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Kenko TA-910 picture issue - a great disappointment

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#51 ngc7319_20

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Posted 15 February 2021 - 09:12 PM

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

Yes, I input astigmatism, pinch, and de-focus, and it makes an image.  The defocus is easy to guess in "waves" units, since it is roughly the number of rings in the star image.  Setting the astigmatism and pinch takes some trial and error.



#52 Kokatha man

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Posted 15 February 2021 - 09:14 PM

Quite honestly Luke, given the scope variabilities as well as those when eliciting a defocused star (artificial or not) image I don't think you have much too complain about with that last image & really think you need to get out & do some more comparisons with other "classics" in your arsenal to gauge it again now that you have fiddled with it...

 

Adjusting the black-point & converting to grayscale throws up what I would deem an acceptable image in the circumstances from your last image. It might be worth checking the orthogonality of the tube wrt the focuser Vs objective cell perhaps as another check, a simple task. 

 

Lukes.png

 

..I display a couple of (real) defocused star images using the C14 here on our website: https://momilika.net...3Processing.htm   - I should add several more variants tbh - but with those sorts of defocused star patterns, images like this compressed selection of the recent 2020 Mars of mine below don't suffer - I have a personal "vendetta" lol.gif against all the pc-generated images of defocused star patterns & Airy Disks the internet is replete with, that give utterly false notions of what we should be seeing in nearly all scopes amateurs use! wink.gif

 

SmallMarsComp.jpg

 

 

 

 


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#53 LukaszLu

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Posted 16 February 2021 - 05:29 AM

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.  

I've noticed that the disc shape is very sensitive to eye position. Contrary to Vixen, which gives regular, round shape that does not change.

 

 



#54 LukaszLu

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Posted 16 February 2021 - 06:20 AM

 

Yes, I input astigmatism, pinch, and de-focus, and it makes an image.  The defocus is easy to guess in "waves" units, since it is roughly the number of rings in the star image.  Setting the astigmatism and pinch takes some trial and error.

Thanks a lot. What a great tool! I didn't know it's possible to get such detailed diagnosis based on a single photo. I'll have to play with this software...

 

 

Quite honestly Luke, given the scope variabilities as well as those when eliciting a defocused star (artificial or not) image I don't think you have much too complain about with that last image & really think you need to get out & do some more comparisons with other "classics" in your arsenal to gauge it again now that you have fiddled with it...

 

Adjusting the black-point & converting to grayscale throws up what I would deem an acceptable image in the circumstances from your last image. It might be worth checking the orthogonality of the tube wrt the focuser Vs objective cell perhaps as another check, a simple task.

 

..I display a couple of (real) defocused star images using the C14 here on our website: https://momilika.net...3Processing.htm   - I should add several more variants tbh - but with those sorts of defocused star patterns, images like this compressed selection of the recent 2020 Mars of mine below don't suffer - I have a personal "vendetta" lol.gif against all the pc-generated images of defocused star patterns & Airy Disks the internet is replete with, that give utterly false notions of what we should be seeing in nearly all scopes amateurs use! wink.gif

 

 

Thanks! Of course you're right: real life tests should be conclusive. Today the weather broke again so I don't know when I will have the opportunity to point my lens at Mars and the stars, but one thing worries me: despite the promising results of the tests carried out during the day, the night tests on the spot images of stars or the small Mars disk show that the smaller and the brighter the object - the worse. In the case of such objects, I constantly see a very clear difference compared to Vixen 60/900. I chose it as a reference because not only does it have similar parameters, but it is a refractor of very average quality. I would expect not only to catch up with it, but a much better picture.

Of course, I am aware that computer-generated images of diffraction disks are just a model, but the truth is that even this average Vixen produces a beautiful, regular circle. More importantly - it remains a circle regardless of the degree of defocus. In Kenko's case, the disc loses its roundness and becomes "angular" the smaller the disc is.

Perhaps the lenses are indeed distorted and the distorted image of the diffraction disc is one of the symptoms of this defect. Perhaps this particular symptom can be compensated by changing the relative position of the lenses, while other effects of improper lens geometry do emerge - for example in the case of such point objects?

