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Eyepeice Abberations

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#26 Asbytec

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Posted 26 July 2019 - 01:59 AM

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


So, a flat field eyepeice does not necessarily have a flat field at its own field of curvature, but a surface of best focus that closely matches the objective?

Putting your comment in line with the informatative exchange between Howard and Vla, the idea should dawn on me in the shower tonight. :)

#27 Ernest_SPB

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Posted 26 July 2019 - 02:18 AM

> a flat field eyepiece does not necessarily have a flat field at its own field of curvature, but a surface of best focus that closely matches the objective?

 

I do not think so... Eyepiece claimed as FF shall have own focal plane flat.

Compensation is another stuff.


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#28 Vla

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Posted 26 July 2019 - 06:32 AM

 Eyepiece claimed as FF shall have own focal plane flat.

Compensation is another stuff.

I second that.



#29 Jon Isaacs

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Posted 26 July 2019 - 07:00 AM

So, a flat field eyepeice does not necessarily have a flat field at its own field of curvature, but a surface of best focus that closely matches the objective?

Putting your comment in line with the informatative exchange between Howard and Vla, the idea should dawn on me in the shower tonight. smile.gif

 

My statement starts with the objective having a flat field. 

 

As I said, my thinking is simple minded. An eyepiece is a sophisticated magnifying glass.

 

If I look at at written page flat on a table and the entire page is in focus simultaneously, that is a flat field magnifying glass. Exchange the flat piece of paper for a flat focal plane and you're at the limit of my comprehension.

 

If the paper happens to be deformed so it has a spherical curvature, then for the entire paper to be in focus, the magnifying glass will have an identically curved focal plane.

 

Jon 



#30 Asbytec

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Posted 26 July 2019 - 07:14 AM

My statement starts with the objective having a flat field. 

 

As I said, my thinking is simple minded. An eyepiece is a sophisticated magnifying glass.

 

If I look at at written page flat on a table and the entire page is in focus simultaneously, that is a flat field magnifying glass. Exchange the flat piece of paper for a flat focal plane and you're at the limit of my comprehension.

 

If the paper happens to be deformed so it has a spherical curvature, then for the entire paper to be in focus, the magnifying glass will have an identically curved focal plane.

 

Jon 

Mine, too, Jon. There seems to be, to the limit of my ignorance, more than few ways to imagine a flat field. That's why I asked above, I was or still am confused a little by what it means. smile.gif

 

Yes, but the same curved focal plane and same curved paper form a flat field at the exit pupil..."collimated pencils" we can focus simultaneously. From Telescope op[tics.net, "Flat eyepiece field means that all off-axis pencils exit the eye lens collimated, enabling the eye to focus on all points across the field simultaneously".

 

The above definition makes sense to me, even if the front focus of the eyepiece is curved. A Petzval system is curved, but the image at the exit pupil is flat and without astigmatism, yes? But, so is a Nagler used in a system with off axis astigmatism. I think so, anyway. It looks flat and probably without field curvature, which is also flat...because....I dunno why. Yet. smile.gif


Edited by Asbytec, 26 July 2019 - 07:16 AM.


#31 Asbytec

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Posted 26 July 2019 - 07:53 AM

> a flat field eyepiece does not necessarily have a flat field at its own field of curvature, but a surface of best focus that closely matches the objective?

I do not think so... Eyepiece claimed as FF shall have own focal plane flat.
Compensation is another stuff.


Ok...Now I'm not sure what I am missing here.

#32 Starman1

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Posted 26 July 2019 - 08:26 AM

Norme,

If the eyepiece is claimed to have a flat field, this is independent of the scope in which it's used.

Of course that means the only scopes the eyepiece will have a flat field in after light passage will be those that have flat enough fields the 

curvature falls within the accommodation of the eye.

And it means that if a curved field is seen, it is not from the eyepiece, but from the scope.

Like 1A or 3A in your post #21.


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#33 Asbytec

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Posted 26 July 2019 - 08:50 AM

Ah, OK Don. If you heard a loud clicking sound, that was me finally getting the all too obvious difference hiding in plain sight.

So, flat field means either way, the system itself can be flat as in collimated pencils or just the eyepeice can have a flat field in and of it's own accord regardless of the objective.

So, in the case of f/6, which has essentially flat field curvature over the image scale, a flat field eyepiece should deliver essentially collimated pencils with only the very slight (8 micron) field curvature of the objective remaining.

