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Kenko TA-910 picture issue - a great disappointment

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#76 LukaszLu

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Posted 20 February 2021 - 05:24 PM

I repeated the tests in better weather conditions - the sky was slightly hazy, but no clouds and no strong wind. Same thing - the image of Mars is either multiplied or blurred. The image of the moon is acceptable at magnifications of 50x. Around 100x you can see the same as in the case of Mars - the image is either multiplied or immediately blurred, there is no place between these points where the sharpness would be correct.



#77 Terra Nova

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Posted 21 February 2021 - 09:09 AM

I repeated the tests in better weather conditions - the sky was slightly hazy, but no clouds and no strong wind. Same thing - the image of Mars is either multiplied or blurred. The image of the moon is acceptable at magnifications of 50x. Around 100x you can see the same as in the case of Mars - the image is either multiplied or immediately blurred, there is no place between these points where the sharpness would be correct.

Are you positive that the elements of the objective aren’t flipped? This sounds exactly like that effect.



#78 LukaszLu

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Posted 21 February 2021 - 11:15 AM

As for the Crown, it cannot be flipped over because it has spacers attached to it. In turn, the Flint fits it perfectly, which is confirmed by the pattern of Newton's rings. I could try to flip the Flint - but what if the lenses get scratched during such an experiment...? I'm a bit afraid of that to be honest...



#79 ccwemyss

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Posted 21 February 2021 - 11:24 AM

If the lenses are bad, there's not much to lose by trying. If you flip it and there are no Newton's rings, you'll know immediately that it's not meant to be that way. 

 

Chip W. 



#80 Piggyback

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Posted 21 February 2021 - 11:53 AM

Lukasc,

This is my Revue 60/910mm lens setup. Works like a charm. This how your setup looks?

 

Revue 60 auf 910mm Achromat Objektivaufbaured.jpg



#81 Kasmos

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Posted 21 February 2021 - 01:45 PM

There's a slight chance that the Crown was flipped at the factory hence the spaces glued to it. My Swift 839 came without spacers and when I installed them I somehow got the crown flipped. The images were weird. Some say they will look ok but not as sharp as they should be. In my case both Saturn and Jupiter had sizeable halos. Silly me, when I went to check it I somehow put it back wrong again. Boy was I relieved when I got it right! 

 

Yes you have nothing to lose, but first check what side is more convex by doing the tilt test with the crown. If it's been correct, it will save you from unnecessary removal of the spacers.



#82 LukaszLu

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Posted 21 February 2021 - 02:41 PM

Lukasc,

This is my Revue 60/910mm lens setup. Works like a charm. This how your setup looks?

 

attachicon.gifRevue 60 auf 910mm Achromat Objektivaufbaured.jpg

Well... wait a minute! Mine looks this way:

 

20210215_000102.jpg

 

.... would it be possible that both lenses are flipped? Which one should be the first??



#83 Kasmos

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Posted 21 February 2021 - 02:55 PM

Well... wait a minute! Mine looks this way:

 

attachicon.gif20210215_000102.jpg

 

.... would it be possible that both lenses are flipped? Which one should be the first??

Yes they both could be in backwards.

You always have to think and double check it depending on if the cell loads from the front or the back.

If from the back, the crown is on the bottom.

 

Once in, it should be like this

Lens Crown-Flint.jpg

with the arrow is pointing towards the focuser


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#84 LukaszLu

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Posted 21 February 2021 - 03:02 PM

In my case the black plastic ring is inside the tube, and the bottom lens is the first. I thought that was correct because Flint usually has a wider side - like here. But...



#85 Piggyback

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Posted 21 February 2021 - 04:45 PM

My lens is facing up towards the sky. Both your lenses seem to point in the wrong direction.


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#86 davidc135

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Posted 21 February 2021 - 05:08 PM

Suspense! Is it flipped?



#87 ngc7319_20

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Posted 21 February 2021 - 06:00 PM

Lukasc,

This is my Revue 60/910mm lens setup. Works like a charm. This how your setup looks?

 

post-222475-0-88836800-1613926398 piggy.jpg

Piggyback:  Which side goes towards sky?  

 

As I have followed the thread... his cell loads from inside the scope... so the sky is towards the bottom of the stack.

 

post-343566-0-55322700-1613936352 xx.jpg


Edited by ngc7319_20, 21 February 2021 - 06:03 PM.


#88 LukaszLu

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Posted 21 February 2021 - 06:19 PM

What a ride :-) Looks like Terra has opened Pandora's Box ...

@Stefan, ngc is right - my lenses point in the same direction as yours. BUT I've just changed the direction to see what happens. I've also restored the original mutual position of the lenses.

The triangular shape of the diffraction disk has returned. The image is perhaps a bit better than it was in terms of sharpness, but it probably has to do with the relative position of the lenses, not the direction they are pointing at. It seems there's some glow around the Moon disc I've not observed before.


Edited by LukaszLu, 21 February 2021 - 06:22 PM.

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#89 LukaszLu

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Posted 22 February 2021 - 12:39 PM

I disassembled the lens again and tried to assess the curvature of the lenses by placing the edge of a piece of paper against their surface. It seems to be the most classic correct construction of achromatic lens: the more convex side of the Crown faces the inside of the lens. The Flint is concave on the side of the Crown, and on the other side - towards the eyepiece - slightly convex.

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#90 Terra Nova

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Posted 22 February 2021 - 06:34 PM

the image of Mars is either multiplied or blurred. The image of the moon is acceptable at magnifications of 50x. Around 100x you can see the same as in the case of Mars - the image is either multiplied or immediately blurred, there is no place between these points where the sharpness would be correct.”

