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Nagler 22 mm (Type 4) in APO: Would I see field curvature?

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#1 db2005

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Posted 22 July 2019 - 12:45 AM

Hello all,

 

I'm considering getting myself a Nagler 22 mm Type-4 eyepiece, and I would appreciate your advice.

 

The Nagler 22mm would likely become my primary wide-field eyepiece and would be used mainly for my SD81S f/7.7 and Tak FC-100 f/8. I already own a Bresser 40 mm 2" eyepiece but the exit pupil is a bit too large for my skies (around 5.2 mm), and I think I see too much field curvature in this eyepiece.

 

I do like the prospects about the Nagler 22 because it offers a generous 19 mm eye relief as I need to wear eyeglasses correcting for astigmatism. However, I've read posts about CN'ers reporting seeing field curvature with the Nagler 22, but these reports seem to be from observers using reflectors without a Paracorr. So, I'd like to ask the collective wisdom and experience on CN for advice and insights: Should I expect to see field curvature when using a Nagler 22 Type 4 in my APOs?

 

There is a small catch: I already own a Pentax XW 20 mm which is in many ways an excellent 70-degree eyepiece. However, I think it too has a bit too much field curvature when comparing with my Delos 14, so I'm wondering if the Nagler 22 would be better. The ES 92 degree 17 mm eyepiece is also an outsider-option under consideration, but I'm somewhat put off by its massive size and weight, but on the other hand it does seem to get rave reviews by owners. By contrast, the Nagler 22 is an older design but still gets much love from its owners.

 

Any insights will be appreciated.

 

CS,

Daniel


Edited by db2005, 22 July 2019 - 01:20 AM.


#2 Ernest_SPB

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Posted 22 July 2019 - 03:24 AM

Scope with most curved FOV is yours Vixen SD81S

But even it has small FC appearance within 30 mm FS of the Nagler. 

You will not see it 


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#3 CHASLX200

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Posted 22 July 2019 - 05:57 AM

At F/7.7 or slower you should be fine. At below F/6 you will see it


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#4 db2005

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Posted 22 July 2019 - 06:06 AM

It is my understanding that field curvature in a refractor is a function of focal length rather than focal ratio. Since my APOs are 625 and 800 mm focal length, I was wondering if the relatively short focal length might pose a problem despite being significantly slower than f/7. But on the other hand... if the Nagler 22 didn't work well in my doublet APOs I guess it wouldn't work well in the APOs made by TV either, and I find that very unlikely... hmm.gif

 

Still, if anyone can share their first-hand experience using the Nagler 22 in an APO I'd appreciate it.


Edited by db2005, 22 July 2019 - 06:06 AM.

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#5 Axunator

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Posted 22 July 2019 - 06:27 AM

Yet another poster without first-hand experience with the mentioned combo, apologies for that. However, I have this to say:

1) while FC is a function of focal length, the defocused blur size of a star is a function of focal ratio. I.e., in a faster scope, similar amount of defocus due to FC leads to larger/blurrier star images than in a slower scope. That’s why f-ratio affects the visibility of the phenomenon.

2) Visibility of FC depends strongly on the accommodation ability of the observer (which, by and large, is a function of age in most human populations, with high interindividual variability).

Because of 2), you will not know the answer before you have tried it yourself.

I’m 47, and already have quite a bit of presbyopia, yet I enjoy the views of Nagler 26T5 (and even Pan 41) in my 500 mm fl (f/6.25) and 740 mm fl (f/7.4) APOs, without FC killing the pleasure at all. Since modern Naglers (including Type 4, of which I have 12 and 17 mm) are quite well corrected for EP-related FC, I find it difficult to believe that 22T4 would be particularly notorious for internal FC. Pentax 20XW, on the other hand, does have quite a bit in my understanding (please feel free to correct me if I’m wrong), so you can’t really base your presumptions for 22T4 on your experiences with 20XW.
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#6 db2005

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Posted 22 July 2019 - 06:41 AM

That's interesting... things are often a bit more complex than one believes at first waytogo.gif. Thanks for the additional info on field curvature.

 

And you are right about the Pentax XW: the 20 mm and the 14 mm versions are well known for having noticeable field curvature. The 10 mm and 7 mm versions are brilliant though, with great edge-to-edge sharpness in my scopes. The field curvature in my XW 20 is one of the reasons I'm looking into the N22T4, but since trying before buying isn't an option for me, I'd like to have some confidence that my views will be noticeably improved in exchange for the extra $ and the additional effort involved in using a 2" eyepiece among all my 1.25" eyepieces. If a ~20-25 mm 1.25" Delos existed that too would be considered.



