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Kenko TA-910 picture issue - a great disappointment

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#26 LukaszLu

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Posted 14 February 2021 - 08:07 AM

Thanks for all these detailed advice and explanations. Unfortunately - the baffles are in the best condition, the holes are regular and round.

 

I think experimenting with additional pressure on lenses to deform them in a controlled manner is a last resort. As the discussion of thermal conditions has shown, changes in temperature could possibly affect the final result and possibly even break the lens.

 

Perhaps it's worth trying the lens rotation method mentioned by Chip. Honestly speaking, I do not have high hopes for it - it seems to me that it helps to reduce spherical or chromatic aberration, but is it able to help compensate for the defect of the lens geometry? It must be remembered that it is present when the lenses rest almost loosely in the cell, there is no firm pressure on their edges.

 

Bad luck ... The easiest way would be to try return this telescope to the seller. But I've put more than 2 weeks of work into it, cleaned it, changed the grease, completely disassembled the entire mount, removed the damaged paint from the tripod, sanded it for a few days and brought it to a state where it was possible to expose the natural texture of the wood.  It was really hard work and I can still feel it in my spine and aching muscles in my back :-(


Edited by LukaszLu, 14 February 2021 - 12:01 PM.

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#27 PawPaw

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Posted 14 February 2021 - 09:07 AM

Here are a couple of pics of how Newtons rings should look.  First one is Edmund second is a Unitron.   Note both of these have a small rattle and are loose in the cell.  Good luck!

 

Cheers

 

Don

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Edited by PawPaw, 14 February 2021 - 09:23 AM.

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#28 Piggyback

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Posted 14 February 2021 - 09:59 AM

Thanks for all these detailed advice and explanations. Unfortunately - the baffles are in the best condition, the holes are regular and round.

 

I think experimenting with additional pressure on lenses to deform them in a controlled manner is a last resort. As the discussion of thermal conditions has shown, changes in temperature could possibly affect the final result and possibly even break the lens.

 

Perhaps it's worth trying the lens rotation method mentioned by Chip. Honestly speaking, I do not have high hopes for it - it seems to me that it helps to reduce spherical or chromatic aberration, but is it able to help compensate for the defect of the lens geometry? It must be remembered that it is present when the lenses rest almost loosely in the cell, there is no firm pressure on their edges.

 

Bad luck ... The easiest way would be to try return this telescope to the seller. But I've put more than 2 weeks of work into it, cleaned it, changed the lubricants, completely disassembled the entire mount, removed the damaged paint from the tripod, sanded it for a few days and brought it to a state where it was possible to expose the natural texture of the wood.  It was really hard work and I can still feel it in my spine and aching muscles in my back :-(

 

I can hear you only too well. It is these moments when you figure all effort may have been in vain. It is rather unusual with 60/910mm achromats from that era, though. I had a few of these as they are offered frequently in Germany. Never have I come across a faulty one. I recommend forgetting about your lens and finding yourself another one. I will have a lookout and shout if something shows up on my radar. 


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#29 LukaszLu

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Posted 14 February 2021 - 12:15 PM

Thanks a lot for the words of consolation Stefan, I bought this copy in Germany - for me it is the closest source of Japanese classic telescopes :-)

 

Today I had the opportunity to test it on the Moon. Conclusion: the picture is average. Still just average. Unlike the Vixen 60/900, which I used as a benchmark again, you never get those short, exciting moments of great seeing when the image gets razor-sharp. Here the image is only average, a bit less contrasting and more hazy, softer - although I cannot say that there are any evident hues or hazes around the contrasting details on the surface.

 

It is worth noting that the Vixen 60L is not a high-class device. I don't know much about this model, but it looks more like an education-grade telescope.

 

And by the way - If you can, take your scope outside and admire the Moon and the trinity of Langrenus-Vendelinus-Petavius craters today! Vendelinus - an ancient crater that can be nearly 4.5 billion years old, the same age as the Moon and Earth - is so heavily eroded that it usually disappears from sight. Today it is beautifully displayed thanks to specific lighting!


Edited by LukaszLu, 14 February 2021 - 12:51 PM.

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#30 ngc7319_20

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Posted 14 February 2021 - 12:50 PM

... I recommend forgetting about your lens and finding yourself another one. I will have a lookout and shout if something shows up on my radar. 

Yes, I think this is maybe the best idea to get another lens.  Perhaps there is a damaged scope somewhere, but the lens is OK.  