 

I’ve had two scopes that had flipped objectives when I got them, (the whole thing turned around) and they both exhibit effects such as these. You also say that the crown had the spacer tabs affixed to it. With just about every standard Fraunhofer achromat I’ve ever had, the spacers have been on the flint (the bottom element, as in towards the focucer). It’s most unusual to have a flint-forward situation in this type of a scope. That is usually associated with the Steinhill design and I’ve only encountered that with more expensive scopes like Gotos.



#91 LukaszLu

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Posted 22 February 2021 - 06:50 PM

Thanks Terra for all your valuable comments and help! What would you try to do if you were in my place? I've already tried to flip both lenses and placed the Flint towards the sky - with result described yesterday. I call the lens with spacers "the Flint" because it's concave - I've never heard of Crown that is concave, no matter what the lens design.

 

Honestly, I have no idea what could be wrong here. It happens that Flint is turned outwards with a concave surface, but then no one would try to solve the problem of the air gap with such flat spacers. Probably a ring of appropriate thickness would be used. If spacers are used, it means that the concave surface of the Flint is supposed to face the convex surface of the Crown. Te hypothesis that the factory has mistakenly attached spacers from the wrong side of the Flint, in my opinion, is wrong.



#92 photoracer18

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Posted 22 February 2021 - 07:09 PM

When you talk about a 3 lobe issue you need to remind yourself that there are exactly 3 spacers between the lenses.  If its tight that is where the 3 stress points will be.



#93 LukaszLu

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Posted 22 February 2021 - 07:23 PM

When you talk about a 3 lobe issue you need to remind yourself that there are exactly 3 spacers between the lenses.  If its tight that is where the 3 stress points will be.

I've chceked the diffraction pattern with lenses being so loose that they rettled when I shook the cell :-) It did not help to improve the shape of the diffraction disk or the image quality.

 

I've just flipped the Crown. The less convex surface now faces Flint. The scope is cooling down...



#94 LukaszLu

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Posted 22 February 2021 - 07:50 PM

... and I've just finished a quick test, performed despite the heavy mist and very unstable air. Well - looking at Moon... I'm not sure. Maybe it's a bit better, bu the picture is so unstable that i can't be sure. Looking at street lamps - seems to be better, however it's too late for me to continue with comparisons to other instruments. I'll try to get back to tests tomorrow - it's almost 2 a.m. in Poland :-)



#95 Kasmos

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Posted 22 February 2021 - 11:56 PM

 Te hypothesis that the factory has mistakenly attached spacers from the wrong side of the Flint, in my opinion, is wrong.

It's hard to fully understand that sentence as written. In post #78 you said the spacers were glued to the Crown, which is unusual.

 

Anyway, keep the crown on the sky side and just make sure it's facing the flint correctly as shown by your drawing in post #89. If it still doesn't work, it's probably time to give up on that objective.


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#96 LukaszLu

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Posted 23 February 2021 - 07:54 AM

It's hard to fully understand that sentence as written. In post #78 you said the spacers were glued to the Crown, which is unusual.

 

Anyway, keep the crown on the sky side and just make sure it's facing the flint correctly as shown by your drawing in post #89. If it still doesn't work, it's probably time to give up on that objective.

I've referred to your suggestion saying "There's a slight chance that the Crown was flipped at the factory hence the spaces glued to it." - maybe I misunderstood it.

 

Yes - I think I'm approaching the point where I will have to give up. Today I will continue testing the version with the Crown flipped more convex side out, but I think if it was the solution - I would have seen it after the first few tests. It's a bit weird that the image seems to be similar and just as bad no matter how I position and whether I reverse the lenses ... This objective seems completely immune to changes in its construction :-)


Edited by LukaszLu, 23 February 2021 - 07:59 AM.


#97 LukaszLu

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Posted 23 February 2021 - 02:58 PM

I have looked at the Moon, also flipped the whole lens set - with the same result: completely immune to changes in construction...

 

I'm putting lenses in original order. The Flint is now rotated 180 degrees from it's initial position. It's chilly but no frost. The moon covers the garden with soft light. The wind has stopped. We're going with Kenko out, into the dark of the night ... ;-)



#98 Kasmos

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Posted 23 February 2021 - 04:24 PM

My 63/1000mm Soligor was the same way. I could flip the crown or the whole assembly and it didn't really change anything that much. I hated to give up on it, but have since replaced it with a very good lens from a Tasco 7TE-5.


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#99 Terra Nova

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Posted 23 February 2021 - 04:32 PM

Thanks Terra for all your valuable comments and help! What would you try to do if you were in my place?

I would do this, and if that didn’t work, I would look for another objective that would fit if you think the scope is worth saving.

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Edited by Terra Nova, 23 February 2021 - 04:33 PM.

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#100 LukaszLu

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Posted 23 February 2021 - 05:27 PM

I would do this, and if that didn’t work, I would look for another objective that would fit if you think the scope is worth saving.

This is exactly what I've returned to, after over a dozen different settings. With the same effect - picture happens to catch moments of sharpness in certain points, but you still have the impression that's there's something wrong with your eye. Endless attempts to turn focuser knobs don't help.

 

Do I think the scope is worth saving? Well - it has terrible wooden tripod - far below any quality standards. BUT it has a wonderful mount. The scope as a whole looks great - grey mount casting is a beautiful quintessence of the concepts of "scientific" and "60's". It's just great to have it in sight and enjoy its appearance. So yes - I think I'll try to find a replacement lens.


Edited by LukaszLu, 24 February 2021 - 08:39 AM.

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