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Redshirt
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Reged: 06/17/13

Unexpected viewing with the ES 24mm 68°
      #5938568 - 06/24/13 07:19 PM

I've spent the past few weeks trying out all three AFOV offerings by ES in a 16" 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?


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5938666 - 06/24/13 08:28 PM

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,


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AhBok
professor emeritus


Reged: 12/02/10

Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5938675 - 06/24/13 08:34 PM

Yes, I found the 24 ES68 gave a lot of seagulls in my scopes. The 24ES82 performs splendidly, however.

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Allan...
sage


Reged: 10/24/12

Loc: Penticton B.C. Canada
Re: Unexpected viewing with the ES 24mm 68° new [Re: AhBok]
      #5938702 - 06/24/13 08:56 PM

I received my 24mm ES 68° two weeks ago and have not been able to use it yet due to clouds. I sure hope I didn't make a mistake with ordering (and waiting 6 months) it.

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GOLGO13
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Reged: 11/05/05

Loc: St. Louis area
Re: Unexpected viewing with the ES 24mm 68° new [Re: Allan...]
      #5938709 - 06/24/13 09:05 PM

If it's like the Meade SWA 5000 24mm 68 degree it shouldn't be a bad eyepiece. I own it and it's pretty nice. I'll have to look for field curvature. May depend on the scope.

I've seen a lot of people compare it to the 24mm Panoptic and a lot of people prefer the SWA. Since you mentioned it I will give it a test in my refractor when I test my ES14mm 100 tonight. Will be a good comparison of similar fields of view.


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tomharri
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Reged: 09/19/08

Loc: USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: GOLGO13]
      #5938767 - 06/24/13 09:49 PM

I liked the ES24/68 so much that I sold the Pan24. Super sharp stars in center that continued all the way to edge, and this is in f/4.8 and f/5's. Maybe it's your mirror?

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Redshirt
member


Reged: 06/17/13

Re: Unexpected viewing with the ES 24mm 68° new [Re: tomharri]
      #5938813 - 06/24/13 10:16 PM

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 Comparison

Perhaps a clue there, though. The other option is a bad egg...


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GOLGO13
Carpal Tunnel


Reged: 11/05/05

Loc: St. Louis area
Re: Unexpected viewing with the ES 24mm 68° new [Re: tomharri]
      #5938864 - 06/24/13 10:57 PM

might be a bad sample you have. Again assuming the same as the 24mm meade SWA. Mine is quite sharp almost all the ways to the edge in my 103mm F7.7 refractor. I will say the ES14 100 seems a bit better overall...but hard to do a comparison with the eyepieces that far apart.

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russell23
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Reged: 05/31/09

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Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5939285 - 06/25/13 08:24 AM

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?




The ES68's work great with my petzval refractor. The field seems pretty flat to me. But others have reported problems with edge performance using these eyepieces in fast dobs.

Dave


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russell23
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5939291 - 06/25/13 08:27 AM

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,




Mark, I find the ES68's to be very consistent in performance. Since you didn't like the 28mm I'm sure you would have the same issues with the 24mm.

Dave


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Scanning4Comets
Markus
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Reged: 12/26/04

Loc: Ontario, Canada
Re: Unexpected viewing with the ES 24mm 68° new [Re: russell23]
      #5939364 - 06/25/13 09:25 AM

Dave,

You're more than likely right! Looks like my next purchase will have to be the mighty 30mm ES 82!

If it's anything like the Meade 5000 30mm 2" UWA, I'd like the 30mm ES 82!


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5940253 - 06/25/13 06:23 PM

Field curvature is worst in:
--refractors (especially short focal length refractors)
--short focal length newtonians (say, under 1000mm FL)
--SCTs

Refractors often have field flatteners available. SCTs (not the ACF or Edge) can be used with a f/6.3 reducer/corrector, and newtonians with focal lengths longer than 1000mm (39.37") are flat enough that the FC you see is probably in the eyepiece.

You can mitigate field curvature by focusing on a star away from the center and letting your eye accommodate for the center and edge. I used to do this with a TeleVue 35 Panoptic in my dob, and it worked well. Older people lack enough accommodation in their eyes sometimes, so this technique is not a universal one. It worked for me, though.