Yes, just like the graphic above. Who posted that anyway? :)

If that's right, I guess I knew that but failed to catch it. If it's not right, this post will self destruct immediately to hide the fact I have a thick skull. :)

Edited by Asbytec, 26 July 2019 - 08:52 AM.

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#34 howardcano

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Posted 26 July 2019 - 10:30 AM

It's not the F/6 ratio alone that determines flatness, it's more the focal length and telescope type.  An 1200mm FL Newtonian has minimal field curvature.  A 1200mm FL refractor has field curvature about three times worse.

 

The focal ratio doesn't much change the field curvature, but it does change the defocusing aberration (depth of field) for a given curvature.


Edited by howardcano, 26 July 2019 - 10:31 AM.

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#35 Asbytec

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Posted 26 July 2019 - 10:43 AM

Thank you, Howard. I actually think that makes sense. Most of this stuff is still new to me. Still getting my feet wet. Its interesting. This is something I never really delved into until now. Just getting to know our hobby a little more. :)

#36 Jon Isaacs

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Posted 26 July 2019 - 11:18 AM

It's not the F/6 ratio alone that determines flatness, it's more the focal length and telescope type.  An 1200mm FL Newtonian has minimal field curvature.  A 1200mm FL refractor has field curvature about three times worse.

 

The focal ratio doesn't much change the field curvature, but it does change the defocusing aberration (depth of field) for a given curvature.

 

The focal ratio determines the depth of focus which is related to defocused blur.

 

The focal length and scope type determines the radius of curvature of the focal plane but the aberration seen depends on both the focal length and the focal ratio.

 

One thing that maybe confusing Norme is that TV NP series are modified Petzvals. Petzvals are early flat field camera lenses. 

 

The Petzval curvature is something else.

 

Jon


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#37 howardcano

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Posted 26 July 2019 - 11:34 AM

The focal ratio determines the depth of focus which is related to defocused blur.

 

The focal length and scope type determines the radius of curvature of the focal plane but the aberration seen depends on both the focal length and the focal ratio.

 

One thing that maybe confusing Norme is that TV NP series are modified Petzvals. Petzvals are early flat field camera lenses. 

 

The Petzval curvature is something else.

 

Jon

Yup, and I learned all that from your many helpful posts!

 

I probably should have also mentioned that the scope's focal ratio can affect the field curvature displayed by the eyepiece (by affecting the eyepiece astigmatism), just like a Barlow can do.


Edited by howardcano, 26 July 2019 - 11:42 AM.


#38 Starman1

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Posted 26 July 2019 - 11:46 AM

The focal ratio determines the depth of focus which is related to defocused blur.

 

 

Jon

One of the reasons I will never own an f/3 scope is that the depth of focus is just too small.

Not only does seeing affect the focus more in the f/3 scope than it does at, say, f/5, but the sensitivity of focus is so exacting that if there is ANY aberration in the system,

like uncorrected coma, spherical aberration, miscollimation, or even a bit of d├ębris on the lens, it can affect your ability to achieve a perfect focus.

I've used some scopes of f/2.8 to f/3.6, and found them all too twitchy with regard to focus.  For me personally, by f/4.2 I no longer notice this issue, so that is my "bedrock" f/ratio.

Perhaps if my vision could accommodate more diopter change, or I observed in perfect seeing more often, I'd feel differently about it.


Edited by Starman1, 26 July 2019 - 11:47 AM.

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#39 Vla

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Posted 26 July 2019 - 06:46 PM

Yes, but the same curved focal plane and same curved paper form a flat field at the exit pupil..."collimated pencils" we can focus simultaneously. From Telescope op[tics.net, "Flat eyepiece field means that all off-axis pencils exit the eye lens collimated, enabling the eye to focus on all points across the field simultaneously".

That should be obvious, but reversed raytracing may be confusing with aberrated eyepiece image coming out of perfect collimated pencils entering the eye lens. It simply means that eyepiece generates aberrations, and since the direction of light is reversed, so is the sign of aberrations (including distortion). Visually, the longitudinal aberration curve should be turned around by 180 degrees. Also, it makes it easier to look at the wavefront. A perfect eyepiece w/perfect objective will produce collimated exiting pencils, i.e. flat wavefronts. If the points of wavefront are out of phase, so that the wavefront is spherical (converging or diverging), the aberration is defocus; if the wavefront is astigmatic, it's astigmatism, if comatic it's coma etc. The actual sign of wavefront aberration is also opposite to that in reversed raytracing.