I will report the results of further tests :-) 

 

 



#55 davidc135

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Posted 16 February 2021 - 09:27 AM

The two lenses are separated by the three spacers. One thing that could be checked or tried is to ensure that the flint rests on the cell at three corresponding spots and also that the front ring contacts the crown similarly such that any force is transmitted down through the glasses into the cell without flexing either lens. I doubt whether there is enough force involved for this to be the explanation but three thin cardboard tabs supporting the flint would be worth a try.

 

David


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#56 Paul Sweeney

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Posted 16 February 2021 - 09:55 AM

I've noticed that the disc shape is very sensitive to eye position. Contrary to Vixen, which gives regular, round shape that does not change.


I had an Apollo 60/910 that was also very sensitive to eye position. If you were not dead on axis, it had triangular stars. I tried everything: rotating the lenses, flipping the lenses, changing the spacing, loosening the retaining ring, all to no avail. I improved it, but I couldn't fix it.

Sometimes glass warps and there is no way to correct it.
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#57 Terra Nova

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Posted 16 February 2021 - 12:12 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.

By Jove! I think you’ve got it!”

 

I think this is by far the best permutation/iteration you’ve come up with! It may nit get any better than this which is really pretty good. Way better than a 60mm x 800mm Towa I was unfortunate enough to briefly possess.


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#58 LukaszLu

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Posted 16 February 2021 - 03:10 PM

By Jove! I think you’ve got it!”

 

I think this is by far the best permutation/iteration you’ve come up with! It may nit get any better than this which is really pretty good. Way better than a 60mm x 800mm Towa I was unfortunate enough to briefly possess.

Terra, I'm really grateful for your words of encouragement and support :-) I know your opinion on the Towa optics as I have carefully read many of your comments on this topic. As for me, I have incomparably less experience than you, but I have two Towa refractors and I generally agree with your assessment - this is mid-range equipment at best. But for this very reason - also after reading many of your opinions - I was counting on something more from Kenko. That is why I would like to give this lens a chance and finish my tests. I have gone so far that it is worth checking what can be obtained by the method of rotating of the lenses - if only so that others who find this topic could obtain some conclusions from it.

 

So I will try a few more settings in the vicinity of the two positions where the diffraction pattern seemed the best.

 

I had an Apollo 60/910 that was also very sensitive to eye position. If you were not dead on axis, it had triangular stars. I tried everything: rotating the lenses, flipping the lenses, changing the spacing, loosening the retaining ring, all to no avail. I improved it, but I couldn't fix it.

Sometimes glass warps and there is no way to correct it.

 

Well, maybe this thread will become a turning point, prompting to verify opinions on this model? Maybe this lens is not a deviation from the norm at all? The very fact that there are almost no anti-reflective layers raises a question about the quality of this optics. When I saw it, I thought immediately: "gosh, Towa would never allow something like this" ... ;-)

 


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#59 LukaszLu

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Posted 16 February 2021 - 03:16 PM

The two lenses are separated by the three spacers. One thing that could be checked or tried is to ensure that the flint rests on the cell at three corresponding spots and also that the front ring contacts the crown similarly such that any force is transmitted down through the glasses into the cell without flexing either lens. I doubt whether there is enough force involved for this to be the explanation but three thin cardboard tabs supporting the flint would be worth a try.

 

David

Thanks David! I'm sure that no external force is transmited through the glass as lenses rest quite loosely in the cell. I've checked the situations when they rattled when the cell was shaken, therefore I exclude the possibility of improper pressure from the rings closing the cell at the top or bottom. What's more, they came to me in this state, which may suggest that for many years no significant pressure was exerted on the lenses...


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#60 davidc135

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Posted 16 February 2021 - 04:34 PM

Thanks David! I'm sure that no external force is transmited through the glass as lenses rest quite loosely in the cell. I've checked the situations when they rattled when the cell was shaken, therefore I exclude the possibility of improper pressure from the rings closing the cell at the top or bottom. What's more, they came to me in this state, which may suggest that for many years no significant pressure was exerted on the lenses...