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rguasto
scholastic sledgehammer
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Reged: 11/18/10

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Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5940535 - 06/25/13 09:45 PM

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.

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Sarkikos
Postmaster
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5940652 - 06/25/13 11:14 PM

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?




Thanks for this report. I don't have the ES 24 68° but every once in a while I think about getting one because - judging from many previous reports - they are the Poor Man's 24 Pan. Now that I know about the FC, I'll permanently delete it from my Wish List.

I have a 10" f/5 Dob. As Markus said, the FC should appear even worse in a 10" f/5 than your Newtonian, since FC is inversely related to focal length in these scopes.

So far I've owned an ES 100° 9, ES 100° 14, ES 82° 30 and an ES 82° 14. Among these eyepieces, the only one in which the FC was obvious to my eyes was the ES 82° 14. That was one of the few eyepieces I've ever sold because an aberration was so bad it actually proved an obstacle to observing. Even with a 1.9x OCS in a binoviewer, the FC was so terrible I had to refocus three times as Jupiter drifted across the FOV.

I have an ES 100° 20 on order now. I did not have any obvious FC problem with the other two ES 100° eyepieces, so I have high hopes for the 20mm.

Mike


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5940657 - 06/25/13 11:22 PM

After reading through this and other threads on FC in eyepieces, I've come to the conclusion that the best test for FC in an eyepiece is a combination of a Newt with a long focal length and an observer who's eyes have poor focus accommodation (usually an old stargeezer). My telescope and I fit that description very well.


Mike


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Ernest_SPB
sage


Reged: 11/13/10

Loc: St.-Petersburg, Russia
Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5940870 - 06/26/13 03:14 AM

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.


According to my measuring eyepieces from 68-degree line of Explore Scientific have very moderate quality of field aberrations (curvature and astigmatism). Only 16 mm ep of the line has more or less balanced image quality at the edge of FOV, but it has very tight eyerelief.
The eyepieces could be recommended only for telescopes with slow F-ratio, like 1:8 and less.


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Ernest_SPB]
      #5940896 - 06/26/13 04:33 AM

Ernest,

Define "very moderate." I can't wrap my mind around that idiom. It's like saying, "very lukewarm."

According to your measurements does the ES 68° 24mm have little, moderate or severe field curvature? Malenkii, ymerennei ili cilnei?

Have you measured the ES 82° 14mm? To my eyes, the FC from that eyepiece was severe.

Mike


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kkokkolis
Carpal Tunnel
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Reged: 09/23/09

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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5940915 - 06/26/13 05:19 AM

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.


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Ernest_SPB
sage


Reged: 11/13/10

Loc: St.-Petersburg, Russia
Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5940916 - 06/26/13 05:27 AM

Hm...
English is not my strong side.
For me "moderate" applied to aberration correction is in a middle between "good" and "poor". "Very moderate" correction of aberration shift evaluation to "poor" (bad).

Measures for 24 mm 68ES showed size of aberration spot near 20 angular minutes 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.


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Jon Isaacs
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Reged: 06/16/04

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Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5940933 - 06/26/13 06:21 AM

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 Comparison

Perhaps a clue there, though. The other option is a bad egg...




Dave:

What scope(s) are you seeing the field curvature with?

I own the 24mm Meade version of this eyepiece and I have used with a number of scopes, most quite fast. It's not a perfect performer in the NP-101 (101mm F/5.4 Petzval), there is some slight residual what I remember to be astigmatism, it's not a clean as the 31mm Nagler, but it is a very good performing eyepiece, there aren't many better.

I spent one evening comparing the 24 mm Meade SWA to my 20mm Type 2 Nagler in a 16 inch F/4.42 + Paracorr, it was close... it's a good eyepiece in a fast scope.

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


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T1R2
Carpal Tunnel


Reged: 06/11/13

Loc: NeverWhere, 35*N
Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5940947 - 06/26/13 06:45 AM

That's not good, I was planning to get the 24, 20, and 16 for my AR127, there advertised as flat field ep's, how would they perform in ES's F/6.5 achro's? I would think they would work good from 6.5 and slower, any thoughts on this set up?

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Miliu
member


Reged: 05/13/13

Loc: Japan
Re: Unexpected viewing with the ES 24mm 68° new [Re: kkokkolis]
      #5940948 - 06/26/13 06:46 AM

Quote:

I found the ES far better than the Hyperion, especially at f/4, and almost as good as the TV and Vixen.




