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Eyepeice Abberations

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#1 Asbytec

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Posted 24 July 2019 - 02:59 AM

I'm getting up to speed studying eyepeice aberrations, specifically near f/6. Hopefully a nice concurrent thread on the topic will help. A place to ask questions.

Initially, I'm kind of interested in what 'they' mean by a flat field. Low distortion or better star images toward the edge and how this all comes together with matching field curvature with the scope. Is a flat field eyepeice going to closely match the field curvature and produce, what, no astigmatism or defocus. What, specifically is a flat field?

Im also curious about any affect a Barlow or Focal Extender might have on eyepeice performance, inuding the Smyth group. It sounds interesting because the Smyth lens group apparently adds overcorrection (to my undercorrected system) and may add astigmatism of opposite sign to a positive lens with astigmatism at faster focal ratios.

I also understand a Barlow or FE do not correct for aberrations of the fast primary, but they may do so as part of the eyepeice. Most eyepeice aberrations are proportional to the eyepeice focal length as well as the focal ratio of the objective. Seems short focal lenghts are more immune to aberration, and if so does a Barlow help? How does all this play out to offer a flat field, either distortion, and off axis astigmatism.

I think this is important information to have when looking for a good eyepeice for our scopes. Thanks.

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#2 Ernest_SPB

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Posted 24 July 2019 - 05:46 AM

> what 'they' mean by a flat field

 

two sense (depends from context): (1) uniform (good) image quality over whole field, (2) not curved surface of the best focus (absence of defocus on edge FOV comparing to axial point).

 

> any affect a Barlow or Focal Extender

 

increase (1) magnification and (2) F-number, so reduce appearance of aberrations, but (3) can introduce own aberrations and light scatter and (4) break telecentricity in off-axial beams of light falling to EP what introduces some artificial EP aberrations  

 

> Barlow or FE do not correct for aberrations of the fast primary, but they may do so as part of the eyepeice

 

make a difference between Barlow/Smith lens and frontal negative component in complex EP schema. That is similar but different stuff.


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#3 HarryRik9

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Posted 24 July 2019 - 06:16 AM

The questions you are asking are not clearly defined. Have you read this?https://www.cloudyni...nternal-barlow/

#4 MartinPond

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Posted 24 July 2019 - 06:17 AM

From what I have seen,

'Smyth' is just a generalization of a lens or group

  that performs the Barlow-type function but also

  applies a compensation to the aberrations of the 

  primary group.   This means that you cannot really

  get a 'Smyth' multiplier alone, since it tuned to the

   other elements in the eyepiece. 

 

A good example would be the 9mm "Expanse"

 and clones.  Clearly, the groups near the eye

 function when the negative section is taken out, but at lower power,

  and the field edges are worse.


Edited by MartinPond, 24 July 2019 - 06:18 AM.

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#5 Asbytec

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Posted 24 July 2019 - 06:32 AM

The questions you are asking are not clearly defined. Have you read this?https://www.cloudyni...nternal-barlow/

No, but I will now. I may not know enough to frame a question properly. Thanks for the link.

I read all comments, pondering your input.

Edit: Read the link. Got the difference between Barlow and Smyth lenses. Still wanna explore how image amplifiers interact with eyepeices. I understand they do not correct for aberrations of the objective even if the focal ratio of the objective is effectively slower. So, some consider them to be more closely related to the eyepeice. So, do they affect eyepeice abberations in that abberations are proportional to the eyepeice focal lenght?

Edited by Asbytec, 24 July 2019 - 07:17 AM.


#6 Ernest_SPB

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Posted 24 July 2019 - 06:52 AM

'Smyth' is just a generalization...

Smyth lens is single negative lens near focal surface that corrects field curvature of prev. optical system



#7 Asbytec

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Posted 24 July 2019 - 07:07 AM

Smyth lens is single negative lens near focal surface that corrects field curvature of prev. optical system


Does flattening the field (curvature) also correct for, or minimize, astigmatism? I understand a flat field means minimal defocus error, but isn't field curvature related to sagital and tangental astigmatism? Ideally on the Petzval curve? (Can you tell I don't know, yet, what all this means? :) )

Edited by Asbytec, 24 July 2019 - 07:08 AM.


