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Equipment Discussions >> Eyepieces

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T1R2
Pooh-Bah


Reged: 06/11/13

Loc: NeverWhere
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
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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
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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
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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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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
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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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Reged: 06/24/03

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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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Reged: 12/18/07

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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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Reged: 06/16/04

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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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Redshirt
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Reged: 06/17/13

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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Jon Isaacs
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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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Jon Isaacs
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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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Reged: 01/20/13

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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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great_bear
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Reged: 07/05/09

Loc: Walthamstow, London, UK
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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