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Explore Scientific 25mm 100 degree eyepiece

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#1 Charlie Hein

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Posted 13 July 2013 - 11:21 AM

Explore Scientific 25mm 100 degree eyepiece

By Richard Lines

#2 smallscopefanLeo

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Posted 13 July 2013 - 03:09 PM

Interesting review, thanks for sharing your take on these heavy hitters. For the time being, I doubt that I will go quite -so- wide with my own glass. Since I am mostly self-relegated to the more diminutive end of the telescope spectrum, I tend to prize lightness, compactness and simplicity in eyepiece design.

BUT, I would be lying if I said that I don't daydream a decent bit about a great Dob every here and there (and also about the accompanying eyepieces that would enable me to luxuriously slew by hand at low to moderate powers with it). So it is with ample interest that I like to hear what others have to say about these meaty 2" oculars.

Based on my own experiences, and on those of countless ones shared here on our forums, I am gradually learning that tolerances for edges in various fields of views vary tremendously. Perhaps I am a fairly tolerant one when it comes to such things, for I have not found different sorts of slightly mushy, partly coma-y edges to really bother or hinder me too much overall. Sure, I care about more than just what's up front and centered, but I have to wonder as to just how much of a deal-breaker this would be for many folks out there. I understand that some are very sensitive and particular to these details, and more finely attuned to slight changes in them from eyepiece to eyepiece.

Right now I see that one can buy the ES 20mm AND the ES 25mm for about the price of one 21mm Ethos. So I wonder, without having owned any of these - is the 21mm Ethos so superior to the 25mm ES (or other ES eyepieces for that matter) that it is worth so much more? The Ethos also has an extra 9th element, I assume? (It would be fascinating to see how they would both test out on-axis.)

I too am eager to hear from others who have had experiences with the ES 25mm. We eyepiece connoisseurs are certainly spoiled for choice these days with all of the dizzying arrays of glass combinations that are now being produced!

#3 star drop

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Posted 13 July 2013 - 04:10 PM

Thank you, Richard. You have saved me from a disappointing purchase.

#4 Starman1

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Posted 13 July 2013 - 09:49 PM

I'll add to Richard's excellent review with a few comments of my own, and some suggestions:

I spent a fair amount of time with the 25mm in my 12.5" under dark skies, too.

My comments could be similar to yours, since I use both a 31 Nagler and a 21 Ethos, in a 12.5" f/5 scope with Paracorr.II.

Here is what I found:

1)distortion--obviously corrected for AMD, because RD is noticeable. It's obviously corrected for nighttime, astronomical observation. This is not a negative, just a comment.

2)vignetting--obviously significant, since it's easily visible in both day AND night viewing, starting about 80% of the way to the edge. Not unexpected, since the field stop is a little larger than would be expected in a 2" barrel with this AFOV (probably why TeleVue stayed away from this focal length), but also minimally noticeable at night, and pretty much only if you let the moon drift toward the edge of the field.

3)light scatter--not great. When the crescent Moon is in the field, the light from the lighted part bleeds over to the earthlit part and reduces contrast. Averted vision shows this.
When the Moon is outside the field, it's obvious which direction the Moon lies.
More aggressive baffling would have reduced the FOV, I believe. Not a great lunar eyepiece, therefore, but since the eyepiece is highly unlikely to be used as a lunar or planetary eyepiece in any scope, this may be a non-issue.
Suggestion: use for all purposes except Moon viewing.

4)astigmatism correction--not the best. Even in the Paracorr, star image degradation starts a little more than half-way to the edge and gets suddenly worse at about the 90% point.
Going back and forth through focus on a star near the edge, the astigmatism at the edge is obvious.
It's better than many 80 degree eyepieces, though.
Suggestion: Use as a finder eyepiece for really low power.

5)lateral color. Star images become prismatic near the edge, and moving the eye only partially eliminates this. Holding the eye in the wrong place with an Ethos can do this too, but changing the position of the eye eliminates it. In this case, it can't be eliminated, AND the blue ring at the edge is very noticeable on the Moon; otherwise not at all.
Suggestion: Experimenting with eye placement is essential to reduce this effect.

6)internal reflections. With Sirius slightly outside the field of view, there is a semi-circular ghost in the field. The center of that circle is where Sirius is.
The ghost is not evident when Sirius is in the field, but it's obvious that reflection from the interior wall of the eyepiece barrel, or the side of a lens, or the filter threads at the bottom, or a modestly bright spacer ring visible near the bottom causes this. It could have been fixed with a baffle, probably, but wasn't, and for the reasons I mentioned above.
Suggestion: I would recommend darkening the filter threads on the bottom of the eyepiece and the bottom of the barrel itself.