#8 Starman1

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Posted 24 July 2019 - 03:03 PM

I'm getting up to speed studying eyepeice aberrations, specifically near f/6. Hopefully a nice concurrent thread on the topic will help. A place to ask questions.

Initially, I'm kind of interested in what 'they' mean by a flat field. Low distortion or better star images toward the edge and how this all comes together with matching field curvature with the scope. Is a flat field eyepeice going to closely match the field curvature and produce, what, no astigmatism or defocus. What, specifically is a flat field?

 

You've gotten an answer on this, but the usual meaning is that the edge of the field is in focus at the same time as the center.

Im also curious about any affect a Barlow or Focal Extender might have on eyepeice performance, inuding the Smyth group. It sounds interesting because the Smyth lens group apparently adds overcorrection (to my undercorrected system) and may add astigmatism of opposite sign to a positive lens with astigmatism at faster focal ratios.

 

any lens in front of the eyepiece that straightens the rays entering the eyepiece will reduce or eliminate the aberrations in the eyepiece that are caused by the angle of the rays entering the eyepiece.  If the eyepiece itself has uncorrected aberrations, these will not be corrected by straightening the rays entering the eyepiece.

I also understand a Barlow or FE do not correct for aberrations of the fast primary, but they may do so as part of the eyepeice.

 

No.  The f/5 scope's coma is still f/5 coma, and is magnified by the Barlow lens.  It will be visible whether the negative lens is external to the eyepiece or included inside it.  A 2x Barlow does not correct an f/5 scope and make it f/10, it merely changes the angles of the rays entering the eyepiece from f/5 to f/10.  That's why it's better to think of a Barlow as a device that shortens the focal length of the eyepiece, not modify the f/ratio of the scope.

 

Most eyepeice aberrations are proportional to the eyepeice focal length as well as the focal ratio of the objective.

 

No, not really.  Some long focal length eyepieces are well-corrected, some short focal length eyepieces are not.

 

Seems short focal lenghts are more immune to aberration, and if so does a Barlow help?

 

Only if you are talking about aberrations induced by the f/ratio and not talking about aberrations inherent in the eyepiece.

 

How does all this play out to offer a flat field, either distortion, and off axis astigmatism.

 

A flat field can be distorted and have astigmatism, or it can be curved and have no astigmatism.  It's all in the design.  Usually, though, any eyepiece wider than a 30-40° field will have distortion, and wider fields will have more.

But, they can be flat and free from astigmatism,

I think this is important information to have when looking for a good eyepeice for our scopes. Thanks.

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Edited by Starman1, 24 July 2019 - 03:05 PM.

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#9 Starman1

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Posted 24 July 2019 - 03:07 PM

Does flattening the field (curvature) also correct for, or minimize, astigmatism? I understand a flat field means minimal defocus error, but isn't field curvature related to sagital and tangental astigmatism? Ideally on the Petzval curve? (Can you tell I don't know, yet, what all this means? smile.gif )

If an astigmatic image happens to be out of focus as well, it will look worse.  Coma is the same--it looks worse when out of focus.

So though neither is corrected merely by flattening the field, the appearance may improve compared to an eyepiece with a curved focal plane.


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#10 sg6

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Posted 24 July 2019 - 03:46 PM

When you say "eyepiece aberration", do you really mean "system aberrations" ? As in objective+Barlow+Eyepiece

 

If I recall correctly a refractor has image curvature one way, whereas a reflector has it the other, neither being flat. So which way does an eyepiece take into account? Probably neither and just gets designed (maybe) for a flat image, which you do not get.

 

I would suggest that other then TV no barlow or Extender is designed for a specific scope or range of scopes. Maybe ES are but unsure. Since TV make refractors, unaware of a newtonian, I wonder if their eyepieces are biased towards refractor field curvature? Interesting aspect.

 

In effect that barlow was never specifically designed to operate with the scope and neither was the eyepiece designed to operate with the barlow or Extender. Or really with the scope.

 

What the result produced is therefore a bit of gamble/compormise/luck.

 

For eyepiece aberration you need to have only the eyepiece and a suitable test screen at the object plain of the eyepiece. Nothing else. Anything else means you are not determining just the eyepiece aberrations.



