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Kenko TA-910 picture issue - a great disappointment

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#1 LukaszLu

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Posted 12 February 2021 - 01:49 PM

After several weeks of waiting for good weather, today I finally had the opportunity to try this telescope for the first time. I had high expectations after reading very good opinions about it, and also remembering the wonderful pictures I got with the recently purchased Royal Astro R-74 refractor. Unfortunately, what I saw turned out to be a huge disappointment.

 

Even at maximum sharpening, the image of Mars was dim and contained very distinct streaks radiating from the planet's small disc in three directions. After defocusing, the diffraction pattern looked exactly like case number 7 shown at https://www.telescop...aberrations.htm and resembled a triangular shield with rounded corners or a coat of arms field.

 

The description on this page says:
"7 - Pinched optics can produce various pattern deformations. The pattern shown - trefoil - would be caused by a support or retaining elements (clips) having near-perfect 3-sided symmetry (0.18λ and 0.36λ  P-V)."

 

The description therefore suggests that the optics is pressed incorrectly in some way - probably in 3 points? The question is how is this possible, since the lens inside is pressed with a ring that acts evenly on it. Are there any collimation screws hidden somewhere that I don't know about? What do you think? Does anyone have any suggestions on what to do with it? In this state, the telescope can only play a decorative role, there is no reason to reach out to it for observation.

 

The temperature outside was quite low: -10 ° C (14 ° F), I gave the telescope 30-45 minutes to cool down.

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#2 ccwemyss

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Posted 12 February 2021 - 01:54 PM

Some classic scopes have three raised bumps on the side of the cell opposite the ring. The spacers are supposed to be positioned over these. If they are not, the ring can cause flexure in the elements.

 

Chip W. 


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#3 ngc7319_20

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Posted 12 February 2021 - 02:39 PM

Probably twice a day we are getting questions about pinched optics.   Metal shrinks 2 to 3 times faster than glass when it gets cold.  Its mid-February.   Usually these problems go away as soon as the weather warms up.  Solutions: (1) wait for warmer weather (2) make a mask with a hole slightly (1/4") smaller than the aperture and put it over the front of the lens, (3) loosen the front retaining ring slightly and see if it helps.


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#4 Kasmos

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Posted 12 February 2021 - 02:40 PM

I have about a dozen 60mm telescopes. Of the total, two had noticeably bad optics and one of those was a Kenko 710mm TA-710 (Circle k). I know it's not a fair judgement but its made me leary of them. It is a very attactive kit, but it wasn't even close to being useable (or sellable), so I replaced it's objective.

 

Also, if the OP's scope is like mine, it won't have those bumps in the cell.


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#5 LukaszLu

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Posted 12 February 2021 - 02:59 PM

Thanks for the hints and ideas. I also quietly hope that this is a result of the fairly low temperature, although the telescope has been outside for a relatively long time - perhaps long enough for the changes in glass and metal sizes to equalize.

 

This is what the lens cell looks like:

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#6 ngc7319_20

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Posted 12 February 2021 - 03:21 PM

... although the telescope has been outside for a relatively long time - perhaps long enough for the changes in glass and metal sizes to equalize.

 

No, equalize will not fix the pinch problem.  It is the cold temperature that will make pinch -- not uneven temperature.  

 

Uneven temperature will make air currents, but this is not about air currents.

 

You could try to loosen the retaining ring a tiny bit and see if that helps.  Careful to not scratch the glass.

 

post-343566-0-73872100-1613159952 x.jpg


Edited by ngc7319_20, 12 February 2021 - 03:27 PM.


#7 Kasmos

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Posted 12 February 2021 - 03:26 PM

That's completely different than the Circle K cells I've seen.

 

Kenko-Cell.jpg

They usually look like this.

 

Kenko-Objective.jpg

Some have their markings on the outside of the cell.


Edited by Kasmos, 12 February 2021 - 09:50 PM.


