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Eyepeice Abberations

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#76 MartinPond

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Posted 31 July 2019 - 05:55 AM

It;s safe to assume that almost any telescope and eyepiece

    is far better than the night sky when seeing is bad.

   That means...don't worry about inside the scope,

   other than focus. The seeing is worse.

 

Correcting for atmospheric disturbance is parking

   the focuser at the best average....if the turbulence

   is high up (most common).

Correcting for "rounder" and longer lasting disturbances

  would mean you could chase the best focal position.

The fact that you can chase focus at all means that

  a disturbance is fairly close (100--1000 ft)....

  making the apparent location closer---nearer-closer.

  So...ordinary depth of in-focus would predict how the

   focuser has to react.  Long and short scopes would

   have trouble, but the higher f-ratio would be in focus over

   a longer range in that near-field distance....like a camera 

  And...less sensitive  to the focuser, of course.

 

At the center of a hot parking lot would be the best spot

   to be less disturbed.  At the edges, the rising air

   would be more turbulent, and you would be looking

   into it sideways....the worst possibility.

 

 

    .  


Edited by MartinPond, 31 July 2019 - 05:56 AM.

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#77 howardcano

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Posted 31 July 2019 - 08:17 AM

I know the field curvature of the objective is Rp = R/2, in other words the same radius as the focal length. I guess this is the Petzval curve Rp. But, we're really concerned about the best focus surface for astigmatism. Is it different form the Petzval curve?

 

Yes, it is usually different.  Being both old and uneducated in optics, it took me a while to wrap my brain around that!

 

What we see as field curvature is actually the shape of the surface of best focus.  In the presence of astigmatism, the surface of best focus may not coincide with the Petzval surface.

 

So, what good is the Petzval sum and Petzval surface?  If one is looking to create a "flat-field" system with all aberrations (including astigmatism) being vanishingly small, then the surface of best focus IS on the Petzval surface.  In this case it is important that the Petzval surface have very little curvature (in other words, a very large radius).

 

The Petzval sum is a cool mathematical tool.  For instance, the order of the optical components doesn't change the sum!


Edited by howardcano, 31 July 2019 - 08:23 AM.

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#78 Asbytec

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Posted 31 July 2019 - 09:39 AM

Thank you, Howard. Reading again...I ran across the idea, again, that the best median surface (best focus, smallest blur) in a Newt has about the same shape, but of opposite sign, as a hypothetical astigmatism free parabola (which does not exist with the field stop at the surface). So, in the presence of astigmatism, then, the median surface has a radius of R/2 = focal length, except it is concave toward the objective while the ideal Petzval curve is convex toward the primary. This seems to be the actual field curvature of the objective that matters. The medial surface in a Newt is 1/Rm = -2/R, in my 200mm f/6, that's -1200mm (negative being opposite sign, as you know). 

 

As it turns out, I think, an eyepiece free of astigmatism has a Petzval curve that is concave to the eyepiece and convex to the objective. However, most simple eyepieces have astigmatism, especially at longer focal lengths, so their focal plane is also of opposite sign and thus, also, concave toward the objective as is the best focus surface of the primary. My guess is, one has a much steeper curve than the other, so the fields do not match closely, so we see astigmatism of the same sign(?). Need to think that through. 

 

I suspect a flat field eyepiece and a slightly curved medial surface is about as good as it get's, short of a custom made eyepiece, and why I am replacing my simple eyepieces with (hopefully) better corrected ones I can afford. They may not be perfect, but I think they will be an improvement. 

 

Yea, this is taking a bit of effort to understand not having much in the way of knowledge of this otherwise interesting field. I appreciate your input...from day one, really, when you responded to another of my posts. I like to think I am getting it, but you can bet I'd be the one student in a classroom asking the proverbial "no" dumb questions. smile.gif


Edited by Asbytec, 31 July 2019 - 09:48 AM.


#79 Vla

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Posted 31 July 2019 - 11:56 AM

Norme, look at post #25. This same simple rules apply to astigmatism/field curvature in general, eyepiece, objective,or anything else.



#80 Asbytec

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Posted 31 July 2019 - 06:49 PM

Thank you, yea that was a good post. 

 

What I took away from telescope optics.net was "Real world <simple> eyepieces, however, produce strong astigmatism, particularly the conventional types. Usually, this astigmatism is of opposite sign to the Petzval, thus abaxial points form astigmatic surfaces less curved relative to the Petzval up to a certain level. At the point when the sagittal astigmatic surface is half as curved as Petzval curvature, best image surface is nearly flat (FIG. 211B). Further increase in the astigmatism causes best image surface to become increasingly curved to the opposite side (FIG. 211C)." These illustrations show eyepiece astigmatic curves more strongly curved concave toward the objective. 