#7 Vla

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Posted 22 July 2019 - 06:42 AM

What someone else sees is not necessarily what you would. It depends on your power of accommodation, and also your personal aberration acuity/tolerance. You are right about image field curvature, with refractors it is about 0.35-0.40 of the focal length. So, if we take, say, 250mm as the worst case scenario, it is matched with 18-19mm field stop radius of the Nagler. that gives the sagitta for the field edge of about 2/3 mm. With 10mm ep, it would require more than 6 diopters accommodation, with 22 less than half as much, or about 3 diopters. That corresponds to refocusing from infinity to about 20 times the eye focal length, or 35cm. Since the Nagler's field is practically flat, and the refractor's image is convex toward the eye, it bows away from the image plane of the eyepiece, which means that the outer field points will produce converging (instead of parallel) exit pencils. That requires the eye lens to relax beyond the shape needed for infinity focus, which is significantly harder for it than the natural, negative accommodation. So, you are likely to notice field curvature, but how much depends on you individual eye function and sensitivity.


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#8 CHASLX200

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Posted 22 July 2019 - 06:45 AM

What someone else sees is not necessarily what you would. It depends on your power of accommodation, and also your personal aberration acuity/tolerance. You are right about image field curvature, with refractors it is about 0.35-0.40 of the focal length. So, if we take, say, 250mm as the worst case scenario, it is matched with 18-19mm field stop radius of the Nagler. that gives the sagitta for the field edge of about 2/3 mm. With 10mm ep, it would require more than 6 diopters accommodation, with 22 less than half as much, or about 3 diopters. That corresponds to refocusing from infinity to about 20 times the eye focal length, or 35cm. Since the Nagler's field is practically flat, and the refractor's image is convex toward the eye, it bows away from the image plane of the eyepiece, which means that the outer field points will produce converging (instead of parallel) exit pencils. That requires the eye lens to relax beyond the shape needed for infinity focus, which is significantly harder for it than the natural, negative accommodation. So, you are likely to notice field curvature, but how much depends on you individual eye function and sensitivity.

True. FC drives me nuts. Some eyepieces make it even worse. Try the older TV wide fields or the Meade 4000 SWA's in a sub F/6 fract while sweeping.


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#9 RAKing

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Posted 22 July 2019 - 07:34 AM

The effects of field curvature are dependent on your eyes - so no one can answer your question.  Some people can easily adapt to it, others cannot.  The Pentax XW14 has a reputation here on CN for lots of field curvature, but I think it's a wonderful eyepiece and don't have any problems with it.

 

I use - and love - the TV 22mm T4 Nagler in all of my refractors (listed below) and it works great!  I do not have to wear glasses, but I went through cataract surgery a few years ago and now I feel much better using eyepieces with lots of eye relief.  The 22T4 is comfortable and its FOV is just wide enough for me to enjoy.  (Sadly, Ethos are no longer comfortable for me.)  If there is any field curvature, it's not enough to bother me.

 

FWIW - I also used the 22T4 Nagler in my old 10-inch f/4.7 Dob to star hop my way to the Astro League's Messier certificate many years ago.  I did use a Paracorr to fix the coma and the eyepiece worked great in that scope, too. smile.gif

 

Cheers,

 

Ron


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#10 SteveG

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Posted 22 July 2019 - 08:44 PM

It is my understanding that field curvature in a refractor is a function of focal length rather than focal ratio. 

That is correct, and coincides with what I've seen at the eyepiece. As others have noted, how much you will see is dependent on the person.

 

I had an 80 mm apo that I purchased for extreme wide fields. It had a 545 mm focal length, and the field curvature was terrible to my eyes, using a 27 mm Panoptic. I also had a 110 mm scope with a 660 mm focal length. As I recall, the field curvature was much less pronounced, but again my widest field eyepiece was the 27 Pan.

 

You have good reason to be concerned with your 625 mm focal length refractor. I can say with confidence that the 22NT4 will not introduce any other artifacts, such as astigmatism or field curvature of its own. And with that, I can highly recommend it, as it will be phenomenal in your Tak.

 

ps: The N22 is a great eyepiece to couple with a Dioptrx. I find it much sharper that using my plastic lens (progressive) eye glasses.


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#11 Jon Isaacs

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Posted 22 July 2019 - 11:10 PM

That is correct, and coincides with what I've seen at the eyepiece. As others have noted, how much you will see is dependent on the person.

 

I had an 80 mm apo that I purchased for extreme wide fields. It had a 545 mm focal length, and the field curvature was terrible to my eyes, using a 27 mm Panoptic. I also had a 110 mm scope with a 660 mm focal length. As I recall, the field curvature was much less pronounced, but again my widest field eyepiece was the 27 Pan.

 

You have good reason to be concerned with your 625 mm focal length refractor. I can say with confidence that the 22NT4 will not introduce any other artifacts, such as astigmatism or field curvature of its own. And with that, I can highly recommend it, as it will be phenomenal in your Tak.