#11 Starman1

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Posted 24 July 2019 - 04:36 PM

1. When you say "eyepiece aberration", do you really mean "system aberrations" ? As in objective+Barlow+Eyepiece

 

2. If I recall correctly a refractor has image curvature one way, whereas a reflector has it the other, neither being flat. So which way does an eyepiece take into account? Probably neither and just gets designed (maybe) for a flat image, which you do not get.

 

3. I would suggest that other then TV no barlow or Extender is designed for a specific scope or range of scopes. Maybe ES are but unsure. Since TV make refractors, unaware of a newtonian, I wonder if their eyepieces are biased towards refractor field curvature? Interesting aspect.

 

4. In effect that barlow was never specifically designed to operate with the scope and neither was the eyepiece designed to operate with the barlow or Extender. Or really with the scope.

 

What the result produced is therefore a bit of gamble/compormise/luck.

 

5. For eyepiece aberration you need to have only the eyepiece and a suitable test screen at the object plain of the eyepiece. Nothing else. Anything else means you are not determining just the eyepiece aberrations.

1) No, eyepiece aberrations.  System aberrations are something else.  I used the term "induced" to describe aberrations in the eyepiece cause by the telescope.

2) If the eyepiece is flat, then only the curvature of the scope need be accommodated, and the focal plane of most scopes, especially dobs, is flat enough that a flat field eyepiece is best.

That being said, though, few eyepieces have truly flat fields.  On example is the Pentax XW series, where the 4 longer focal lengths had positive field curvature and the 4 shorter ones had negative field curvature.

Not surprising the 4 shorter focal lengths got better press.

3) TeleVue makes certain their products work in their scopes, but not exclusively for their scopes.  Also, their refractors are designed to have flat fields.

TeleVue's philosophy is low to zero angular magnification distortion, no astigmatism, and flat fields.

4) Think of a Barlow as an extra lens added to the eyepiece.  It may not give optimum results, but usually the results are good or better.  Many, if not most, eyepieces are improved by having light rays enter the field lens straighter.

It's one of the reasons why telecentric Barlows perform so well.

5) You're right, of course, but if the eyepiece is well-designed, and well-designed for the f/ratio the eyepiece is being used in, the aberrations you see will be predominantly from the telescope.


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#12 howardcano

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Posted 24 July 2019 - 04:49 PM

If I recall correctly a refractor has image curvature one way, whereas a reflector has it the other, neither being flat.

The mirror's inherent astigmatism causes the surface of best focus for a Newtonian scope to be the same shape as for a refractor, although with a larger radius than a refractor (given the same focal length).  Both are convex when viewed from the eyepiece.

 

https://www.telescop...t/curvature.htm

 

See Figure 61.


Edited by howardcano, 24 July 2019 - 05:20 PM.

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#13 Asbytec

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Posted 24 July 2019 - 08:06 PM

Thank you. Reading and thinking through your replies.

#14 MartinPond

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Posted 24 July 2019 - 09:03 PM

One great way to examine variious aberrations  

  is to move a star (or an articial star) across the field..

 

At the center you can test the spot spread of the whole scope,

but near the edges you can see what aberrations are doing,

 the chromatics, the astigmatism (radial or circumference stretch),

 the fuzzing of the spherical aberration...

 

Great way to check EPs at the field edge..

Scanning a star, real of artificial, across the field.



#15 Jon Isaacs

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Posted 25 July 2019 - 07:12 AM

Initially, I'm kind of interested in what 'they' mean by a flat field. Low distortion or better star images toward the edge and how this all comes together with matching field curvature with the scope. Is a flat field eyepeice going to closely match the field curvature and produce, what, no astigmatism or defocus. What, specifically is a flat field?

 

 

Norme:

 

- I think of it as an advertising term. 

 

Technically, I think it means a focal plane that is flat rather than curved.  That does not mean the eyepiece is free of other aberrations like astigmatism.  

 

Don wrote:

 

"3) TeleVue makes certain their products work in their scopes, but not exclusively for their scopes.  Also, their refractors are designed to have flat fields.

TeleVue's philosophy is low to zero angular magnification distortion, no astigmatism, and flat fields."