#8 LukaszLu

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Posted 12 February 2021 - 03:41 PM

No, equalize will not fix the pinch problem.  It is the cold temperature that will make pinch -- not uneven temperature.  

 

Uneven temperature will make air currents, but this is not about air currents.

 

You could try to loosen the retaining ring a tiny bit and see if that helps.  Careful to not scratch the glass.

Well, I had to tighten this retaining ring as it was loose when the telescope arrived. The lenses rattled as the tube moved. However, I tried to do it gently, only to the first noticeable resistance. I will try to loosen it, of course.

 

Honestly, I have never experienced such an effect, although it is true that I tend to avoid observing at temperatures below -5 ° C (23 ° F) - partly because of laziness and partly because of the limitations of the electronics I use.

 

What is, in your opinion, the temperature limit at which such effects begin to become visible?



#9 LukaszLu

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Posted 12 February 2021 - 03:55 PM

That's completely different than the Circle K cells I've seen.

I've noticed that some have the lettering on the front ring and some have not, but it seems that all 910 mm version have the same angular ring.



#10 39cross

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Posted 12 February 2021 - 04:53 PM

Well, I had to tighten this retaining ring as it was loose when the telescope arrived. The lenses rattled as the tube moved. However, I tried to do it gently, only to the first noticeable resistance. I will try to loosen it, of course.

Maybe the lenses rotated slightly and are not aligned correctly now, if they were loose. Maybe that's worth checking.

Edited by 39cross, 12 February 2021 - 04:53 PM.


#11 ngc7319_20

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Posted 12 February 2021 - 05:09 PM

Well, I had to tighten this retaining ring as it was loose when the telescope arrived. The lenses rattled as the tube moved. However, I tried to do it gently, only to the first noticeable resistance. I will try to loosen it, of course.

 

Honestly, I have never experienced such an effect, although it is true that I tend to avoid observing at temperatures below -5 ° C (23 ° F) - partly because of laziness and partly because of the limitations of the electronics I use.

 

What is, in your opinion, the temperature limit at which such effects begin to become visible?

I can't say there is some specific temperature where pinching will start.  It will depend on a case by case basis on how closely the lens cell is machined, how tight the retaining ring is, what metals were used, etc.  I have personally seen lenses that start to pinch around 40F and were very noticeable (like you describe) by 20F.  But that was only 2 cases.  Most are OK colder than that.

 

OK, just try to loosen the ring and see if it helps.  A small rattle will be OK. 

 

If you are worried about the retaining ring coming loose, you could put a small piece of tape on the cell threads right where the ring is, or a tiny drop of nail polish.

 

OK, good luck, and let us know how it turns out.


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#12 Van Do9:3

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Posted 12 February 2021 - 05:26 PM

I have read on CN threads that the objective lens should shift a little in the cell. My Selsi 80mm does but ever so slightly, it doesn’t sound completely loose. Please check out DavidG’s post how he determined the correct retaining ring pressure via interferences under a monochrome light (compact fluorescent lamp), https://www.cloudyni...ens-adjustment/

Good luck and clear skies. 
 

My



#13 LukaszLu

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Posted 12 February 2021 - 05:48 PM

I thought about the possibility of lens rotation, but as far as I understand this kind diffraction pattern suggests rather the geometric deformation of the surface than aliging problem.

 

I made an artificial star out of a flashlight covered with aluminum foil. After the lens has warmed up, the effect may not be as strong, but the diffraction pattern is still not round and it deforms towards the shape of the egg. The axis of this shape rotates as the lens cell rotates.

 

I unscrewed the cell. The tightening ring was almost loose - I turned it without applying any force. I tightened it just enough to hear only slight rattling of the lenses and put the cell back on. The pattern slightly changed and became symmetrical in 2 axis - but still not round shape. The same artificial star in the Vixen 60/900 refractor gives the correct round image.


Edited by LukaszLu, 12 February 2021 - 06:24 PM.