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#40 Vla

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Posted 26 July 2019 - 06:49 PM

Not only does seeing affect the focus more in the f/3 scope than it does at, say, f/5...

Seeing error is independent of focal ratio.


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#41 Starman1

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Posted 26 July 2019 - 11:42 PM

Vlad,
The visible effects of seeing do vary by f/ratio, though.
There is a longitudinal effect in seeing that causes focus to move in and out. You can actually "chase" the focus with the fine focus knob.
At f/3 I spent an inordinate amount of time focusing and refocusing. At f/5, I don't.
I think this is related to the depth of focus issue.
Seeing would be the same at all f/ratios but the visible effects are different.
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#42 Asbytec

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Posted 27 July 2019 - 03:46 AM

Okay, I think it helped (me) to go back and look at what field curvature is. Basically, the Petzval curve is the surface of zero astigmatism and, in a Newt, it is concave toward the eyepiece with a radius of R/2. However, it's kind of a fictitious curve because a parabola has off axis astigmatism whose longitudinal separation between sagital and tangential focus is proportional to the square of the objective field angle. The best focus (smallest blur) location is midway between them, and its field curvature is defined by 1/R(medial) = - 2/R. In other words, the field radius of best focus has the same radius as Petzcal surface, but it has the opposite sign and is convex toward the eyepiece. 

 

This is the objective field we hope a flat field eyepiece approximates to reduce astigmatism, if I understand that correctly (I still need to understand how the eyepiece is corrected for astigmatism). This is the condition of figure 1a in the illustration above. Since there is a slight divergence, there will likely be some small amount of field curvature (as Jon calculated above at less than 8 microns) and thus a tiny bit of astigmatism at the edge of the field. The diffraction limit for astigmatism is 0.37 waves PV, so it might be possible to figure how large the diffraction limited FOV might be. But, I am not there yet.

 

I guess it helps when understanding eyepiece aberrations observed at the exit pupil, it helps to understand what they have to deal with. I hope I am closer to really understanding this. Thank you. 

 

Don, I have been plagued with chasing focus in seeing conditions, best I can tell. I thought my focuser was slipping under the weight of the heavy eyepiece. That turned out not to be the case. The focuser position did not budge when it was locked down and tension is adequate on the draw tube to hold the weight. Yet focus would drift over time. If I waited long enough (not long), focus would return. It had to be the seeing conditions. 


Edited by Asbytec, 27 July 2019 - 03:48 AM.


#43 Jon Isaacs

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Posted 27 July 2019 - 07:22 AM

Vlad,
The visible effects of seeing do vary by f/ratio, though.
There is a longitudinal effect in seeing that causes focus to move in and out. You can actually "chase" the focus with the fine focus knob.
At f/3 I spent an inordinate amount of time focusing and refocusing. At f/5, I don't.
I think this is related to the depth of focus issue.
Seeing would be the same at all f/ratios but the visible effects are different.

 

Were both scopes of the same aperture?

 

https://www.fpi-prot...reer/seeing.htm

 

I did a simple analysis based on depth of focus and depth of field and came to the same conclusion as Bryan. 

 

Depth of focus depends on focal ratio but sensitivity to defocus depends on focal length and both are second order functions.  

 

This means for a given aperture, the effect of the seeing is independent of focal ratio.

 

But it also says that larger aperprures with show more shift in focus. 

 

Jon


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#44 Vla

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Posted 27 July 2019 - 08:39 AM

Vlad,
The visible effects of seeing do vary by f/ratio, though.
There is a longitudinal effect in seeing that causes focus to move in and out. You can actually "chase" the focus with the fine focus knob.
At f/3 I spent an inordinate amount of time focusing and refocusing. At f/5, I don't.
I think this is related to the depth of focus issue.
Seeing would be the same at all f/ratios but the visible effects are different.

I already posted this illustration recently, showing the effects of a hypothetical wavefront bent into spherical by seeing. Slower scope will have larger linear focus deviation, but the wavefront error remains exactly the same - and that is what determines what you see. The actual wavefront deformed by seeing is not spherical, not even close. In the smaller telescopes dominates roughness, in the large ones tilt, and in between the tilt error gradually increasing from near negligible to dominant. Neither of these two main components can be affected by refocusing.