The ES68 24mm and LVW 22mm cost about the same here. The LVW can get a bit cheaper when on sale. The ES68 24mm was on my list but I wonder whether I should go for the LVW instead as there seems to be contradictory reports for the ES.

Miliu


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Redshirt
member


Reged: 06/17/13

Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5941001 - 06/26/13 08:06 AM

Quote:

What scope(s) are you seeing the field curvature with?



Jon - the scope is a 16" f/5 (see opening post).


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Redshirt
member


Reged: 06/17/13

Re: Unexpected viewing with the ES 24mm 68° new [Re: rguasto]
      #5941017 - 06/26/13 08:19 AM

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.



Thanks for the report, rguasto. "Obvious" and "annoying" are also how I would describe it.


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Sarkikos
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Reged: 12/18/07

Loc: Scotophobe Maryland, USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: kkokkolis]
      #5941138 - 06/26/13 09:59 AM

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.




"Seagulls" would be from coma which is from the primary mirror, not the eyepiece. What the OP is talking about is field curvature. This occurs when you need to focus separately for on-axis and off-axis. If you focus for on-axis, off-axis will be fuzzy. If you focus for off-axis, on-axis will be fuzzy. This is not coma, not astigmatism, not spherical aberration, not etc. ... it is field curvature.

A Newtonian over about 1000mm focal length will have essentially zero field curvature. If you see FC when using an eyepiece in such a telescope, the FC will be from the eyepiece. BUT - and this is important but seems to be often forgotten or ignored or not known at all - some observers' eyes can compensate for the FC, while other observers' eyes cannot. Like many older folks, I cannot compensate for the FC. If FC is in the eyepiece, I will see it. Guaranteed.

So you have to keep in mind the variance in native FC coming from different types of telescopes, and the variance in the ability of different observers to compensate for FC when present in an eyepiece. Otherwise you can end up with opposing arguments over a misunderstanding.

The bottomline is that sometimes an observer cannot see FC in an eyepiece, even though it definitely is there.

You say you see bloated stars near the edge with the ES 68 24 in an f/5 Dob? Do these stars at the edge come into focus if you focus for them? And when you do that, do stars on-axis become bloated? If so, you are seeing FC.

Also keep in mind that FC is inversely related to the focal length. The shorter the focal length, the worse the FC. FC does not depend on f number.

I think I've said enough about FC.


Mike


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5941158 - 06/26/13 10:14 AM

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


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Ernest_SPB]
      #5941161 - 06/26/13 10:17 AM

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.




Could you detect field curvature?

Mike


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5941200 - 06/26/13 10:42 AM

Mike,
Typical limit resolution for the eye is about 3 arc minutes.
Above that limit, aberrations gradually become visible, e.g. coma, astigmatism, field curvature, etc.
So, given average vision, a 6' aberrations would barely be visible to most observers, if at all. A 20' aberration would be visible to everyone.


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Sarkikos
Postmaster
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Loc: Scotophobe Maryland, USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5941219 - 06/26/13 10:54 AM

OK... you're coming closer to giving me a direct answer. So you're saying the OP would probably not have been able to see field curvature in the ES 68 24? He said the FC was "annoying," which strongly implies that it was obvious, and not just "barely visible." He also said, "The amount of adjustment required to alternate between on-axis and edge-of-field sharpness was surprising." I tend to allow competent field observations to trump bench tests and stats derived from neural-typicals.

I wonder how the FC in that eyepiece compares to the definite, obvious and annoying FC I saw in the ES 82 14.

Mike


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Starman1
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5941264 - 06/26/13 11:12 AM

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.

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!]
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.

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.


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Sarkikos
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Loc: Scotophobe Maryland, USA
Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5941286 - 06/26/13 11:28 AM

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.




I should have referred back to Earnest's figures and figured that out for myself. But I have this annoying habit of preferring direct answers to simple questions, even when math is involved ... or maybe especially when math is involved.

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!]




The 35 Pan: another eyepiece to put on my Deathwish List!

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.




That trick will work for me with some eyepieces, but not with others. I could not accommodate the ES 82 14 with this half-way trick, not even in a Paracorr. It worked very well for me with the XW 20 once I put it in a Paracorr.

