10" F/4.7 Modified Skywatcher Reflector, 38mm Orion Q70, 17mm Modified Ultima LX, 10mm TeleVue Delos, 7mm Pentax XL.
Quote:Maybe it's your mirror?
Quote:I've spent the past few weeks trying out all three AFOV offerings by ES in a 14" f/5 scope. Overall a pleasant experience, particularly with the 100° flavor.The most unexpected finding was with the 24mm 68°, which seems to get mainly great reviews. While I'm usually tolerant of a bit of field curvature, I found this EP's FC to be very annoying.I was constantly reaching for the focuser knob to alternate between sharp stars in the middle of the field and the periphery. It was impossible not to notice the extent of the field curvature.The amount of adjustment required to alternate between on-axis and edge-of-field sharpness was surprising. Has anyone else experienced field curvature with this eyepiece?
Vixen 140mm Neo-achro, 2" AP Maxbright diagonal, 40mm Orion Optilux, 35mm, 30mm, 18mm, and 15mm Ultrascopic/Ultima, 28mm & 20mm ES 68, 19mm TV Panoptic, 5.5mm Meade UWA, 2.4x 2" Dakin barlow (prototype barrel),1.6x Antares barlow.
Quote:Wow!I am so glad I read this! I almost ordered a 24mm ES 68 last week, but on the last whim, I ordered a 20mm Meade 2" waterproof 5000 UWA. I can't stand field curvature. My scope is a 10" F/4.7, so it would be even worse in my scope!Thanks for the heads up!Cheers,
C10NGT, Z8, 150 Rumak, XLT 150, C6, C5, SW5 Newt, 4.5 Ball, C102GT, C90, ST80, A70LF; 15x70, 25x100; Burgess BV; Paracorr II; T6 2.5, XO 2.58/5.1, Ethos-SX 3.7, Delos 4.5, TV Plossl 7.4-26, BCO 10, Hutech HC 12.5, Sterling 12.5-25, ES100 14, CZJ H 16/25, CZJ O 16, M5k UWA 24, T5 31, Ultrascopic 35, Titan-II 40; Bino Pairs M5k UWA 6.7, Baader Zoom 8-24, M5k SWA 24, TV Plossl 26, RKE 28.7; Zooms NZ 2-4, NZ 3-6, Leica ASPH 8.9-17.8, Baader 8-24; Baader Zoom Barlow, VIP Barlow
Quote:The most unexpected finding was with the 24mm 68°, which seems to get mainly great reviews. While I'm usually tolerant of a bit of field curvature, I found this EP's FC to be very annoying.
Quote:Quote:Maybe it's your mirror? It's just the one eyepiece that I was having the FC issue with, so I'm ruling that out for now.I took a closer look at Bill's 24mm-26mm review, and he does mention some noticeable residual field curvature after using a Paracorr at about 80% from center FOV. Hard to say how much is there without the Paracorr as he groups other moderate-to-severe aberrations in at about 45%-50% away from center FOV.24-26 mm Eyepiece ComparisonPerhaps a clue there, though. The other option is a bad egg...
Keeper of the Swamp Gas Observatory "This R2 unit has a bad motivator" AR127, CG4 CGE1100 1984 tasco 60mm 49TR F/13.3 refractor 1976 Vixen Polaris 80mm F/15 refractor Denkmeier Big Easy BV'er Astro-Tech Titan 38mm 70* Astro-Tech Titan 20mm 70* Astro-Tech SR6 12mm 60* Meade MA25mm Jap. Meade MA9mm Jap. TMBpII 5mm ES 1.25" FX
Quote: I found the ES far better than the Hyperion, especially at f/4, and almost as good as the TV and Vixen.
Quote:What scope(s) are you seeing the field curvature with?
Quote:I have noticed the field curvature also. It's the only eyepiece I own where FC is obvious. I find it a little annoying. Oh and its in my 8" F8 NEWT.
Quote:Last summer I tried a friends ES68 24mm on my SW 12" f/4.9, 66mm WO f/6 ED and SV 50mm f/4 finder. I aslo had my Hyperion 24mm and a borrowed Panoptic 24 and Vixen LVW 24mm. I found the ES far better than the Hyperion, especially at f/4, and almost as good as the TV and Vixen. There were some seagulls but I thought it was much more transparent than the Pan (the sky was very dark, 1600 meters high).
