Explore Scientific 20mm 100° field waterproof 2" eyepiece


Availability: In stock

The 100° apparent field of the Explore Scientific 20mm waterproof 2" eyepiece provides superb maximum field/medium power observing in all scopes, from small apo refractors to the largest Dobsonians and catadioptrics . . .
Our Product #: ES20
Explore Scientific Product #: EPWP10020-00
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Product Description

The 20mm Explore Scientific 100° field 2" eyepiece is O-ring sealed and nitrogen-purged to be 100% waterproof. To insure a perfect seal, Explore Scientific tests every eyepiece in one meter of water for 30 minutes before shipping.

Internally sealing the eyepiece and purging it with inert, dry nitrogen gas prevents internal fogging in the eyepiece when observing at sub-freezing temperatures. For observers in dusty climates, sealing keeps wind-blown dust and fine particulates from getting into the eyepiece body. For observers in very humid climates, sealing prevents the growth of fungus on the internal lens elements. For all observers, complete sealing maximizes the life of the internal optical coatings and makes the eyepiece easier to clean, as there is no risk of cleaning solution migrating from the top of the lens and seeping around edges to be trapped in between elements.

The 20mm Explore Scientific eyepiece, has high contrast, comfortable eye relief, and full field sharpness - with well-controlled astigmatism, field curvature, and lateral color. Its nine element optical system (using lenses of dense crown, light crown, dense flint, and lanthanum rare earth optical glasses) is designed for maximum field/medium power deep space use with telescopes having focal ratios down as low as f/4. It is exceptionally effective on nebulas with fast f/ratio reflectors, such as Dobsonians, as it gives both an immense field and a bright exit pupil (almost 4.5mm with the typical f/4.5 Dob). Its very wide field also makes it convenient for moderate power lunar and globular cluster observing with Dobsonians, as you have much longer to observe before the planet drifts out of the field and the scope has to be repositioned than you do with a 60° or 70° (or even 82°) field eyepiece. In addition to Dobs, the new 20mm Explore Scientific Explore Scientific is marvelous for observing open clusters, nebulas, galaxy clusters, and globular clusters with any telescope capable of using a 2" eyepiece.

The 100° apparent field of this new 20mm Explore Scientific eyepiece is nearly 50% larger in area than an 82° field eyepiece. You could get a similar true field with a longer focal length (but narrower apparent field) conventional eyepiece, but the 20mm Explore Scientific Explore Scientific has the benefits of the higher power and darker sky backgrounds that are inherent in its shorter focal length. You can use the field of view calculator in the "formula" tab above to see just how much sky the Explore Scientific can reveal with your telescope.

There is a soft rolldown eyecup to shield your eye from ambient light (from a neighbor's security light, for example) and improve the image contrast. With an eye relief of 14.5mm, the 20mm Explore Scientific 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 20mm Explore Scientific fits 2" focusers only. A safety groove is machined into the 2" chrome barrel to engage your focuser thumbscrew. This helps prevent the eyepiece from falling should the thumbscrew accidentally loosen while observing.

This individually serial numbered Explore Scientific eyepiece is backedby a limited one year warranty, which will be extended to an ExploreScientific transferable product lifetime limited warranty when its warranty card is registered with Explore Scientific within 60 days of purchase.With the extended product lifetime warranty, this Explore Scientificeyepiece will be protected for as long as it continues to bemanufactured, plus an additional five years. Full details on this unique transferable lifetime warranty.

Tech Details

Barrel Size 2"
Eye Relief 14.5 mm
Field of view 100
Focal Length 20mm
Number of optical elements 9
Weight 2 lbs. 2 oz.
Warranty 1 year, extendable to lifetime


Review by:
I am using this EP with a 22 inch f/4.2 reflector. It's fine. The edge correction is not as good as competing 21mm but it's not $300 worse! I use it with a paracorr BTW. Build quality is excellent. One drawback, it's heavy. Make sure your able to balance it and you focuser is up to the task. (Posted on 8/23/2019)
Review by:
I own the 20mm 14mm 9mm and 5.5mm ES 100deg. Wonderful field of view with enough magnification to make the background black velvet. Wonderful views. (Posted on 12/31/2018)
Review by:
This eyepiece is my favorite out of all my ES 100deg. It offers a wide field of view with great magnification. Truly black velvet backgrounds. Xray images show the design of these to be remarkably similar to that of the Ethos. (Posted on 12/31/2018)
Review by:
One of my favorite eyepieces in all my scopes F4.8, F5 and F6. It replaced my ES 24/82 - same FOV but more magnification. It's my go to eyepiece for wide FOV. I also have an ES30/82 which is also a great eyepiece but I prefer the extra mag in the 20mm. It's a superb eyepiece for a dob. (Posted on 12/30/2018)
Review by:
This eyepiece surprised me when I used it to view planets. One would not expect an ultra wide eyepiece to provide pleasing planetary views, but the 20mm 100 degree eyepiece does just that. Saturn and Jupiter float in a field full of stars. The view are very high contrast and crisp. Stars are pinpoint to the very edge, especially with an ACF scope. Great product! (Posted on 12/11/2018)
Review by:
the E/S 20mm 100* is truly a wonderful viewing eyepiece in my 120mm refractor - super wide FOV lets you experience the " … window in a space ship effect …" - you actually find yourself looking all around at the space vista . snaps into focus nicely and is sharp to the edges - you really do not need any more than this / a favorite of many telescope viewers !!! (Posted on 11/24/2018)
Review by:
Incredible views through this EP. The immersive, expansive views draw you in. I particularly enjoy the view of clusters and double stars through my Celestron 9.25". The whole disk of the moon fits in the eyepiece, and it is an impressive sight. You will never be disappointed with this spectacular eyepiece! (Posted on 10/31/2018)
Review by:
This eyepiece has a HUGE fov! Enough that I have to move my head around in order to see it all. It is very sharp to the edges with no aberrations that my eye can pick up on. I use this in my 12" f-4.9 dob. At almost half of the cost of other large fov eyepieces like this one, its hard to pass up. (Posted on 10/30/2018)
Review by:
This is the best eyepiece I own. It works perfectly in my 16" f/4.5 dob as an initial "finder" eyepiece and for larger star clusters. Correction across the field is fantastic, eye placement is easy, and eye relief is not problem (I don't wear glasses). (Posted on 10/26/2018)
Review by:
I can't bring myself to pay for the other 100 degree EP on the market, but the ES 20 mm at the 300 dollar price point made me bite. Good gravy, is that view wide. And clear and sharp. The EP is also extraordinarily large and heavy and well made. I eventually purchased all four in the line up. They work well in all my scopes and I'm pleased with the purchase(s). (Posted on 8/4/2017)
Review by:
The views through this 20mm is simply outstanding. This EP is perfect for open clusters, double clusters, galaxies and big nebula. I'm a wide FOV nut so the 100* series is my favorite. The image is crystal clear and tack sharp all the way to the edge of the FOV. The images have great contrast with pinpoint stars in the whole FOV. I have the ES 100* 9mm and the ES 82* 30mm and they too are spectacular EP's but I always find myself going back to the 20mm. The 20mm also barlows exceptionally well too. Its a perfect addition to my fast 10" Dobsonian. (Posted on 8/4/2017)
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