 

I do not know if Unitron or Polarex 60 / 900 lens would fit in the cell, but often those lenses appear for sale at reasonable prices.  Probably someone could measure their Unitron 60 / 900 lens to see if it might fit.


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#31 LukaszLu

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Posted 14 February 2021 - 01:41 PM

Well, I'm currently waiting for the Polarex 114 lens that I've sent to professional cleaning service as it was too old and had too much fungus to believe I can do the cleaning the safe way myself. They managed to remove the fungus, unfortunately traces of damages in anti-reflective layer can be seen. Anyway this is a part of another project I'm currently working on: restoring ancient copy of Polarex 114 that came to me in very poor and incomplete condition. So I won't use it.

 

Still I have an impression, that this Kenko model is much more common in Germany than Unitron/Polarex. I suppose that Weltblick 60/910 mm which is also quite popular in Germany is the same scope with only cosmetic differences. So maybe I could find exactly the same type of lens.


Edited by LukaszLu, 14 February 2021 - 01:43 PM.

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#32 Kasmos

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Posted 14 February 2021 - 02:35 PM

This is why more than once it's been said, "make sure a scope performs well before you put time into it".....

 

....but I feel your pain. I've had to replace two objectives and one was after spending several weeks of work on it.

On that particular scope, it was rare, so I probably would have done it anyway.

 

BTW, my C60 Celestron/Vixen is probably the best of my several 60/900mm telescopes.



#33 ngc7319_20

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Posted 14 February 2021 - 03:07 PM

 

Still I have an impression, that this Kenko model is much more common in Germany than Unitron/Polarex. I suppose that Weltblick 60/910 mm which is also quite popular in Germany is the same scope with only cosmetic differences. So maybe I could find exactly the same type of lens.

Yes maybe there is it easier to find Kenko and Weltblick lenses.  Also sometimes only one of the two glasses in the objective is bad.  If you are very lucky sometimes two "bad" or damaged objectives can be used to make one excellent objective.


Edited by ngc7319_20, 14 February 2021 - 03:07 PM.

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#34 LukaszLu

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Posted 14 February 2021 - 03:29 PM

I've managed to make better pictures of Newton rings. I think they can be described as round and regular - contrary to the image I got earlier. This time, however, I looked at them closely - not just in the photo - and I see no signs of distortion. Does this mean that the lenses are properly aligned and there's no use twisting them?

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#35 ngc7319_20

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Posted 14 February 2021 - 03:54 PM

It means the spacing is uniform (not wedged).  And the air space is symmetric about the axis.  This is good news.

 

However, it is still possible both glasses are warped by the same amount -- so the air looks OK but the glasses are still bad.    Or there are internal problems with the glass -- there could be variation in composition within the glass.

 

But this test looks very nice.  Maybe you should test the lens again with the artificial star?  Maybe you got lucky and fixed it somehow?



#36 Kasmos

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Posted 14 February 2021 - 03:56 PM

It looks very good now, so maybe another observational test is needed?

 

One of my telescopes had what thought was a bad lens and I bought two complete sets of replacements. It has a mysterious problem. Most of the times when the lenses were taken out of the cell and reset, the images were terrible. After dozens of times of checking and rechecking I somehow got them to sit right. Now it gives excellent images (better than the two replacements), so I'm afraid to ever touch it again. Perhaps you'll have the same results.



#37 LukaszLu

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Posted 14 February 2021 - 05:37 PM

Unfortunately - the diffraction disc still looks as usual. Photo taken with an SLR camera.

I've watched Mars today - the same terrible three-way glow...

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Edited by LukaszLu, 14 February 2021 - 05:38 PM.


#38 LukaszLu

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Posted 14 February 2021 - 06:31 PM

WOW! I've disassembled the lens and flipped them against each other and... something changed! Unfortunately there are no stars in the sky any more but I'm very excited with this discovery. Chip was right! Maybe this is a way to save this lens, although I didn't want to believe that it would bring results... Tomorrow I will be making more attempts, maybe it will be possible to make the shape of the disc more regular, although the progress seems already clear.

 

Obviously, a question arises - are the lenses really curved and rotating them simply compensates for these errors, or is it not true that this type of deformation of the diffraction disc must mean a distortion of the glass geometry ...?