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.




But we can do some things to try to narrow down the probable eyepiece aberrations. One is to use a Paracorr in a Newt to reduce coma and flatten the field somewhat. To eliminate the scope as a source of FC, we should use a Newt with a focal length over 1000mm or a very-well-corrected refractor. AFAIK, when testing for FC, I don't trust achromats because their FC can be positive, negative, who knows? Also, for a field test, it's much better if the observer is sensitive to FC, i.e., does not have very good focus accommodation.

Mike


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Jon Isaacs
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5941304 - 06/26/13 11:40 AM

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




Mike:

I am speaking from experience here, I am an old geezer whse eyes are close to fixed focal length. I do have an NP-101 as well as a variety of fast Newtonians plus a Paracorr. A Newtonian with a Paracorr and eyepieces like the 31mm Nagler still show some residual off-axis aberrations such as off-axis astigmatism inherent in a Newtonian, stars are not perfectly round at the edge of the field.

It is true that coma is the primary off-axis aberration in a Newtonian and it is also true that a Paracorr goes a long way to correcting the coma but in a fast Newtonian, there are other visible, residual off-axis uncorrected aberrations.

I am sure this is also true of any optic but in my experience, the NP-101 and probably other scopes like the Takahashi FSQ series do provide the tightest off-axis stars for fast scopes.

It was also not clear whether the original poster was using a coma corrector with the 24mm ES. Don has pointed out that often a combination of aberrations is more distracting than a single aberration, I suspect that might be the case here. The combination of coma and astigmatism can appear very much like field curvature, when there are multiple aberrations present, it is more difficult to precisely identify the components.

I spent about an hour one evening under dark skies comparing the 16mm and 24mm Meade Series 5000 eyepieces to the 16mm and 20mm type 2 Naglers. I used my 16 inch F/4.42, mostly with a Paracorr so the effective focal ratio was F/5.07. With the Paracorr, both eyepieces were very similar in edge correction to the Naglers.

As I wrote previously, the 24mm SWA is a very good but not perfect performer in the NP-101. My recollection is that it showed a small amount of off-axis astigmatism but if all goes as planned, I will know tonight...

Jon


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5941320 - 06/26/13 11:51 AM

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


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Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5941344 - 06/26/13 12:11 PM

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.

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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5941363 - 06/26/13 12:28 PM

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




Mike:

In my experience, the problem is that there is likely a mix of the three aberrations, so achieve best focus off-axis means finding the place where the sum of the aberrations is a mininum. The fact that this minimum may not occur when the center is optimally focus does not necessarily mean that there is significant field curvature. A Paracorr certainly helps but it's still not perfect but that's why I pointed to my experience with the Meade 24mm SWA and the 16 inch operating at F/5.07 with the Paracorr, it's a very good performer in a coma corrected Newtonian.

But situations like this are why I think that scopes like the NP-101 are the toughest on eyepieces, an eyepiece can't behind the inherent aberrations in the scope.

Jon


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5941396 - 06/26/13 12:53 PM

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.




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


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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5941420 - 06/26/13 01:12 PM

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




It's a lot better than coma or astigmatism. With FC, you can refocus.

Jon


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5941591 - 06/26/13 03:07 PM

Not IME. Most of the eyepieces I use at moderate to high power in a binoviewer have a comparatively narrow AFOV, which will in itself limit the degree of coma seen. I use eyepieces with comparatively narrow AFOVs because those tend to be ones which give me the sharpest images and work best in my binoviewer. (Disclaimer: Not all eyepieces with comparatively narrow AFOV - or "simple glass," for that matter - will be sharper!) Also, they tend to be well corrected for astigmatism. What I see is predominately FC - if FC is present in the eyepiece.

My larger scopes do not have mounts that track. I would rather not have to refocus three or more times as the planet's disk drifts past ... while I'm trying to tease out fine surface details and perhaps even working on a sketch. IME, FC is the very worst of the common aberrations for an eyepiece to have during such an observing session. If FC is present, it's time to sell the eyepieces - or only use them for deep sky with a Paracorr and hope that the Paracorr flattens the field enough for my eyes.