#14 LukaszLu

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Posted 12 February 2021 - 06:19 PM

This is the out of focus shape - photographed through the eyepiece...

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Edited by LukaszLu, 12 February 2021 - 06:20 PM.


#15 ngc7319_20

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Posted 12 February 2021 - 06:38 PM

Hmmm.... Well seems there may be more than one problem.  So this is not just the shape of the pinhole?  Vixen 60/900 was ok?  How far was the pinhole?  Does moving the pinhole twice as far away make any improvement?  Sometimes the shape of the pinhole gets mixed up in the lens problems.

 

Can you make shorter exposure where the diffraction rings (Fresnel rings) are visible inside and outside focus?  Might give a few more clues...

 

It is possible the glass itself was bad.  Either not ground properly, or not annealed and warped over years.  That would be a sad result.

 

I think lens rotation is not the problem here.  This looks like a very large error.... I would expect only a small astigmatism for rotation problem.


Edited by ngc7319_20, 12 February 2021 - 06:40 PM.


#16 LukaszLu

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Posted 12 February 2021 - 07:30 PM

I think that what you can see in the photo is the diffraction disc, but overexposed. I took the picture with a mobile phone, sorry for the quality, but I had no way of reducing the amount of light to make the diffraction rings visible. They line regularly towards the center of this shape. I'll try to take pictures with my SLR tomorrow - maybe I can show the rings.

 

This is not the shape of the hole in the foil, because in Vixen the defocused image is round, and secondly, the shape rotates as the lens rotates. If it was the shape of a pinhole, its position would not change as the lens rotates. I can't move the pinhole any further - that's the entire length of my house :-)

 

I reckon it may be a more serious drawback of the glass, but oddly enough, it is quite regular and three-direction deformation. It is hard to imagine that the problem with grounding, and even more so with the annealing of glass, worked in such a special way ...


Edited by LukaszLu, 12 February 2021 - 07:33 PM.


#17 ngc7319_20

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Posted 12 February 2021 - 07:57 PM

... the shape rotates as the lens rotates. If it was the shape of a pinhole, its position would not change as the lens rotates. I can't move the pinhole any further - that's the entire length of my house :-)

 

Yes, of course.  If the pattern goes around with the lens cell, the problem is probably in the lens cell.  I wish we had a good solution for you, but I don't see one right now.



#18 LukaszLu

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Posted 12 February 2021 - 08:18 PM

Well - I'm also afraid that there is no point in doing any experiments with taking the lens apart because I can't see anything pressing against the lenses at the moment, that could cause this effect.

 

The worst thing is that around bright objects, there are nasty radial glows that show the outlines of diffraction discs - also spreading in three directions. This effect can sometimes be observed with extremely bright objects such as Venus. Here it even applied to a faintly glowing Mars. The image is simply nasty and well below expectations even in the case of average quality optics.

 

Perhaps this is a defect that has been present from the very beginning - I do not know the exact history of this telescope, but if it was used by children or as an occasional "family" gadget, of course such a defect could have gone unnoticed for many years.



#19 PETER DREW

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Posted 13 February 2021 - 07:54 AM

A pronounced glow around bright objects could be indicative that the lens has been removed at some point for cleaning and the components incorrectly assembled before refitting.


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#20 LukaszLu

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Posted 13 February 2021 - 08:16 AM

A pronounced glow around bright objects could be indicative that the lens has been removed at some point for cleaning and the components incorrectly assembled before refitting.

It looks exactly as shown on the picture in my first post, maybe a little less regular - it's brighter towards one side. Mars looks like this:

 

mars.jpg

 

This also applies to artificial star picture. There are no traces of such glow on 60/900 Vixen at the same time.


Edited by LukaszLu, 13 February 2021 - 08:49 AM.