Obviously, there will be change in the best focus location, but it has nothing to do with defocus sensitivity itself and, like on the illustration below, will be smaller in the faster scope, in proportion to the square of the focal ratio. But this shift is very small. We can get a rough idea from the % of defocus term in the seeing error broken down to Zernikes. For the roughness component alone, defocus makes 10% of the RMS error, and for the full blown error with the entire tilt component only 3%. Taking medium-to-large aperture, with, say, 0.15-0.2 wave RMS seeing error, and ~6% of it defocus, gives about 0.01 wave RMS, or 1/30 wave p-v. Even if you could take it out by refocusing, what difference would that make? Some people claim they can chase down seeing, some are ridiculing it, both from their own experience. Looking at it, what comes to mind is the saying: "Whether you think you can, or you can not, you're right."

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#45 Starman1

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Posted 27 July 2019 - 08:49 AM

Were both scopes of the same aperture?

 

https://www.fpi-prot...reer/seeing.htm

 

I did a simple analysis based on depth of focus and depth of field and came to the same conclusion as Bryan. 

 

Depth of focus depends on focal ratio but sensitivity to defocus depends on focal length and both are second order functions.  

 

This means for a given aperture, the effect of the seeing is independent of focal ratio.

 

But it also says that larger apertures with show more shift in focus. 

 

Jon

Yes and no.

The f/5 scopes were 12.5" and 15", f/4.2 was 28" and the f/3 was a 20" and f/3.3 and 3.6 were 32"

So perhaps I was noticing the sensitivity of the largest aperture combined with the short depth of focus?

I just noted the f/3 to f/3.6 were the ones where focusing was constantly needed.


Edited by Starman1, 27 July 2019 - 08:51 AM.


#46 Asbytec

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Posted 27 July 2019 - 09:25 AM

"Obviously, there will be change in the best focus location, but it has nothing to do with defocus sensitivity itself...

Some people claim they can chase down seeing, some are ridiculing it, both from their own experience."

I have to confess little success at chasing focus, only momentary improvement before focus changes again. It's better to resist the urge and just wait it out.

Using the L = 8 Lambda F^2 approximation in Vla's graphic, at f/6 I figure 0.15 waves. Since I do see some focus shift, I'd guess that amount of error is noticeable. And interestingly, roughly on the same time scales as the variance of R0 in excellent seeing as shown above.

Don does not claim to see it in larger apertures at slower than F/5, but I see it in a smaller 8" aperture at f/6 in pretty good seeing. I may have seen it in 6" f/13 as I sketched Jupiter slightly out of focus once (maybe some of the time, anyway). I wondered why I had to, and could, refocus at that moment. I started the sketch in focus, I'm sure.

Bottom line is, something seems to be going on with focus in seeing.

Edited by Asbytec, 27 July 2019 - 09:42 AM.


#47 Vla

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Posted 27 July 2019 - 10:26 AM

Using the L = 8 Lambda F^2 approximation in Vla's graphic, at f/6 I figure 0.15 waves. Since I do see some focus shift, I'd guess that amount of error is noticeable. And interestingly, roughly on the same time scales as the variance of R0 in excellent seeing as shown above.

Norme, what you see is the change in aberration magnitude, not defocus. Of course it will cause visible effect on image definition if the aberration level changes from, say, 1/4 wave p-v SA level to 1/2 wave, within 2-3 seconds. How do you focus out change in aberration magnitude? You don't. But nothing prevents you from trying and, when it accidentally coincides with the drop in aberration magnitude, to think that you did it.

 

Btw. the "excellent seeing" on the graph is at about 0.5 arc second. You are probably much closer to the average, 2 arcsec seeing.

 

Forgot to add, the expression is exact, not an approximation.


Edited by Vla, 27 July 2019 - 10:28 AM.


#48 Asbytec

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Posted 27 July 2019 - 06:29 PM

How do you focus out change in aberration magnitude? You don't. But nothing prevents you from trying and, when it accidentally coincides with the drop in aberration magnitude, to think that you did it.

 

Btw. the "excellent seeing" on the graph is at about 0.5 arc second. You are probably much closer to the average, 2 arcsec seeing.

 

Forgot to add, the expression is exact, not an approximation.