7)image sharpness on axis and in the center 50% of the field is very good. If a user mainly looks here, it will be an engaging eyepiece to use.
Suggestion: Use for objects that can be framed well by the field of view.

8)field curvature. In the Paracorr, I didn't really notice anything that I couldn't eliminate by careful focusing (I'm 62). But without the Paracorr, though coma dominated the edge, (and astigmatism), I could focus the edge a little better and when I did, it defocused the center. Since my scope has a 1587mm focal length sans Paracorr (1825mm with), I would bet the field curvature of the eyepiece matches the scope and augments it to create a noticeable curvature. It really didn't matter, though, since FC wasn't the biggest issue at the edge, and it was fully eliminated in the Paracorr by focusing partway out from center.
Suggestion: Don't focus on a star in the center, but partway out and the entire field will be in better focus. Astigmatism and lateral chomaticism are reduced by having a better focus over the entire field.

9)coloration. Well, I'm pretty insensitive to this. Even if the tint were slightly yellow, my eye would see it as white 2 seconds after looking through the eyepiece. It's one of the reasons I regard any discussion of tint as a "tempest in a teapot". You might notice it if quickly changing eyepieces, like in a turret, but otherwise? It's only important to me if it reduces the ability to see colors in stars or objects. if I see a tint, it's STRONG. But in eyepieces? REALLY subtle. I saw none in this eyepiece, even on the Moon.

It would be interesting to see if a simple black ring on the bottom of the eyepiece might eliminate an internal reflection (as it has done on some other eyepieces) or whether simple blackening of the filter threads helps. Had internal baffling been more aggressive, and the FOV reduced to, say, 90 degrees, this eyepiece could easily have earned a better review.

[Excerpted from a March 17th post and somewhat enhanced]

#5 astrophile

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Posted 14 July 2013 - 09:22 AM

Richard (and Don), thank you for the cautionary tale. I've always wished TeleVue could produce an Ethos 25/26, and this ES would have been a tempting dead end for me. I'll have to continue being satisfied with the Nagler 31 for my widest-feasible good FOV (which, by the way, can be had used/like new for much less than a new ES 25)...

#6 rlines

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Posted 14 July 2013 - 05:14 PM

Gentlemen...many thanks for your feedback. There are some interesting points made. It might be worth mentioning that while I have been fortunate enough to own some of the best eyepieces around, I'm not generally that critical of minor optical shortcomings. The ethos is not necessarily the most practical choice, especially with small scopes. I keep a selection of 1.25" eyepieces; Meade 24mm 4000 series, TV 24mm widefield (1980s), Orion megavista 10.5mm, Meade 8.8mm and 6.7mm ultrawides....all of these have minor defects which do not detract much from the viewing experience. All have been used in a wide variety of scopes. All were bought secondhand for not very much money and have given many years of service. In spite of 40 years in the hobby I couldn't tell you which is best for planetry observation from an ethos 8mm, TV 8mmm Plossl, or Baader or Zeiss orthos of the same FL. I know what the consensus opinion would be, but to be honest my eyes don't see that much difference. My beef with the ES25 is that the defects do detract from the observational experience, to the extent that I could not sell on the eyepiece with a clear conscience. I'm pretty certain that the next owner would be similarly disappointed. This was the main motivation for submitting the review. ES on thair website go out of their way to invite comparison with the best (by which they mean TV in this case?). This one eyepiece in their portfolio is found to be rather wanting on that basis....I suppose they would argue that there is no comparable product.
The value-for-money argument is a good one and rather different over here. In the UK all US imports become dollars-to-pounds plus dealer factor X. Thus there is an incentive to import directly from the US. In the UK the ES prices are certainly lower than TV but the relative differential is less, certainly for the 100 degree series. It's very much a premium product.
Just a little extra perspective...
RL

#7 star drop

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Posted 14 July 2013 - 05:44 PM

Hi Richard and welcome to Cloudy Nights.

#8 Starman1

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Posted 14 July 2013 - 05:49 PM

Richard doesn't mention the f/ratio of his scope (I suspect f/5), but this eyepiece seems to like f/6 much better than f/5 and it detests f/4.
I used it in a friend's f/6.5 scope a while back, and it was OK--much of the complaints that Richard and I had about the eyepiece were considerably reduced and a much larger % of the field was in sharp focus.
Of course, most of the users of this eyepiece will have short f/ratios and larger scopes, so that doesn't help, but I did want to point that out.

#9 russell23

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Posted 15 July 2013 - 08:39 AM

After reading the review I find myself wondering if Richard's speculation about incorrect assembly is correct - particularly because he says that the stars are even bloated in the center of the field. I've had numerous ES eyepieces including a 14mm ES100 and I have never seen one that had bloated stars at sharpest focus. In fact, my 20mm ES68 comes to sharper focus than the 20mm XW.