 

Don is using the technical definition for a flat field, that is, the focal plane is in flat and will be infocus across the entire field of view.  TeleVue does not call their eyepieces Flat FIeld but I think they have the flattest fields and the least astigmatism of any out there because they are designed for and tested in their modified Petzvals which also have flat focal planes. 

 

-The field curvature one sees depends on the field stop of the eyepiece and the field curvature of the telescope.  The curvature of the field is a second power of the off-axis distance so it quickly diminishes at the magnification is increased and the field of view reduced.  

 

I calculate that for your 8 inch F/6, the curvature of the field across an 82 degree eyepiece is less than 8 microns.  The depth of focus at F/6 is about 80 microns so for observing the planets, field curvature of your telescope is not a concern.  For viewing the planets, you want eyepieces that have flat fields and are free of off-axis astigmatism at fast focal ratios.  

 

I use the T-6 Naglers and the seem sharp across the field based on close doubles.

 

Jon


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#16 Eddgie

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Posted 25 July 2019 - 10:19 AM

The field of almost all telescopes will not be flat, but for reflectors, it is essentially flat. In refractors and SCT though, the curvature is quite strong. 

 

Here is a great explanation.  See figure 211 and the figure directly below it...

 

https://www.telescop...errations_1.htm

 

In the drawing below Figure 211, exibit 1b shows the case with refractors and how an eyepiece with some curvature could in theory make the field appear flat to the observer. 

 

 

So, everyone wants a flat field eyepiece, but in reality, some curvature is often beneficial and eyepiece designers know this.  For reflectors that are essentially flat as compared so most amateur sized refractors, a flat field or even a bit of curvature towards the observer is not going to be easy to see, but if the curve is as shown in figure 1b is present as is the case with most refractors, it can improve the off axis focus as seen by the observer. 

It is impossible to separate the eyepiece out from the telescope because the telescope can greatly modify the appearance of aberrations.  A perfectly in focus abberated blur may be to small to resolve the shape of the blur, but if it is out of focus, the expanded size makes any aberration present much easier to detect.  


Edited by Eddgie, 25 July 2019 - 10:40 AM.

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#17 Starman1

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Posted 25 July 2019 - 12:04 PM

I admit I don't understand the drawings in Vlad's figure in the next illustration AFTER figure 211.

I thought that if the curvature of the eyepiece was in equal amount, but opposite in sign, to the curvature of the scope, the result would be a flat surface after the eyepiece.

His drawings show that if the signs are opposite, the result is LARGER field curvature after the eyepiece, which is the opposite of intuitive.

If the edge curvature of one is -1 and the other is a +1, how does this not result in 0.0?



#18 Vla

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Posted 25 July 2019 - 02:02 PM

Doesn't work like that, Don. When eyepiece field is curved, it is tied up with the parallel exit pencils. In other words, field point in its front plane has to be displaced from the plane in order to produce a parallel exit pencil. So when eyepiece curvature coincides with that of the objective (i.e. when they are of the same sign), we get parallel exit pencils, and zero curvature. If the objective's image point is closer, exiting pencil will be diverging, and the eye will need to accommodate to bring it to focus on the retina, and when objective's image point is farther away, exiting pencil will bi converging. Either way it produces field curvature effect.


Edited by Vla, 25 July 2019 - 02:08 PM.

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#19 Asbytec

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Posted 25 July 2019 - 06:08 PM

Yes, I have been pretty silent in my own thread studying the text and images in telescope optics.net Edggie posted above. It's quite interesting, but a bit hard to put all the information together between the astigmatic curves and the Petzval curve and what to expect with a "flat field" eyepiece and field curvature at f/6. Coupled by the idea we don't know much more about the eyepiece unless we ray trace it, I guess.

 

Understanding it is just going to take some more brain power and maybe even some observation in the real world. That includes observation with a Barlow and a focal extender to see how they interact with the eyepiece. I'm a little concerned I got the 5x extender causing me to rely on longer focal length eyepieces, where aberrations seem to be proportional to eyepiece focal length (in the simple designs, apparently) and the focal ratio. But, the idea that any eyepiece might be easier to use or better corrected with shallower converging light cone form the Barlow/Extender. 