Mike


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Jarrod
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Redshirt]
      #5941811 - 06/26/13 05:33 PM

This is consistent with my findings. It's a good and very well-constructed eyepiece for the price, but it's not perfect. And why does this thread already have like 4x the views and umpteen more replies than my review does? I put a lot of effort into that after I think three different people asked me to do it! I got crickets...

Edited by Jarrod (06/26/13 06:02 PM)


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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5941898 - 06/26/13 06:29 PM

Quote:


"Seagulls" would be from coma which is from the primary mirror, not the eyepiece.




Coma produces comet-tailed aberrations, not seagulls.
Seagulls are astigmatism - very much an eyepiece issue.


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Starman1
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Re: Unexpected viewing with the ES 24mm 68° new [Re: great_bear]
      #5941953 - 06/26/13 06:51 PM

Quote:

Quote:


"Seagulls" would be from coma which is from the primary mirror, not the eyepiece.




Coma produces comet-tailed aberrations, not seagulls.
Seagulls are astigmatism - very much an eyepiece issue.



Well, an actual seagull shape can be a combination of coma and astigmatism.
If the sky background is bright, rather than look like a comet, coma may look like a "V" with its point aimed at the center of the field. The "V" will be narrow, though, and not as wide as >
If that "V" is widened until it has a seagull shape, it is likely to be a combination of coma and astigmatism.


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Scanning4Comets
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5942141 - 06/26/13 09:11 PM

ASTIGMATISM:

http://tinypic.com/view.php?pic=hvuqzr&s=5

COMA:

http://tinypic.com/view.php?pic=6tkyzm&s=5


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csrlice12
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5942186 - 06/26/13 09:38 PM

Thanks Markus. Sometimes a picture is worth a thousand words.

Edited by csrlice12 (06/26/13 09:40 PM)


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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5942195 - 06/26/13 09:45 PM

When you combine EYEPIECE astigmatism with coma, it does make "seagulls". TELESCOPE astigmatism produces images that are elongated in one axis that then flip to elongation of the other axis depending on focal position. Slightly inside of focus the astigmatic image will elongate on one axis and on the other side it will elongate on the other axis.

EYEPIECE astigmatism take takes a comet-shaped comatic image from the telescope and spreads out or fans the light of the "tail," forming seagulls and other similar distortions of the comet-like shape. Here are some examples:

EYEPIECE astigmatism:

http://www.telescope-optics.net/images/epa.PNG

TELESCOPE astigmatism:

http://www.telescope-optics.net/images/13d.PNG

TELESCOPE coma:

http://www.telescope-optics.net/images/11d.PNG

You can see how combining the first and third would produce fans or seagulls. Eyepiece astigmatism is common when a less well corrected eyepiece encounters a fast light cone. Most Dobs are fast scopes delivering a fast light cone. Fast dobs also show telescope coma from the steeply parabolized mirror. Telescope coma plus eyepiece astigmatism gives you little fans or seagulls off axis depending on the degree of each aberration and which one predominates. Haven't really checked to see how eyepiece astigmatism, telescope coma and telescope defocus manifest in off-axis SCT images. For me I think the defocus is so pronounced with wide field eyepieces that encourage me to care about off-axis image quality in my SCTs that I can't tell in what other ways the blobby defocused stars are also distorted.

- Jim


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Sarkikos
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Re: Unexpected viewing with the ES 24mm 68° new [Re: jrbarnett]
      #5942280 - 06/26/13 10:55 PM

Very instructive pics and text, Jim and Markus!

Mike


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Scanning4Comets
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Sarkikos]
      #5942328 - 06/26/13 11:32 PM

Great additions and images there Jim

Cheers,


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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #5948270 - 06/30/13 03:48 PM

Regarding astigmatism:

Astigmatism means that an optic has two different focal lengths which are at 90 degrees to one another. In the absence of other aberrations, it means that when one orientation is focused, the other orientation will be out of focus, creating a line. "Best focus" is somewhere in the middle.

Anyway I spent the past 4 nights, out under dark skies and did some comparisons and evaluations with the Meade 24mm SWA. Basically it's a good eyepiece.

16 inch F/4.42: Without the Paracorr, coma was visible.. maybe some other stuff. With the Paracorr (F/5.07), I saw no field curvature, the edge correction was about as good as it gets in a fast Newtonian.