Dave

#10 obrazell

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Posted 15 July 2013 - 10:45 AM

I bought one recently at the IAS show and tested it on an F6 TMB triplet (80mm) I found that the centre of the field was sharp but towards the edge the focus went very quickly. As Don says the ring of fire effect was making it unusable on the Moon which is about all we get a shot at here in northern latitudes until August. My main scopes are F4.2 so Don's comments suggest the eyepiece will not be that usable. There was obviously a good reason why Al said it could not be done :-)

Owen

#11 Zamboni

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Posted 15 July 2013 - 01:10 PM

Unfortunately, the problem with long focal length for Ethos class eyepieces (and the ES100 EPs are optically clones of the Ethos) is that they are physically constrained to around 22mm due to the physical limitations of a 2" barrel. In order to make a longer focal length, compromises have to be made resulting in edge distortion which is something ES is apparently OK with doing, but TeleVue refuses to do.

This is why ES is introducing a new 3" barrel size for the 30mm 100* eyepiece, although I think it may ultimately be a fool's errand. Of course, that's what people said about Al Nagler's namesake eyepieces when they first came out.

#12 rlines

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Posted 15 July 2013 - 05:34 PM

russell23...
The focus is generally ok in the middle, although there is a touch of ambiguity in the exact focus which under other circumstances would pass un-noticed at this level of magnification. The issues really start about half way out. The scope is a 12" f/4 Newtonian so, yes, it is a tough test. But doubling the focal ratio with a barlow lens (of known good quality) fails to provide much of an inprovement! The main market for this ep will be big heavy dobs that can brush off the balance problems. My own comments after 2 weeks of ownership mirror virtually exactly what Don has said....it's not a one-off. Other comments are coming in to the same effect.; no-one has replied saying they've got a good one. They're all like this...You might as well take a 25mm 2" plossl and remove the field stop. RL

#13 Starman1

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Posted 15 July 2013 - 06:05 PM

Well, I don't know how much accommodation you have in your eyes, but try focusing on a star away from the center (try 25%, 50%) to see if that tames some of the field curvature. If you can still focus the center after you do so (and I suspect your accommodation will allow it), that does seem to reduce the edge of field aberrations somewhat. Because the edge issues are exacerbated by defocus when the center is dialed in.

I've had other eyepieces like that (35 Panoptic comes to mind), where focusing half way out allowed me to see an entire field more-or-less in focus.

And since the Paracorr will tame the coma and flatten the field slightly, that just might work to give you a larger portion of the field in good focus.

If you can get 75% of the field into sharp focus and only the edge deteriorates, then most of any edge problems will be outside the normal viewing field.

#14 JimP

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Posted 15 July 2013 - 08:42 PM

Hi Don,
Interesting that you say, "If you can get 75% of the field into sharp focus ... most of the edge problems will be outside the normal viewing field". That helps explain, perhaps, why I like eyepieces with a 70 degree FOV so much. Of course wider FOV eyepieces are nice because the remaining FOV (over 70 degrees) adds to the overall observing experience. It's like looking forward at the bookshelf and seeing the rest of the room with my peripheral vision which is not in sharp focus.

best,

JimP

#15 obrazell

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Posted 16 July 2013 - 01:02 PM

Looks like this one might be a pup :-( I had bought any ES eyepieces before and it looks like I won't again.

Owen

#16 obrazell

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Posted 16 July 2013 - 01:03 PM

Post deleted by iceblaze

#17 Shneor

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Posted 16 July 2013 - 02:28 PM

The review and following comments in this thread underscore the need to match an eyepiece with a specific purpose appropriate for the telescope or telescopes it is intended for. I own this eyepiece. It's an excellent eyepiece in my 22" for extended objects, especially when used with a filter, in my case, normally an NPB. For me, this eyepiece is far and away the best eyepiece I own for framing large objects. It gives superb views of M8, the Veil and M42, for starters. It's pretty useless as a finder eyepiece (in any case, I use my 9mm ES120 for that) or for other purposes in my 22".

Having said that, I own other ES eyepieces; they are uniformly excellent performers. I also own several Televue eyepieces, which are also excellent performers. For the best possible views, better than either ES or Televue products but with a smaller apparent field, I own a set of Baader Classic Orthos (see list below). And just a word about my 9mm ES120 (which actually has a true AFOV or almost 140 degrees) which is simply revolutionary, and also is as good as a Delos in the central field (same 72 degrees).

I won't bash Televue except to complain about their high prices, but ES has produced eyepieces rivalling Televue's quality at much more reasonable prices. Bashing ES and Scott Roberts - who I have met and who is not only a gentleman but a true innovator - is just off the mark. Hopefully, this competition will spur more innovation and even better prices all around.