There is the weight of the crown acting down when viewing Mars but yes, very slight and even that wouldn't apply if the scope was in a horizontal testing position so probably not the explanation.  David



#61 LukaszLu

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Posted 16 February 2021 - 07:03 PM

OK, enough is enough... I'm finishing my test on something like this today, and I'm going to be testing outside tomorrow:

 

_1010839.jpg

 

Unfortunately, from what I can see, the chances of clear skies this week are pretty slim.

 

I have found it difficult to control the mutual alignment of the lenses because tightening the retaining ring causes the lenses to rotate a little. If you disassemble this type of lens and mark the position of the lenses, reassembling them will most likely require the markers to be moved a little to compensate for this rotation.

 

@David, I also made a few tests to make sure that the pressure of the retaining ring did not affect the shape of the diffraction disk. It did not :-)


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#62 ngc7319_20

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Posted 16 February 2021 - 07:37 PM

 

 

I have found it difficult to control the mutual alignment of the lenses because tightening the retaining ring causes the lenses to rotate a little. If you disassemble this type of lens and mark the position of the lenses, reassembling them will most likely require the markers to be moved a little to compensate for this rotation.

 

 

 

That looks really nice!

 

Maybe put small piece(s) of tape across the edges of the lenses to hold the rotation?  Perhaps some thin tape.  Here we have "Scotch Magic Tape" which is pretty thin.  If it is too thick, the lenses will be tight in the cell.


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#63 ccwemyss

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Posted 16 February 2021 - 08:21 PM

I sometimes find that I need to press gently on the lens with a lens cloth to keep it from rotating, while snugging up the retaining ring with a fingernail of the other hand.

 

Chip W. 


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#64 Kokatha man

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Posted 16 February 2021 - 11:32 PM

...if you haven't already get one of the variations on this tool (a lens spanner/wrench) - very cheap btw!

 

ScreenHunter_2266 Feb. 17 15.00.jpg


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#65 ccwemyss

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Posted 17 February 2021 - 12:26 AM

I have an excellent spanner - made by a small company that also did repairs on large format photography lenses. Beautifully machined, super stiff, and very precise. But I sometimes find, when I get very close, and a lens is somewhat sensitive to pressure, that the feeling from a fingernail lets me get it just right. Of course, it only works when the ring fits well enough to be moved easily by one of the notches. 

 

Chip W. 



#66 LukaszLu

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Posted 17 February 2021 - 10:45 AM

Maybe put small piece(s) of tape across the edges of the lenses to hold the rotation?  Perhaps some thin tape.  Here we have "Scotch Magic Tape" which is pretty thin.  If it is too thick, the lenses will be tight in the cell.

A good idea. Although I am always afraid of the proximity of adhesives and paints to the optics covered with anti-reflective layers, there are almost no such layers here, so there is almost no need to worry :-)

 

 

I have an excellent spanner - made by a small company that also did repairs on large format photography lenses. Beautifully machined, super stiff, and very precise. But I sometimes find, when I get very close, and a lens is somewhat sensitive to pressure, that the feeling from a fingernail lets me get it just right. Of course, it only works when the ring fits well enough to be moved easily by one of the notches.

In the absence of such a tool, an ordinary caliper works quite well.



#67 Kasmos

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Posted 18 February 2021 - 03:03 AM

I can't remember the make but more than once a lens has been posted that had a piece of tape (put there by the manufacturer), to keep them aligned. I might be mistaken but  kind of doubt a piece of Scotch tape would hurt the coatings.


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#68 LukaszLu

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Posted 18 February 2021 - 07:30 AM

You are probably right, but in the case of old magnesium fluoride coatings, I prefer to keep the limited principle of trust :-)



#69 LukaszLu

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Posted 19 February 2021 - 05:35 PM

The snow stopped falling, Mars appeared among the clouds for a while, so today I had the opportunity to test the last lens positioning, which I believe gives the most regular diffraction pattern.

 

One word: massacre. The image can hardly be sharpened. It goes from the phase of each object doubled to out of focus. A nasty radiant glow is still visible around bright objects such as Mars. I tried to observe other glowing objects, such as street lamps. Same thing - no sharpness, not even decent contrast. It confirms what I've noticed before - the brighter and smaller the object, the worse.