 

Most of my eyepieces now are not simple designs, they have more elements and a Smyth lens group (as I understand it, a negative field flattener that may have some Barlow "effect", but is not a Barlow, per se, but coupled with a positive lens). As I read it, this group adds over correction (to my under corrected system), so that's good. It also improves other aberrations in some way and by some amount. I just need a bout of clear weather to go out and look at their real world performance to determine what is present. But, it seems to me, the field of the telescope is going to be astigmatic, i.e., concave toward the primary and the eyepiece is "flat", so I might not expect collimated pencils at the exit pupil. That's not so disheartening because, I suppose, the parabolic primary is essentially flat over the image scale. So, aberrations might be minor and only toward the edge. 

 

I'm kind of ignorant about eyepiece aberrations, so it's something new to learn. So, I do not know how to frame many questions. But, I am primarily looking at astigmatism and field curvature at the moment by understanding whats going on then going out to look. Then, of course, get a good view of Jupiter and Saturn while it's clear. But, that's hard to do during the monsoon season. 


Edited by Asbytec, 25 July 2019 - 06:37 PM.

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#20 Starman1

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Posted 25 July 2019 - 07:01 PM

Astigmatism in eyepieces is my personal bête-noire, along with any visible barrel distortion.

I cannot tolerate it at all.  I've sold innumerable eyepieces because they displayed it somewhere in the field,

and I've gotten rid of telescopes that had it in the objective's optics.

If I can see it in a star test, something's gotta go.

Field curvature is an old man's issue.  When we're young, we can accommodate a lot of it.

When we're old (and especially after cataract surgery), we can see very small amounts of it.

I don't find it a coincidence so many eyepieces are being advertised as having "flat fields".gramps.gif


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#21 Asbytec

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Posted 25 July 2019 - 07:04 PM

TeleVue does not call their eyepieces Flat FIeld but I think they have the flattest fields and the least astigmatism of any out there because they are designed for and tested in their modified Petzvals which also have flat focal planes. 

 

-The field curvature one sees depends on the field stop of the eyepiece and the field curvature of the telescope.  The curvature of the field is a second power of the off-axis distance so it quickly diminishes at the magnification is increased and the field of view reduced.  

 

I calculate that for your 8 inch F/6, the curvature of the field across an 82 degree eyepiece is less than 8 microns.  The depth of focus at F/6 is about 80 microns so for observing the planets, field curvature of your telescope is not a concern.  For viewing the planets, you want eyepieces that have flat fields and are free of off-axis astigmatism at fast focal ratios.  

 

 

Jon

Thanks, Jon. I am actually encouraged by the illustrations in the figure below, as "Most objectives generate curvature concave toward objective, and most eyepieces nowadays have near flat field, in which case the combined visual field has curvature similar to that of the objective (1a)".

 

What seems interesting is the Petzval curve is not flat, and the eyepiece field should match the Petzval curve in order for the field to be flat (collimated pencils) at the exit pupil. WHich confuses the flat field issue for me. Is a flat field flat on the front focal plane (I think so) or flat at the exit pupil (probably as Tele Vue might be). The same matching seems to be true with any astigmatic curve at best focus. ??

 

Image from https://www.telescop...errations_1.htm

image_curvature.png
 

If the eyepiece field is flat, I presume the front focal plane is flat, but the pencils on the exit pupil are (should be?) as flat as the objective field. As you show, that's pretty flat at less than 8 microns. My understanding is, a Newt has off axis astigmatism and the best focus curves are shown above. So, there may be some trace astigmatism near the edge unless the eyepiece is corrected for it, too. And, I understand, distortion is also related somehow and there is a trade off between astigmatism and distortion that cannot be corrected. If so, I think I'd rather have some distortion than astigmatism. 


Edited by Asbytec, 25 July 2019 - 07:10 PM.


#22 howardcano

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Posted 25 July 2019 - 07:38 PM

The "surface of best focus" is what we see when we are looking for "field curvature".  Best focus is not necessarily on the Petzval surface.  Astigmatism can shift the best focus away from the Petzval surface.

 

I was myself quite confused when use of a Barlow flattened the field of a Celestron XCel LX eyepiece in my 8" F/6 Newtonian, but did not affect the flat field of a TV Delite at all.  It turns out that the Barlow was reducing the astigmatism displayed by the XCel LX, which in turn changed its surface of best focus, giving a flatter field.  The Delite was unaffected since it had essentially no astigmatism to begin with.