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#39 LukaszLu

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Posted 14 February 2021 - 07:08 PM

I've made a quick test looking at street lamps through the (closed) window. Obviously, test is not very reliable due to the presence of an additional glass along the way, but I have already made such observations and have a comparison with the previous state. Well - the picture seems much, much better! The tiniest details seem to be on the verge of the capabilities of the eye, not the lens. This is a clear change.



#40 ngc7319_20

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Posted 14 February 2021 - 07:11 PM

Good!  Perhaps mark this position so you can find it again.  And then try rotating one of the glasses in 10 or 15 degree steps.  If there are errors in both glasses, there may be hope to make the errors cancel.

 

Do you still see Newton's rings?


Edited by ngc7319_20, 14 February 2021 - 07:13 PM.


#41 LukaszLu

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Posted 14 February 2021 - 07:51 PM

Yes - I can see them under CFL bulb. Is it good or bad?



#42 ngc7319_20

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Posted 14 February 2021 - 09:46 PM

Yes - I can see them under CFL bulb. Is it good or bad?

It is good!



#43 Kasmos

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Posted 15 February 2021 - 03:05 AM

The side of the Crown that is more convex should face the Flint. Viewing it from the edge it can be a little hard to tell what side that is on a 910mm lens so the tilt test is a good way to confirm it. Lay it on a piece of tissue and press the edge with your finger. Check both sides. The side that tilts more will be obvious and should face the flint. BTW, I'm surprised you're getting Newton rings both ways.


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#44 LukaszLu

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Posted 15 February 2021 - 06:24 AM

I'm sorry for my English! I was imprecise. I didn't turn the crown upside down - that's impossible because the spacers are attached to the crown. I turned the crown against the flint. Now I understand why the question is whether I can still see Newton's rings :-)

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Edited by LukaszLu, 15 February 2021 - 06:25 AM.

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#45 LukaszLu

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Posted 15 February 2021 - 02:27 PM

I've made some outdoor tests comparing the picture to Vixen 60/900 again. I observed elements of the distant electrical installation during the day and I've fount it really hard to tell which scope gave better picture. It's a clear progress. When the night came however and I directed the telescope towards Mars, picture occured far below expectations. The contour of the planet's disc was lost once again in the radiant glow that was still visible, though perhaps to a lesser extent. The chromatic aberration was also quite visible on the edge of the dial.

 

Well, I will disassemble the lens again and try another mutual adjustments of the crown and flint.



#46 ngc7319_20

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Posted 15 February 2021 - 02:40 PM

OK you got some nice improvement rotating / turning the crown glass.  Very good!  Probably what I would do is make some reference marks on the sides of the glass, and then try more rotations.  Start a notebook and keep careful notes about which rotations are good and which are bad.  Probably 15 or 20 degree steps would be good...  0, 15, 30, 45, 60, 75... degrees.  Maybe take pictures of star test when you find some good rotations. 

 

Since you have a 3-spoke pattern, there may be rotations at 120 degrees that are similarly good.  Maybe one of them is better than the other two.

 

Sometimes a green filter can help to judge the star test patterns.  It will remove the color effects from the view.


Edited by ngc7319_20, 15 February 2021 - 02:42 PM.


#47 ngc7319_20

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Posted 15 February 2021 - 03:07 PM

I tried to make a model for the aberrations with the Aberrator software.   It looks like your photos when there is about 0.30 wave P-V or astigmatism, and 0.25 wave P-V pinch.  The Strehl ratio comes out to 0.8 which is not too bad.

 

kenko 2j.jpg

 

kenko 3j.jpg



#48 LukaszLu

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Posted 15 February 2021 - 03:33 PM

Impressive ...! Does this program take into account that this is defocus pattern?



#49 LukaszLu

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Posted 15 February 2021 - 07:57 PM

OK, I've just tried a total of 7 position options, this is probably the best result I could get. The strange thing is that there is no regularity in it. Distortion does not increase or decrease as the lenses rotate relative to each other - they increase suddenly and disappear just as suddenly. Unfortunately, I have the impression that I am still not able to get a satisfactory image of a pinhole (artificial star), which seems to be imprecise, as if multiplied, out of focus, etc.

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#50 ngc7319_20

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Posted 15 February 2021 - 09:08 PM

These last three are the best star images yet.  Good progress.  They are not perfect, but may be "good enough."

 

Yes, the star images will be very sensitive to many things.  So it is possible there is variation.  Maybe how well the cell phone is centered on eyepiece.  Where the star is in the eyepiece.  Also how tight the lens retaining ring is.  




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