 

ps: The N22 is a great eyepiece to couple with a Dioptrx. I find it much sharper that using my plastic lens (progressive) eye glasses.

 

I think the visibility of field curvature depends on both the focal length and the focal ratio.  The radius of curvature depends on the focal length and size of the defocused blur depends on the amount of curvature and the focal ratio.  If the curvature is say 1 mm, the diameter of the blur will be 1mm/f where f is the focal ratio.  

 

Jon


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#12 db2005

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Posted 23 July 2019 - 04:40 AM

Definitely food for thought... but it seems like I can reasonably expect a Nagler 22T4 to perform extremely well in my APOs.

 

Thanks for all the insights! bow.gif



#13 Ernest_SPB

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Posted 23 July 2019 - 07:14 AM

for SD81S f/7.7 at edge of FS meridianal defocus 0.5 mm, sagittal - 0.25 mm, in average 0.37 mm

it produces spot with diameter 0.37/7.7 = 0.05 mm

so observer will see the spot under angle 0.05/22*57.3*60 = 7.8 angular minute 

it much less then own aberration spot of the 22 mm Nagler

so 

You will not see it

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#14 Vla

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Posted 23 July 2019 - 09:12 AM

The exit pupil for 22mm f.l. ep at f/7.7 is 22/7.7=2.9mm. That gives the angular (green) Airy disc size in it as 4.6/2.9=1.6 arc minutes. I'm pretty sure that the ray spot plot of the Nagler, at the field edge, is nowhere close to 7.8 arcmin, let alone much more.


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#15 iKMN

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Posted 23 July 2019 - 10:06 AM

Just another example... I bought a 480Fl 80mm refractor for wide field viewing thinking I was really upgrading from my 600FL 80mm. Oh what a mistake. I’m 48 y/o and my eyes can not adjust to the field curvature. To make matters worse I have astigmatism with exit pupils over 4mm.

I now use a 24mm/68 with that scope as my largest FOV. It’s still around 3 degrees and I can somehow accommodate the FC easier.

Good luck and best wishes
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#16 Starman1

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Posted 23 July 2019 - 03:56 PM

It is my understanding that field curvature in a refractor is a function of focal length rather than focal ratio. Since my APOs are 625 and 800 mm focal length, I was wondering if the relatively short focal length might pose a problem despite being significantly slower than f/7. But on the other hand... if the Nagler 22 didn't work well in my doublet APOs I guess it wouldn't work well in the APOs made by TV either, and I find that very unlikely... hmm.gif

 

Still, if anyone can share their first-hand experience using the Nagler 22 in an APO I'd appreciate it.

TeleVue refractors like the NP101 and NP127 have incorporated field flatteners, so actually have flatter focal planes than most reflectors, even though the radius of curvature of a normal

refractor is about 1/3 of its focal length.

You can calculate the number of millimeters of deviation at the edge of a 31.1mm field in a telescope with a ROC of 1/3 the focal length of your scopes.

And that is smaller than the aberration of the 22mm Nagler, so:

Likely not visible, as Ernest says.

 

And, should it be under some unusual conditions, you can always add a field flattener.


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#17 Vla

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Posted 23 July 2019 - 07:29 PM


You can calculate the number of millimeters of deviation at the edge of a 31.1mm field in a telescope with a ROC of 1/3 the focal length of your scopes.

And that is smaller than the aberration of the 22mm Nagler, so:

Likely not visible, as Ernest says.

How do you figure the defocus of 240-300mm field curvature at f/7.7 and f/8 is smaller than the aberration of the Nagler? That would require it to be 10+ times worse than the 1988 patent.

Attached Thumbnails

  • def22.png

Edited by Vla, 24 July 2019 - 07:05 AM.

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#18 Starman1

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Posted 23 July 2019 - 08:05 PM

Vlad,

Ernest measured the 17mm T4, but not the 22mm.

The 17mm and 12mm are worse than 10 arc minutes at the edge at f/10 and worse than 20 arc minutes at the edge at f/4

The 22mm, at least in my scopes, is no better than the 17mm or 12mm, so I'll assume the same figures for the 22mm T4.

Ernest calculated a 7.8' blur on the OP's f/7.7 scope due to FC. (post #13).

The blur at the edge of the 22mm is somewhere between 10' and 20'

 

Now, if that ADDS to the blur from the eyepiece, then yes, something bad will be visible at the edge.

But it seems like FC isn't the primary issue visible at the edge of the field, here.

 

You calculate an edge error of 0.546 lambda due to FC.

Would that be visible?

 

Perhaps I am interpreting the figures incorrectly, but Ernest's figures imply a far larger error from other aberrations:

http://astro-talks.r...php?f=32&t=1483

Of course, I haven't checked his figures.  They could be wrong.