TV NP-101. As previously noted, I saw no obvious field curvature but there are some small off-axis aberrations, sufficient to be noticeable but small enough that I could not determine their nature. Near as I could tell it was probably astigmatism. With the NP-101, I expect the off-axis stars to be as sharp and clean as those in the center, this eyepiece, this eyepiece was very good but not that good. In comparison, the sometimes maligned 22mm Panoptic showed perfect tight stars across the field of view.

In another forum, another fellow and I went round and round about the 16mm Series 5000 Meade. He claimed it had coma, I thought it must be astigmatism. I finally purchased a 16mm SWA simply to resolve this question. It did seem like it had coma but that it went away when used with a Paracorr.

It does seem unlikely but I wonder if the reverse coma an coma correct adds could also affect the eyepiece.

Jon

Edited by Jon Isaacs (06/30/13 03:51 PM)


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Starman1
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Jon Isaacs]
      #5948407 - 06/30/13 05:32 PM

Quote:

Regarding astigmatism:

Astigmatism means that an optic has two different focal lengths which are at 90 degrees to one another. In the absence of other aberrations, it means that when one orientation is focused, the other orientation will be out of focus, creating a line. "Best focus" is somewhere in the middle.

Anyway I spent the past 4 nights, out under dark skies and did some comparisons and evaluations with the Meade 24mm SWA. Basically it's a good eyepiece.

16 inch F/4.42: Without the Paracorr, coma was visible.. maybe some other stuff. With the Paracorr (F/5.07), I saw no field curvature, the edge correction was about as good as it gets in a fast Newtonian.

TV NP-101. As previously noted, I saw no obvious field curvature but there are some small off-axis aberrations, sufficient to be noticeable but small enough that I could not determine their nature. Near as I could tell it was probably astigmatism. With the NP-101, I expect the off-axis stars to be as sharp and clean as those in the center, this eyepiece, this eyepiece was very good but not that good. In comparison, the sometimes maligned 22mm Panoptic showed perfect tight stars across the field of view.

In another forum, another fellow and I went round and round about the 16mm Series 5000 Meade. He claimed it had coma, I thought it must be astigmatism. I finally purchased a 16mm SWA simply to resolve this question. It did seem like it had coma but that it went away when used with a Paracorr.

It does seem unlikely but I wonder if the reverse coma an coma correct adds could also affect the eyepiece.

Jon



John,
If the coma corrector corrects the coma from the objective, the image provided to the eyepiece will have no coma.
If you saw coma without a coma corrector and no coma with it, it isn't because the coma corrector canceled the coma in the eyepiece--it's because it canceled coma in the objective.
It's not totally impossible for an eyepiece to produce coma, but it is usually a characteristic of extremely simple lens designs, which the 16mm does not qualify for.

I've tested this eyepiece, and it has mild field curvature, a mild-to-moderate edge of field astigmatism, minimal lateral chromatic aberration, and mediocre suppression of off-axis or out-of-field scattered light, but decent suppression of peripheral light reflection off the top of the eyepiece. Its best performance wasn't in the NP101, but in my f/12.8 Maksutov. It was out-performed by a 16 Nagler, but at 3X the price.
If a coma corrector improved the images in this eyepiece and it was not coma you were talking about, it would likely to be the mild field flattening of the Paracorr at work. Or the mild f/ratio change affecting the induced astigmatism in the eyepiece from the light cone.


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Jon Isaacs
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5948860 - 06/30/13 11:11 PM

Quote:

John,
If the coma corrector corrects the coma from the objective, the image provided to the eyepiece will have no coma




Don:

That's how it seemed to me but consider this:

Imagine that you do have an eyepiece with coma. You also have a moderately fast Newtonian and a Paracorr. Now I believe that if the relationship between the Paracorr and the focal plane is not correct, then either too much negative coma is added or not enough negative coma is added. This would seem to depend on which side of the "proper relationship you are."

Now rather than adjusting the distance between the focal plane and the Paracorr so that it provides the appropriate amount of negative coma to correct only the mirror, you experimentally adjust the relationship between the Paracorr and the eyepiece so it minimizes the coma of the mirror + eyepiece.

Of course there are some assumptions about how the Paracorr works but it does seem at least plausible that the Paracorr could be adjusted to so that it adds more negative coma than is necessary to correct the mirror.