Clears,

#18 TechPan6415

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Posted 16 July 2013 - 06:34 PM

I just ordered mine, will compare it to my ES82 30mm which I am pretty happy with but would appreciate the improvement on exit pupil. I intend to use this eyepiece in the manner in which Shneor does above, large, low surface brightness DSO's and low power star field work with my 16".

I live at altitude with some pretty dark skies so I will be curious to see how it measures up to the 82/30mm and my 100/14mm. If it is not to my liking, I will just return it and stick with the 30.

#19 JimMo

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Posted 16 July 2013 - 08:44 PM

I've had one for a while and have used it three times so far in my f4.3 dob w/ Paracorr II. I've tried to critically test it, but find I don't use this wide of a field very often and with observing time scarce I haven't really put it through it paces. That said I haven't noticed any problems with it and I get tight stars across the wide FOV using the same Paracorr II setting (A) as the 17 and 21 Ethos. I've only used it on DSO's at a semi dark observing site. There is a slight vignetting of the last 10% or so, but not objectionable at all. I haven't decided if I will keep it or not, but one or two of my longer f/l wide fields must go.

#20 davidpitre

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Posted 17 July 2013 - 03:11 PM

Bashing ES and Scott Roberts - who I have met and who is not only a gentleman but a true innovator - is just off the mark. Hopefully, this competition will spur more innovation and even better prices all around.

Clears,

Agreed. I have seen and heard nothing but good from Mr Roberts. Like the eyepieces or not, I think we should be respectful.

#21 thetortoise

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Posted 27 July 2013 - 03:05 PM

Such an interesting review that articulates what I don't have the experience to articulate myself! In short, I struggle with the 25mm - I thought for sure this would be an easy replacement for the ES30 82*, but here I am months later and I can't bring myself to sell the 30 (or the 25) but know that one of them "should" go for practical reasons.

Not that much I can add but to confirm this EP really is a nightmare on the moon (I do like to look at the moon wide-field occasionally) with a bright orange ring all the way around. I struggle somewhat with eye-placement as well, black-outs are common as I try to scan the sky - not excessive but it certainly detracts from the immersion experience. I do also find the light scatter to be excessive.

Finding a spot to focus well off-center does seem to create a nice balanced pleasing view, and it's nice to have the extra magnification in such a wide-field when compared to the pinpoint tighter view of the 30mm. Maybe one more night with this review in mind and I can finally come to a conclusion on whether to keep or let someone else try the 25mm...

#22 TechPan6415

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Posted 27 July 2013 - 06:17 PM

I like it a lot for all the reasons I bought it for, low surface brightness large dso's, wide field low power sweeps and a more workable exit pupil.

I also saw the orange on the moon but thankfully won't be using it for that instead opting for binoviewing.

It's not a one size fits all world, it's best to buy eyepieces based on specific needs rather than over enthusiastic consensus in my opinion....

I have since sold my ES 30, another great eyepiece...

#23 thetortoise

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Posted 27 July 2013 - 09:55 PM

Wide field low power sweeps are one of my favorite things. The exit pupil problem does not seem to affect me yet with the 30mm as far as I can tell, though something to consider for the future. My specific need is just the most pleasing subjective aesthetic experience of observing - as of yet I have been unable to determine which of the two EP's "wins" in that regard for me, even after months of observing with each. I'm certainly not looking for an external "consensus" to decide what my best subjective experience is, though reading what the specific phenomena I am experiencing with the EP's are actually called does certainly help with the assessment. After using the 20mm 100 I thought for sure the 25mm would be another "no contest" against the 30mm but it hasn't worked out that way which is disappointing only from a tough decision perspective (there are certainly worse problems in life than having two excellent EP's!). I agree these are two great EP's, which is better? Each observer has to decide for himself. Something I will either do, or, will continue to hold onto both because I just can't decide.

#24 JimMo

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Posted 19 September 2013 - 08:51 PM

As an update to my previous post I ended up selling the ES25/100 for many of the same reasons that the OP and Don P. had. I tried to like it, but the views through my 21 Ethos showed that hands down it a much better eyepiece, although the f/l is different as is the exit pupil. I traded a Nagler 31 for the ES 25 and don't know if I'll try to replace it. Didn't use the 31 as much the 21 Ethos and higher mag eyepieces, but maybe if I had a 22" or larger reflector I could see needing a wider field, but where I'm at in aperture I don't miss either of them at all.

Cheers,

#25 Scott in NC

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Posted 19 September 2013 - 09:17 PM

Thanks for the review, Richard. I really like my ES100 20mm eyepiece, but now I'm pretty sure that that's as wide as I'll go. Not that I was in the market for a *new* ES100 25mm, especially at that price, but if a gently used one had ever come up on the used market at a nice price, well, you never know... :)






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