 

If they are additive, I can understand completely why people say a coma corrector with field flattening characteristics cleans the eyepiece up a lot.


Edited by Starman1, 23 July 2019 - 08:06 PM.

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#19 Vla

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Posted 23 July 2019 - 10:25 PM

Vlad,

Ernest measured the 17mm T4, but not the 22mm.

The 17mm and 12mm are worse than 10 arc minutes at the edge at f/10 and worse than 20 arc minutes at the edge at f/4

The 22mm, at least in my scopes, is no better than the 17mm or 12mm, so I'll assume the same figures for the 22mm T4.

Ernest calculated a 7.8' blur on the OP's f/7.7 scope due to FC. (post #13).

The blur at the edge of the 22mm is somewhere between 10' and 20'

 

Now, if that ADDS to the blur from the eyepiece, then yes, something bad will be visible at the edge.

But it seems like FC isn't the primary issue visible at the edge of the field, here.

 

You calculate an edge error of 0.546 lambda due to FC.

Would that be visible?

 

Perhaps I am interpreting the figures incorrectly, but Ernest's figures imply a far larger error from other aberrations:

http://astro-talks.r...php?f=32&t=1483

Of course, I haven't checked his figures.  They could be wrong.

 

If they are additive, I can understand completely why people say a coma corrector with field flattening characteristics cleans the eyepiece up a lot.

Those are definitely not the ep aberration sizes (the spot for the Nagler 2 should be more relevant than some unspecified measurements). If they were 10 arcmin at f/10, they would have been 20 arcmin at f/5. Obviously, it can't be. I remember from one discussion with Bratislav not long ago, that he was mistakenly assuming Ernest's figures as aberrations, and then realized (kind of small print) that what was measured was actually "scatter" (best translation from Russian). Whatever it is, it is not representative of the Nagler ep blur size. I raytraced all his six patented 82deg's, and don't see why would the latter units have 10 times, or more, worse correction. And, of course, it contradicts users' experience.


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#20 Starman1

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Posted 23 July 2019 - 10:28 PM

Interesting.

#21 rkelley8493

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Posted 23 July 2019 - 10:44 PM

I haven't noticed field curvature, but I have noticed "rolling ball" distortion when sweeping the Milky Way with the 22 Nagler in my f/7 Apo. However, when the scope is at rest, the 22T4 is very clean and one of the most comfortable eyepiece I own. I'll pay a little closer attention the next clear night I have and let you know what I see. By the way, It's my primary wide field eyepiece and one of the most used in my case.


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#22 Vla

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Posted 24 July 2019 - 08:10 AM

There is not much to notice at 910mm f.l. and 22mm f.l ep. Such triplet has about 370mm best image curvature, which gives 0.27mm sagitta at 15mm off. With 10mm ep it would have been about 2.7 diopters accommodation (it is convenient to start with 10mm because for it every 0.1mm of defocus approximates 1 diopter of accommodation), with 22mm less than half as much. That is practically flat for most people (accommodation from infinity to 0.8-0.9m).

Since I initially went with 18-19mm field radius, and the sagitta changes with the square of it, I should recalculate for 15mm radius, which is less than 1mm from the field stop. With 250mm curvature (worst case scenario for OP), it gives 0.45mm sagitta, i.e. defocus at this point, which is about 2 diopters of accommodation with 22mm f.l. eyepiece. That is what most people should be able to handle w/o problems in the case of natural accommodation (contracting eye lens), but question is how it would work in this case, with - I'd call it - inverse accommodation, requiring stretching it out beyond infinity shape (as it is the case with most curved-field telescopes and flat field eyepiece). If judging based on the definition of positive (natural) vs. negative relative accommodation "high value", 3.5 vs. 2.5 diopters, the inverse accommodation would be roughly 50% less efficient. But people with accommodative insufficiency have 2-3 times, or more, less of accommodation range. Age wise, average accommodation goes from 10+ diopters at 20, to less than 2 at 60.



#23 Starman1

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Posted 24 July 2019 - 08:26 AM

So, will FC be seen?

Depends on the age and accommodation of the observer and whether the accommodation is for infinity + or infinity -

I guess it makes it a bit tough to answer the OP's question.

Thanks for doing the math, Vlad.


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#24 Jon Isaacs

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Posted 24 July 2019 - 08:56 AM

So, will FC be seen?

Depends on the age and accommodation of the observer and whether the accommodation is for infinity + or infinity -

I guess it makes it a bit tough to answer the OP's question.

Thanks for doing the math, Vlad.

 

It also depends on the experience of the observer, the expectations.. 

 

Jon


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#25 Vla

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Posted 24 July 2019 - 09:00 AM

There is no straight answer, but if he's 20, he has plenty of reasons to be optimistic.


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