 

Makes sense...and, "Astigmatism modifies the field curvature depending on its sign and magnitude (B,C). If the sign is opposite to the Petzval's - usually the case - sagittal astigmatic surface S forms on the convex side of the Petzval's."

 

And, "Astigmatic surface profile of most amateur telescopes (Newtonian, refractor, Cassegrain) is roughly similar in form to that shown on FIG. 211C, concave toward objective, with the tangential surface closer to it. Newtonian form is identical to it, while in refractors and Cassegrain-like systems - including SCT and Gregory MCT - sagittal surface is also concave toward converging light, and so is the system Petzval..." The astigmatism is also bending to the opposite side of the convex Petzval surface with tangential and sagital surfaces increasingly separated by increasing longitudinal aberration in proportion to the square of the field angle. 

 

From post #25, "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." Here the sign is in relation to the eyepiece Petzval which astigmatism bends to the opposite side and is also concave toward the primary. But, due to the different focal lengths of the eyepiece and the objective, I take it the curvature of each is likely very different and probably astigmatic.

 

What's a little unclear is and I am thinking through is, I think the sign convention used makes the resulting astigmatism in each of the same sign, so there is no cancellation of the aberration? Rather they add. But both curves seem to curve in the same direction as described, "Eyepiece itself can generate curved image surface, either convex or concave toward the eye. If it is convex and nearly coinciding with the image surface of the objective, light from all points will exit eye lens as collimated pencils, i.e. will focus onto the retina, and the visual field will be effectively flat (1b)." But, that is for two positive lenses. I understand a negative lens reduces astigmatism to some degree. 

 

confused1.gif

 

This is going to take some more brain power and reading it again. And again, if necessary.  :)



#81 Vla

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Posted 01 August 2019 - 06:30 AM

Norme, that was explained too (post #39). You probably missed it.



#82 Asbytec

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Posted 01 August 2019 - 08:44 AM

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...

 

Yes, understood.

 

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.

 

Got it...

 

Also, it makes it easier to look at the wavefront.

 

This suggests a test that might be performed indoors while the sky socked in. I'd like to go out and observe a little, but cannot even see the sky for the clouds. Hoping for a brief break in the monsoon. 

 

The actual sign of wavefront aberration is also opposite to that in reversed raytracing.

 

waytogo.gif

 

Thank you, Vla, I didn't miss it as much as I just struggled to put it in context. 



#83 Asbytec

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Posted 02 August 2019 - 10:27 PM

Well Vla's idea of the reverse ray trace sat with me for a while until I decided just to see what I could do. As a quick look, I held the eye lens up to the monitor to magnify the pixels. The results were interesting enough to ask questions. 

 

So, then I grabbed the 133LPI Ronchi screen, a 9x50 finder scope as a beam collimator, and an led flashlight. I set the flashlight some distance from the finder eyepiece and shined into it. I adjusted it until the primary beam from the objective on the wall was close to the same size as the 50mm objective. It may not be perfectly collimated, but it's pretty close. It gave some results.

 

In the Ronchi test, I held the eyepiece close to the Ronchi screen, then backed it away until I could see fewer lines. On the computer monitor, I just played with the distance until I saw something interesting. Not sure what to make of them in terms of any aberrations. It's amazing how similar the computer monitor and Ronchi show the same patterns.  

 

First up is a pretty standard 25mm Plossl: (I believe the colors are coming from the monitor pixels, not any chromatic defect.)

 

25 Plossl.jpg  

25 Plossl (2).jpg  

25 Plossl (3).jpg


Edited by Asbytec, 02 August 2019 - 10:39 PM.


#84 Asbytec

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Posted 02 August 2019 - 10:29 PM

Next up an 18mm HD Ortho:

 

18 HD Ortho.jpg

18 HD Ortho (2).jpg

18 HD Ortho (3).jpg



#85 Asbytec

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Posted 02 August 2019 - 10:32 PM

Now a 15mm Ultra Flat Field:

 

15 UFF.jpg

18 HD Ortho (2).jpg

15 UFF (3).jpg

 

 



#86 Asbytec

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Posted 02 August 2019 - 10:34 PM

The only one that was noticeably different was the 8mm TMB Planetary II:

 

8 TMB.jpg

8 TMB (2).jpg

8 TMB(3).jpg



#87 Starman1

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Posted 02 August 2019 - 11:03 PM

The 8mm TMB, IF the conditions of imaging display the actual image, shows severe figuring problems on the lenses.