The result was that I bought the eyepiece from the man who loaned it to me and sold the Hyperion. I found Hyperion's views brighter than those of the Panoptic but the Pan had the better edges. My general feel is that the ES was a very nice eyepiece and I needed to use it on a f/5 scope with 1.25". I can can see somewhat bloated stars at the edge but no seagulls.
Quote:As far as the toughest test for an eyepiece, I think fast, flat field refractors like the NP-101 that are corrected for field curvature themselves are the toughest because they are essentially free of off-axis aberrations.. a fast Newtonian with a Paracorr is pretty darn good but they are not as perfect, there are some uncorrected aberrations.Jon
Quote:Measures for 24 mm 68ES showed size of aberration spot near 20 angular minuets at edge of FOV in 1:4 scope. E.g. 24 mm Panoptic in the same conditions shows size of aberration spot less then 6 angular minutes.
Quote:Earnest did estimate 6 arc minute star images in the Panoptic 24 versus 20 arc-minutes in the ES 24x68.
I would expect that last figure to be visible to all.
Annoying is a personal evaluation.
Quote:The real issue is how much of the 20' star images' size is due to field curvature, astigmatism, and other aberrations.
Because if only a part of it is due to FC, a field flattener isn't going to help much.
It reminds me of the 35 Panoptic which had hugely bloated star images at the edge in my f/5 scope, exemplifying coma. I put in a coma corrector which, in that f/5 scope, eliminated coma completely. What was left was field curvature.
Since the Paracorr also provided some field flattening, and I STILL saw some FC, the comatic images must have been as bad as they were without coma correction because of defocus due to FC.
[I likened it to the image from the flight deck of the Millenium Falcon as it jumped to light speed. I simply couldn't have imagined anyone being able to put up with that much coma. It was like a widefield eyepiece in an Astroscan!]
Quote: When coma was gone, and the field was flattened slightly, I could still see the edge stars were out of focus compared to the center.
I learned to focus half-way to the edge and get a focused field that my eye could accommodate from center to edge.
Quote:I use that illustration to point out that what we see at the edge of the field is often a combination of aberrations and merely describing the bloated stars, or even their sizes, doesn't necessarily pin down all the causes.
Quote:Quote:As far as the toughest test for an eyepiece, I think fast, flat field refractors like the NP-101 that are corrected for field curvature themselves are the toughest because they are essentially free of off-axis aberrations.. a fast Newtonian with a Paracorr is pretty darn good but they are not as perfect, there are some uncorrected aberrations.Jon This is true, but in favor of Newts is that they are much more readily available for testing eyepieces than NP-101's and the like. For example, I've never seen anyone with an NP-101 or other high-end refractor at my dark site. And most Dobs these days have at least a 1200mm focal length, which is fine for testing FC. The second primary requirement for testing FC is an observer who is sensitive to that aberration. That shouldn't be a problem. There are plenty of old stargeezers around. Just watch out for the young observers who still have a deep range of focus accommodation. They could swear all night that there is no FC in an eyepiece, though a stargeezer would know better.Mike
Quote:Jon,But I thought that a good test of FC is to see whether there is a different focus setting for on-axis vs off-axis. AFAIK, coma and astigmatism cannot be focused out no matter where we set the best focus. Now, sure, if FC is jumbled together with other aberrations, it will be harder to tease out of the mix. But if the eyepiece is put in a Paracorr, what we usually have left in an eyepiece in a Newt is FC, astigmatism and/or distortion. Mike
Quote:Field curvature was by far the dominant aberration here. I say this based on the ability to refocus the edge of the field to sharpness (some mild coma/astigmatism remained - no Paracorr) while the central FOV stars became round out-of-focus blobs.
Quote: That sounds like my experience with the ES 82 14mm. I had bought a pair of these eyepieces for my binoviewer. I cannot use a Paracorr with my binoviewer, but still, even with a 1.9x OCS, the main aberration certainly behaved like field curvature. FC is not a good thing to experience when you're trying to tease out fine surface details for Jupiter as it drifts across the FOV.Mike
Quote:"Seagulls" would be from coma which is from the primary mirror, not the eyepiece.