#21 LukaszLu

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Posted 13 February 2021 - 06:51 PM

Small update. First I unscrewed the rataining ring and checked if both lenses move freely when propped underneath. There's absolutly nothing that could hold or press the glass when retaining ring is not tightened firmly. Second - I've repeated the artificial star experiment without the dewshield to make sure it's not the source of any pressures. The result was the same as yesterday, I managed to make better picture of out of focus diffraction disc:

 

20210214_002028(0).jpg

 

I've also found a picture of the lens I've made a few days ago that shows some kind of interference pattern. The rings are not round however and in my opinion resemble the diffraction disc's shape... Coincidence?

 

obiektyw interferencja.jpg


Edited by LukaszLu, 13 February 2021 - 07:01 PM.

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#22 ccwemyss

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Posted 13 February 2021 - 10:05 PM

You're seeing Newton's Rings. You can get a better view of them using a CFL bulb. Their presence would indicate that the lens elements aren't flipped with respect to each other. When you can see them clearly, you can play around by applying some pressure to the circumference of the lens and see if they move or change shape. If you decide to take them out, you can lay them on a table and see if the rings are different outside of the cell. It's possible that the cell is rim isn't flat or that it is pinching. At that point, you can also try rotating one with respect to the other, to see if there is any difference (be sure to put an alignment mark on the edge before you start, so you know where you began). They should be round and roughly centered. 

 

Chip W. 


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#23 ngc7319_20

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Posted 13 February 2021 - 11:03 PM

Small update. First I unscrewed the rataining ring and checked if both lenses move freely when propped underneath. There's absolutly nothing that could hold or press the glass when retaining ring is not tightened firmly. Second - I've repeated the artificial star experiment without the dewshield to make sure it's not the source of any pressures. The result was the same as yesterday, I managed to make better picture of out of focus diffraction disc:

 

I've also found a picture of the lens I've made a few days ago that shows some kind of interference pattern. The rings are not round however and in my opinion resemble the diffraction disc's shape... Coincidence?

Probably not a coincidence that the interference patterns matches the star test.  The pattern is called Newtons Rings and is caused by the airspace between the glasses being some number of wavelengths of light.   Math is something like: 

 

2 x space = integer x wavelength....   makes bright ring due to constructive interfereince of light waves

 

2 x space = ( integer + 1/2 ) x wavelength....   makes dark ring due to destructive interference

 

Yes, non-circular pattern directly implies that one or both glasses as warped.  For spherical glass surface -- what you should have -- the rings are circular.  Triangular pattern in rings implies triangular warp.  Perhaps scope had the retaining ring too tight for several decades and the glass warped or flowed slightly.  

 

One idea might be to rotate the glasses 60 degrees relative to each other.  Maybe if both are warped, it might help cancel the pattern.  First mark the original orientation.  Put arrow on edge of each for direction light is supposed to go through the glass.  Label front F and rear R also.

 

Maybe you can also watch the ring pattern and see if rotating them can give you more circular ring pattern.

 

Good luck!


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#24 ngc7319_20

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Posted 13 February 2021 - 11:21 PM

I suppose there is some scenario where you could intentionally warp the glass to remove the pattern.  This would involve placing spacers on the front most and rear most surfaces at 120 degrees.  You could use small pieces of tape -- perhaps double thickness -- for these spacers.  These would go 60 degrees to the "points" of the ring pattern.  It appears from the photo you posted that this is about where the current spacers are.   

 

Next you would need to install new thicker spacers (thicker than original spacers) between the glasses 60 degrees to where the original spacers are. 

 

Then re-assemble.  Tightening the cell retaining ring (gently) should slightly deform the glass in the correct direction.  You can probably watch and see if the ring pattern become more circular.  There is probably some small chance to break the glass if it is already damaged.  But done gently and carefully is probably OK.


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#25 ngc7319_20

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Posted 13 February 2021 - 11:23 PM

I guess I would also check that there are no obstructions inside the telescope tube.  Like a baffle that is fallen out of place.  There is little chance that is the problem -- but worth a quick look.


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