Vla, I understand and agree totally that we cannot re focus an increase in aberration magnitude. It would be very nice, though, if we could. The futility of trying is clear. This is exactly what I expect with seeing and why I thought it odd we actually could chase focus with very small focuser movement and refocus the image at a different point. It may have coincided with a drop in aberration magnitude, but refocus was successful every time, apparently, not just once or seemingly coincidental with a drop in aberration magnitude. Maybe, though. 

 

However, each time the image blurred I could adjust focus again and achieve good focus. Every time. I would not expect refocus to be successful in seeing, but it was always successful. I could chase focus over small time intervals, but I found doing so tedious and not worth the effort to continually adjust the focuser. That is not the seeing behavior I am used to or understand observing during our dry season when seeing is often very good. But, successfully chasing focus is why I thought my focuser was slipping under the weight of the eyepiece and focal extender combination. The behavior is not normal seeing where the image is blurred and no amount of defocus would never clean it up. Something else is going on. 

 

Attempts to chase focus may have been coincidental, I cannot rule that out now that you mention it. Next time it happens, I'll pay closer attention. But, I have never really observed during our monsoon season with weather patterns markedly different than our dry season. I have not experienced this weird effect nor remember ever having to chase focus before (except possibly the time I focused on Jupiter to sketch, then found I could actually focus on it again for some reason).

 

Our seeing on most nights if much better than 2" arc in the way I understand average seeing. More often than not, the Airy disc is easily visible most of the time. It's better than average. On the nights in question, seeing was better than Pickering 5/10 which is enough to degrade the image in itself and very much normal. Whatever is happening, and similar to Don's description and anecdotal input form others, it's probably seeing related except we can refocus the image. The behavior is as if the focus is shifting more so than the image is degrading. I do see some image degradation, but during longer periods of degradation is what prompts refocus and it's surprisingly successful. It's almost as if a giant smooth "air lens"  was defocusing the image or, as mentioned, the focuser itself is slipping. The latter was ruled out. 

 

I could be wrong. Whatever the case, I decided not to chase focus, just wait for it to return to normal. It behaves like a momentary blurring of the image due to seeing. If I had not attempted to refocus, I may never have noticed it.  


Edited by Asbytec, 27 July 2019 - 06:47 PM.


#49 Starman1

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Posted 27 July 2019 - 07:41 PM

I see the same thing as Norme, but typically only in shorter f/ratio scopes.



#50 Asbytec

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Posted 27 July 2019 - 10:10 PM

I see the same thing as Norme, but typically only in shorter f/ratio scopes.

I didn't (or might have once) notice it at f/13, but I really notice it at a faster f/6. So, in conjunction with Don's experience, et al, whatever its cause does seem to be related to the focal ratio. I'm sure Vlad is right, it probably has nothing to do with the depth of focus as I understand the article Jon linked above. Or maybe it does, because the amount of defocus needed is also very small. Coincidence? Seeing was otherwise pretty good when I notice it, so descent seeing might have allowed me to notice a change in the otherwise pretty nice image. And to chase focus to another pretty nice image, only to have it drift again slowly over a minute or so. Focus direction was confusing because I'd focus one way then the other, unpredictably, to regain focus. Small amounts in either direction, so I loose track of where the point of best focus was. I am not sure if defocus is always in the same direction or by how much. 

 

But, I believe the phenomenon is seeing related, but not the type of seeing that simply degrades the image to the point no amount of defocus will cure it. There is an otherwise fine seeing induced image to be had, if you care to chase the focus drift. It's not the focuser, I locked it down, adjusted more tension on the draw tube, and tested that it held it's position over long periods with the eyepiece and focal extender load. It holds. So, I do not chase focus anymore because I know the otherwise fine seeing induced image will return in a short while, just like it does when more variable seeing calms down. The real kicker is, there is a fine image just a slight adjustment of the focuser away. Then it drifts again and it can be chased, if one wants to chase it. It has to be atmospheric in origin, unless the OTA is expanding or contracting or some other mechanical part is adjusting to the cooling. But, in my case, the scope is already mostly near thermal equilibrium, on the mirror anyway, as judged by the defocus star test. 

 

I am reading just about every thread and website I can find on eyepiece aberrations. So, I don't mind talking about anything we see and why we might see it. 


Edited by Asbytec, 27 July 2019 - 10:12 PM.



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