 

Not even close to being useable - I think that description given by @Kasmos fits the situation best. After all these tests, I think it's time to acknowledge that this lens has such serious flaws that it is useless for anything but decoration. I'm sorry to say this, but ... Kenko - you've made a total trash! What a shame...


Edited by LukaszLu, 19 February 2021 - 05:36 PM.


#70 ngc7319_20

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Posted 19 February 2021 - 06:02 PM

Sorry to hear it...  Interesting that artificial star test was nice...  Perhaps find a replacement lens...



#71 LukaszLu

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Posted 19 February 2021 - 06:59 PM

Yes - the test was promising, but in terms of the shape of the diffraction disk. Perhaps rotating the lenses in relation to each other compensates for the defects manifested by the incorrect shape of the disk, but does not compensate (or maybe even intensifies) other defects?

 

I talked to a service technician, he said that there is a method of removing the incorrect tension of the glass by heating the lenses to a temperature of 300-400C and cooling slowly, but firstly you would need to know what a defect we are really dealing with, and secondly - there are some limits to the profitability of further fight for this lens. I think I'm close to such a limit...

 

It would be best to find another one, of course, but it looks like a lottery - you can find more descriptions of the terrible quality of the lenses in this model, users complain about the astigmatism that disqualifies them, the multiplication of the image I observe, etc. defects.


Edited by LukaszLu, 19 February 2021 - 07:00 PM.


#72 Kokatha man

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Posted 19 February 2021 - 07:57 PM

Well, Mars is very small & low in the sky now & I think most people would be really struggling to discern much from it: your description sounds very much like a lot of the seeing we experience when the seeing & elevation is bad as it has been since Xmas, & I think I know a thing or two about planetary imaging! :lol:

 

But that of course doesn't excuse the appearance of the street lamps - the suggestion of looking for another lens of similar size & f/l might be worthwhile...Sheldon here on CN has some good small objectives that he sources from various places only he seems to be able to access, might be worth PM'ing as one suggestion, I've obtained stuff from him & I'm in Australia...

 

But of course there are "dogs" from all manufacturers & I've made the comment here on CN several times about getting too carried away with notions of all these 1960's etc small refractors being "Holy Grail's" of the telescope-making arts...I've chronicled my efforts with the Royal Astro 4" f10 Newtonian - still not to the stage of knowing whether the primary which I've spent $200Aus getting re-mirrored is actually of the standard many reverently bless RAO scopes with :lol: but I at least have a good substitute if it is a dud - but let me tell you bro, whoever positioned & cut out the hole in the ota for the secondary/focuser when this scope was made was either (a) drunk (b) totally incompetent or © bent on some sort of vendetta against the company...it was about 10-15mm out of position & on a small secondary that is horrific - so much for RAO standards..!

 

My Dai-Ichi Kogaku, who were mainly OM's for other Japanese branded scopes outperforms my Yamamoto SYW, which is no slouch itself...but I actually think it is a lottery with many of the old so-called "classics" tbh..!


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#73 Kasmos

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Posted 19 February 2021 - 08:00 PM

I replaced the 710mm in my Kenko with a 700mm and didn't notice any difference in the focuser's length or any problems. I realize your options in Poland are much more limited than in the states but very many 900mm Towas were made, so maybe you can find a good objective from one. I was a Towa doubter but two of mine turned out to be very good. Also, if you find one in another country, shipping for just the lens might not be too costly. I suppose it largely depends on how valuable or how good the rest of the kit is. 


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#74 ngc7319_20

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Posted 20 February 2021 - 01:32 AM

Does the "best focus" indoor artificial star test view correspond to what you see outdoors?  Or is the indoor view some how better?

 

Have we ruled out some other problem like the 90 degree diagonal?  Any chance we a looking through trees outside?  Sorry for seemingly obvious questions.