If that were the case, then correcting for coma in an eyepiece would seem to be possible.

Jon


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Scanning4Comets
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Starman1]
      #5948869 - 06/30/13 11:16 PM

Quote:

Dave:

What scope(s) are you seeing the field curvature with?

I own the 24mm Meade version of this eyepiece and I have used with a number of scopes, most quite fast. It's not a perfect performer in the NP-101 (101mm F/5.4 Petzval), there is some slight residual what I remember to be astigmatism, it's not a clean as the 31mm Nagler, but it is a very good performing eyepiece, there aren't many better.

I spent one evening comparing the 24 mm Meade SWA to my 20mm Type 2 Nagler in a 16 inch F/4.42 + Paracorr, it was close... it's a good eyepiece in a fast scope.

Jon




Jon,

I have owned the 28mm Meade SWA and also the 28mm ES 68. You'd think that both were similar performers. However, in my fast scope, the Meade 28mm SWA showed a lot less coma than the ES 68 which is quite similar. I found that weird.

There must be something quite different in the two brands, (ES 68 / Meade SWA Series).

Cheers,


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Scanning4Comets
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #6591190 - 06/19/14 04:59 PM

Quote:

I've spent the past few weeks trying out all three AFOV offerings by ES in a 16" 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?

--------------------
Dave




After selling this very same eyepiece, I tried it again just last week...and the FC was extremely annoying! So, mine was not a bad sample either.

It IS there and it nearly drove me nuts, LOL!

I'm glad I was able to hand it back to the owner and not have to put it in my eyepiece case again!

I don't understand how someone can say they've "got rid of their 24mm Pan" in favor of this EP because as far as sharpness goes, the 24mm pan just kills it from center to edge. You can slice it, dice it, any way you like but there is no way that this eyepiece is anywhere near as good as the 24mm Pan in the sharpness dept in a fast reflector.


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Dave Bush
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #6591322 - 06/19/14 06:24 PM

I read this thread a while back when I was doing research on the ES eyepieces. I ended up getting one and so far, in my C8 at f/10 I see no more FC than any other eyepiece which tells me the FC is from the scope itself (to be expected).

With the f/6.3 RC, as with my other eyepices, the FC goes away (or at least to the point that I can't detect it anymore).

Perhaps there is variation with this one.


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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #6592755 - 06/20/14 02:46 PM

Quote:

Quote:

I've spent the past few weeks trying out all three AFOV offerings by ES in a 16" 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?

--------------------
Dave




After selling this very same eyepiece, I tried it again just last week...and the FC was extremely annoying! So, mine was not a bad sample either.

It IS there and it nearly drove me nuts, LOL!

I'm glad I was able to hand it back to the owner and not have to put it in my eyepiece case again!

I don't understand how someone can say they've "got rid of their 24mm Pan" in favor of this EP because as far as sharpness goes, the 24mm pan just kills it from center to edge. You can slice it, dice it, any way you like but there is no way that this eyepiece is anywhere near as good as the 24mm Pan in the sharpness dept in a fast reflector.




I had and tested all (3); 24 Pan, 24 Meade SWA, 22 Pan. Yes the 24 Pan was the sharpest, and also had the shortest eye relief (no glasses or Dioptrx). I found the Meade to be much easier to use, with ample eye relief. I sold both Pan's.


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Scanning4Comets
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Re: Unexpected viewing with the ES 24mm 68° new [Re: SteveG]
      #6592856 - 06/20/14 03:46 PM

Eyepiece performance is 100% scope dependent.

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Starman1
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Re: Unexpected viewing with the ES 24mm 68° new [Re: Scanning4Comets]
      #6592875 - 06/20/14 03:54 PM

Quote:

Eyepiece performance is 100% scope dependent.



Well, that's an exaggeration. There are lots of eyepieces that perform quite well at f/6 and there are others that don't.
Let's just say that one of the factors influencing the performance of an eyepiece in the outer parts of the field is the f/ratio of the scope.
Given how different eyepieces appear in my f/12.8 Maksutov, and some f/15 refractors, though, I'm inclined to think that eyepiece design is also pretty important.
Eyepieces that are really poor performers at f/5 are usually less than perfect at f/8 too.


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