lens from being scratched during use. An eyepiece might have a “designed” eye relief of 15mm (and the eye relief will truly measure 15mm from the eye lens to where the image forms). However, if the eye lens is recessed 3mm below the eye guard, the Ramsden Disk forms only 12mm above the eyepiece body (the 15mm “designed” eye relief, less the 3mm of eye relief made unusable by having the eye lens recessed into the body of the eyepiece). This “usable” eye relief of 12mm (measured from the rolled-down eyeguard – the closest point you can get your eye to the eye lens – to where the image forms) is the eye relief figure we would measure and list in this website.

Why is it important to list the “usable” eye relief? For those people who don’t wear eyeglasses while observing, a few mm difference between the eye relief they expect from the manufacturer’s literature and the shorter eye relief they actually get in real life doesn’t mean a lot. They can simply move a little closer to the eyepiece to see the full field, and never realize that the eye relief is a little shorter than they expected. However, some people must wear eyeglasses while observing, because of severe astigmatism. These observers can’t move closer to the eyepiece if the eye relief is shorter than expected because their glasses get in the way. For these people, the real life “usable” eye relief is more important than the technically correct but sometimes not fully usable “designed” eye relief. We measure and list the actual usable eye relief so that people in the real world can pick the eyepieces that will work best for them.

15mm
Focal Length:
This is the length of the effective optical path of a telescopeor eyepiece (the distance from the main mirror or lens where the lightis gathered to the point where the prime focus image is formed). Focallength is typically expressed in millimeters.

The longer the focallength, the higher the magnification and the narrower the field of viewwith any given eyepiece. The shorter the focal length, the lower themagnification and the wider the field of view with the same eyepiece.

21mm
Barrel Size:
The industry standard diameter, as of 2011, is 1.25" and is the most common.  Other sizes available are .965" and 2".
2"
Weight:
The weight of this product.
2.25 lbs.
Warranty:
Limited Lifetime
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1. Brandon on 5/18/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
I have many eyepieces. However, the 21mm Ethos is hands down my favorite eyepiece that I own. It looks absolutely stunning through my Celestron 8" SCT. It is the the most versatile eyepiece in my collection. I cannot believe how amazing both wide field targets like the double cluster and small targets like double stars appear in this eyepiece. The contrast through the eyepiece is fantastic. This will make you sold out on TeleVue. If I could only own one eyepiece, this would be it. Don't think, just buy it. P.S. I am not an owner or affiliated in any way with TeleVue!
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2. Avery on 5/7/2013, said: AstronomicsAstronomicsAstronomicsAstronomicsAstronomics
I don't have much experience with with high end eye pieces - so, when I decided to take the plunge I went all-in for TeleVue Ethos. I've read mixed reviews about the "thrill" of 100 degree apparent field of view being overrated... It's not... The view is amazing and the spacewalk view is everything you're going to want. It turns your telescope into something new and exitingly unexpected. And to top it off - as investment goes - I can always add DIOPTRX to compensate for my changing eye stigmatism. Traumatic brain injuries have left me with a sight condition that changes my vision from 20/20 to something much less and akin to a palsy or stignatism. Anywho - I couldn't be happier.
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Telescope Eyepiece Formula
To calculate the magnification and approximate actual field of view of this eyepiece on your telescope, enter the focal length of your telescope below and press the "Calculate" button.

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TeleVue - 21mm 2" 100° field Ethos

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TeleVue - 21mm 2" 100° field Ethos
 AstronomicsAstronomicsAstronomicsAstronomicsAstronomics (Average: 5.00 | Users: 2)  Only registered users can submit ratings - Register Here
Our Product #: TE21
Manufacturer Product #: ETH-21.0
Price: $895.00
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The 100° apparent field of this new TeleVue 21mm Ethos eyepiece provides a field area 64% larger than the “space-walking” 82° field of a 20mm Nagler, and 36% larger than a 22mm Nagler . . .





. . . our 35th year