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#75 LukaszLu

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Posted 20 February 2021 - 06:32 AM

Well, Mars is very small & low in the sky now & I think most people would be really struggling to discern much from it: your description sounds very much like a lot of the seeing we experience when the seeing & elevation is bad as it has been since Xmas, & I think I know a thing or two about planetary imaging! lol.gif

 

But that of course doesn't excuse the appearance of the street lamps - the suggestion of looking for another lens of similar size & f/l might be worthwhile...Sheldon here on CN has some good small objectives that he sources from various places only he seems to be able to access, might be worth PM'ing as one suggestion, I've obtained stuff from him & I'm in Australia...

 

But of course there are "dogs" from all manufacturers & I've made the comment here on CN several times about getting too carried away with notions of all these 1960's etc small refractors being "Holy Grail's" of the telescope-making arts...I've chronicled my efforts with the Royal Astro 4" f10 Newtonian - still not to the stage of knowing whether the primary which I've spent $200Aus getting re-mirrored is actually of the standard many reverently bless RAO scopes with lol.gif but I at least have a good substitute if it is a dud - but let me tell you bro, whoever positioned & cut out the hole in the ota for the secondary/focuser when this scope was made was either (a) drunk (b) totally incompetent or © bent on some sort of vendetta against the company...it was about 10-15mm out of position & on a small secondary that is horrific - so much for RAO standards..!

 

My Dai-Ichi Kogaku, who were mainly OM's for other Japanese branded scopes outperforms my Yamamoto SYW, which is no slouch itself...but I actually think it is a lottery with many of the old so-called "classics" tbh..!

I have very good experience with RAO - my R-74 gives fantastic images and if it weren't so big, heavy and troublesome to carry, I wouldn't even look for something smaller. However, the Kenko is much lighter and more convenient - it is very easy to carry and has a very good and convenient mount. If it could be used for something more than a museum exhibit, it would be a great addition to the observation arsenal.

 

At the moment, Mars is very well visible in Poland - in the evening it was almost 60 ° above the horizon, so it is not the atmospheric dispersion that is responsible for the image distortions. Admittedly, the sky was a bit overcast and the air was restless, so I don't want to make final judgments. However, it did not have the same effect on the pictures given by Vixen 60/900, which I used again as a comparison.The weather is nice since the morning - maybe I'll be able to re-try in clear skies.

 

It is sympromatic that a very similar situation takes place with the tripod. While in the case of the lens, the glass has probably not been properly annealed, the cheapest, unseasoned wood was used for the production of the tripod. Some of the wooden elements are bent.

 

 

 

 

I replaced the 710mm in my Kenko with a 700mm and didn't notice any difference in the focuser's length or any problems. I realize your options in Poland are much more limited than in the states but very many 900mm Towas were made, so maybe you can find a good objective from one. I was a Towa doubter but two of mine turned out to be very good. Also, if you find one in another country, shipping for just the lens might not be too costly. I suppose it largely depends on how valuable or how good the rest of the kit is. 

I found out yesterday that even my quirky, ancient Towa spotting scope whose image quality I criticized here: https://www.cloudyni...epiece-revolver / gave a much more precise and contrasting image. Towa - please forgive me! :-) I think that I will try to monitor offers in Germany - this model is quite popular there, as well as Weltblick, which I think has the same lens.

 

 

 

 

Does the "best focus" indoor artificial star test view correspond to what you see outdoors?  Or is the indoor view some how better?

 

Have we ruled out some other problem like the 90 degree diagonal?  Any chance we a looking through trees outside?  Sorry for seemingly obvious questions.

Unfortunately, yes - the problems with obtaining a precise image of the pinhole in the foil resemble those with the Mars disk - they are of similar angular size.

 

Of course, I've tried to replace or remove the diagonal, and I've tried a few different eyepieces. I am aware of the bending of the light on the branches along the way and to be honest I could not completely eliminate this influence while observing the street lamps, but I tried to change the position to make sure that it would not improve the situation. When the snow melts and I can safely carry the telescope over longer distances, it will be easier for me. Unfortunately, my passion for gardening is in drastic contradiction to my love for astronomy :-)


Edited by LukaszLu, 20 February 2021 - 07:50 AM.



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