The others show barrel distortion in one direction, pincushion distortion in the other, and you can play with the Ronchi figures here:

https://www.bbastrod...com/ronchi.html

And some further sites to look into:

https://www.google.c...SV5q8zYKWO1MSM:



#88 Asbytec

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Posted 02 August 2019 - 11:44 PM

The 8mm TMB, IF the conditions of imaging display the actual image, shows severe figuring problems on the lenses.

The others show barrel distortion in one direction, pincushion distortion in the other, and you can play with the Ronchi figures here:

 

Yes, best match I could find for the TMB is higher order spherical. Interesting in that it gives nice star images. Is it normal for pincushion and barrel distortion to be seen together, just on opposite sides of focus? One is more dominate? I was hoping to see astigmatism, but I do not. At first I thought the images similar to the very top 25mm Plossl might be showing some field curvature. It does look like barrel distortion (top), though, doesn't it. 


Edited by Asbytec, 02 August 2019 - 11:44 PM.


#89 Starman1

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Posted 03 August 2019 - 08:58 AM

Are the barrel and pincushion photos taken in the same direction through the eyepiece?

No, they are not usually seen together in the same eyepiece.

It makes me wonder if what is seen is a trick of the camera lens spacing rather than the eyepiece.



#90 Vla

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Posted 03 August 2019 - 09:39 AM

Norme, what it shows are the effects of eyepiece used as a magnifying glass, not an eyepiece. You are using the entire opening, and that creates enormous spherical aberration, which dwarfs everything else (remember that the wavefront error goes with the 4th power of aperture radius, and the transverse aberration with the 3rd). Also, I don't see how you could use collimated beam to look at the computer monitor. In order to simulate reversed raytracing, you'd need some sort of a transparent mesh (window mesh would do) through which you'd direct the collimated beam (all other light sources should be excluded, so it should be in a box of some kind), directed to the eyepiece eye lens with a diaphragm that would equal the exit pupil size. Even then, you'd be able to evaluate only axial point, since for the field points you'd need collimated pencils coming onto the diaphragm at an angle.



#91 Asbytec

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Posted 03 August 2019 - 09:53 AM

Don,

 

Yes, all were viewed using the same reverse direction, just different distances form the source (for the computer monitor images.) The two images of each computer screen, I went thorough "focus" to the other side. The large barrel like images are further from the screen, the tighter pincushion like images are much closer. I only took one image with the Ronchi, from the reverse direction, starting with the eyepiece eye lens against the Ronchi screen. Then I backed off until I got a few lines. 

 

Interestingly, I checked my ES 5x FE, as well, through both directions with a Ronchi. But, I could not see any Ronchi lines. However, what I did notice was the projection of the source through the negative field lens thorough the second positive set, then onto the wall about 3 feet away. Interestingly, the image of the source converged (grew smaller) as the FE moved away from the wall and toward the source. I might have expected it to remain the same or diverge a little. I recall reading the 5x Powermate diverges a little, more like a Barlow. This is not conclusive because the input beam was not perfectly collimated, just close. 


Edited by Asbytec, 03 August 2019 - 10:00 AM.


#92 Asbytec

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Posted 03 August 2019 - 10:00 AM

Norme, what it shows are the effects of eyepiece used as a magnifying glass, not an eyepiece. You are using the entire opening, and that creates enormous spherical aberration, which dwarfs everything else (remember that the wavefront error goes with the 4th power of aperture radius, and the transverse aberration with the 3rd). Also, I don't see how you could use collimated beam to look at the computer monitor. In order to simulate reversed raytracing, you'd need some sort of a transparent mesh (window mesh would do) through which you'd direct the collimated beam (all other light sources should be excluded, so it should be in a box of some kind), directed to the eyepiece eye lens with a diaphragm that would equal the exit pupil size. Even then, you'd be able to evaluate only axial point, since for the field points you'd need collimated pencils coming onto the diaphragm at an angle.

Ah, okay...a magnifying glass. Go it. Can that still show us something about distortion? 

 

I did not use a collimated beam to look at the monitor, I used my 9 x 50 finder and an LED flashlight to fabricate a collated beam (for what it was) and sent it through the eyepiece from the eye lens side. The bean went through a 133LPI Ronchi screen, but I did not stop down the aperture to any exit pupil diameter. I used, I guess incorrectly, the full 50mm aperture. Probably none of them were off axis, either. Wish I had known how to do that. Need a laser, I guess. I should be able to find one around here. 


Edited by Asbytec, 03 August 2019 - 10:04 AM.



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