Very good overall. Using it on a 10" Skyline Dob, need to leave about 4 mm space from fully seated in the telescope in order to bring it to focus, however. Not a problem, just an observation. Very pleased with the clarity, brightness and wide view at the 138x magnification with the f/5 scope. So far, comparing it with other eyepieces, it does best on fairly compact open clusters and nebulae such as Orion.

Super wide field of view' with power..love it's tack sharp star images, excellent contrast and black background. Anyone looking for a great mid to upper power eyepiece this one you'll love.

I love this eyepiece. The wide field of view is great for finding nearby objects. I use manual setting circles on my 12" f/5 dobsonian and I can usually get an object within the field of view of this eyepiece. It even works great in my Celestron 130SLT, though it's not as necessary with a goto scope. I love the clarity of the eyepiece and brightness the eyepiece gives. I can often see objects better than or equal to my other lower magnification eyepieces (Baader Hyperions).

I bought both ES100 9mm and ES82 11 mm to compare one to another. Contrast and sharpness was similar. At last, I've chosen ES100 though it's two times heavier. 100 degree view is awesome! I believe, this bomb-eyepiece become my favorite with Dob 10 f1/5. A bit of coma right at the edge of the view but it doesn't matter for me.

I bought this eyepiece during the ES winter sale here at OPT at the beginning of the year. It was a little out of my budget (I also bought the 82 degree 11mm and 14mm eyepieces while on sale but OPT's return policy convinced me to take a chance, so I ordered it about 1/2 hour before their free shipping promo expired. The 9mm worked very well in my 12 f/5 LightBridge and a fellow astro club member's 12 Sky-Watcher dob. He owned a 13mm Ethos so we were able to test against one another. It provided a FOV similar to the 14mm 82 degree ES eyepiece that I had also bought, though with more magnification. It was sharp across the field with a little softness creeping in right at the edge. Contrast and sharpness was on-par with the Ethos with a similar softness at the edge, though not as pronounced. Thinking that I might go ahead and return it due to the FOV being similar to the 82 degree 14mm (and being out of my budget, I had one last thing I wanted to try, and decided that if it worked out that I would keep the 9mm and figure out how to balance my budget to afford it. I also do a little astrophotography and had earlier purchased an Antares 2 x0.7 focal reducer for use with a Starshoot CCD camera I have. If ******* on to the end of the ES 9mm it would turn it into a 12.9mm eyepiece (or 14.2mm if I used the ring I had bought for the 17mm Hyperion I also own, but would it work OK or not? The next evening I was able to go out I took the focal reducer with me and was again able to test it against the 13mm Ethos. It worked wonderfully well. The softness at the edge creeped in a tiny bit more, but the contrast and sharpness remained on par with the Ethos. And I definitely enjoyed the larger true FOV at both 12.9mm and 14.2mm than I did at 9mm on many objects! And it makes it a most versatile eyepiece when used with the focal reducer, as some targets are better at 9mm than at the lower magnification with the reducer. I ended up selling my Baader Clickstop Hyperion Zoom eyepiece to balance my budget so I could keep this eyepiece (the purchase of these ES eyepieces made the Clickstop less necessary. I've been very happy with the Explore Scientific eyepieces I have purchased and feel they are an incredible value compared with the their completion. If I had returned the 9mm 100 degree eyepiece I would have at some point in the future purchased the 14mm version from Explore Scientific (another astro club member has one of these and I love it. But I'm real glad things worked at as they have and am happy to be able to use this as a 9mm, as well as a 12.9mm and a 14.2mm. James

Wonderful viewing thrue my 10 ince newtonion reflector, very sharp images all the way to the edge of the f.o.v and no coma. A little bit big and heavy. I will recomend it to a friend.