 

While the contribution of the Barlow to the system's Petzval surface was present when using either eyepiece, its contribution to visible field curvature was far below that due to astigmatism.

 

Now Vla can correct me if I'm wrong!


Edited by howardcano, 25 July 2019 - 07:46 PM.

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#23 Jon Isaacs

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Posted 25 July 2019 - 07:46 PM

Thanks, Jon. I am actually encouraged by the illustrations in the figure below, as "Most objectives generate curvature concave toward objective, and most eyepieces nowadays have near flat field, in which case the combined visual field has curvature similar to that of the objective (1a)".

 

What seems interesting is the Petzval curve is not flat, and the eyepiece field should match the Petzval curve in order for the field to be flat (collimated pencils) at the exit pupil. WHich confuses the flat field issue for me. Is a flat field flat on the front focal plane (I think so) or flat at the exit pupil (probably as Tele Vue might be). The same matching seems to be true with any astigmatic curve at best focus. ??

 

 

Norme:

 

My thinking is simple minded.

 

Objectives have differing amounts of field curvature and even different directions of curvature.  If you are designing binoculars, matching to objective and the eyepiece makes sense, otherwise, it's a losing battle, you can't make eyepieces designed to match the objective's curvature. 

 

If the objective's focal plane is flat, then an eyepiece that is able to focus the entire field simultaneously is by definition a flat field eyepiece.

 

Jon


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#24 Asbytec

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Posted 25 July 2019 - 08:19 PM

Thank you, Jon. Yes, we cannot fabricate a one size fits all eyepeice. Rather just understand them in relation to our scope.

I guess simply, if we have a rather shallow objective field curvature, then we can assume is essentially flat as you show above. Then, to my way of thinking, if the eyepeice field is flat, then we can expect very little or no "old guy's" field curvature both at the eyepeice front focal plane and the exit pupil. So, we should have an essentially flat field.

The only remaining aberrations are astigmatism and distortion (as spherical and EP coma are normally dealt with), but astigmatism is related to the "flat" field. So, I think a flat field means it should be better corrected, at least at best focus, but is likely still present. This is the part I'm trying to understand. I have flat field eyepeices, wanna go see how well they are corrected. Probably not as well as the Nagler, but minimized over a large portion of the field is nice.

Still trying to understand all of this. Thanks for chiming in.

Edited by Asbytec, 25 July 2019 - 08:21 PM.


#25 Vla

Vla

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Posted 25 July 2019 - 09:18 PM

The "surface of best focus" is what we see when we are looking for "field curvature".  Best focus is not necessarily on the Petzval surface.  Astigmatism can shift the best focus away from the Petzval surface.

 

I was myself quite confused when use of a Barlow flattened the field of a Celestron XCel LX eyepiece in my 8" F/6 Newtonian, but did not affect the flat field of a TV Delite at all.  It turns out that the Barlow was reducing the astigmatism displayed by the XCel LX, which in turn changed its surface of best focus, giving a flatter field.  The Delite was unaffected since it had essentially no astigmatism to begin with.

 

While the contribution of the Barlow to the system's Petzval surface was present when using either eyepiece, its contribution to visible field curvature was far below that due to astigmatism.

 

Now Vla can correct me if I'm wrong!

Nothing for me to correct, Howard. We could add a few things about Petzval vs. astigmatism, since it is basically simple. If astigmatism is of the same sign as Petzval, it will make best image surface more strongly curved than the Petzval, so that the separation between the two equals the longitudinal astigmatism. If of opposite sign, it will be bending the opposite way, having flattening effect, again with the best image surface separated from the Petzval by the amount of longitudinal astigmatism. Since Petzval curvature represents the one with zero astigmatism, the lower astigmatism, the closer to the Petzval is the field curvature.

Another thing is how does astigmatism of the eyepiece combine with that of the objective, and the logic is similar to that for field curvature alone. One needs to remember that those astigmatic surfaces of the eyepiece are generated starting with the parallel pencils entering eyepiece from the side of the eye lens (reverse raytracing), so when the astigmatic profile (i.e. curvature, longitudinal aberration and position) of the objective coincides with those of the eyepiece, the exiting pencils will be parallel, i.e. w/o astigmatism or field curvature.


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