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Takahashi TSA102 needs collimation?

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#1 RichardZ

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Posted 30 June 2020 - 04:48 AM

Hi All, this is my first post here but I've been lurking this forum since years!

 

I have a TSA102 triplet and when looking at a very bright star at high power (230x, no diagonal) in good seeing I notice that, in-focus, the airy disk appears sharp but the diffraction pattern is slightly but obviously skewed to the left: the diffraction rings look brighter and "more present" in that direction than in the opposite one. Moreover, if the star is of medium magnitude (i.e. not overly bright) the first diffraction ring appears "C shaped" around the left portion of the airy disk, rather than circular. If I turn the tube 180° in its cradle, I observe the same behavior but reversed (rings brighter on the right portion of the star).

 

As for the out-of-focus pattern, I see similar figures inside and outside focus, with apparently concentric Fresnel rings. Outside focus pattern looks sligthly sharper.

 

I also tried putting a Cheshire eyepiece in the focuser: the green reflections appeared concentric but they looked best (sharpest) if I kept my eye sligthly off-axis. If my eye is perfectly on-axis, the central darker spot looks sligthly blurred and not perfectly black and sharp as it appears with my eye off-axis.

 

This makes me think of possible miscollimation of the scope. Unfortunately, these scopes do not have collimation screws and I don't know what to do...

 

Could you help me diagnose the possible issue?


Edited by RichardZ, 30 June 2020 - 07:59 AM.


#2 Star_Shooter

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Posted 30 June 2020 - 07:12 AM

It sounds like coma on axis. The cause is one of the elements is tilted relative to the other two elements in the lens cell. Your cheshire eyepiece proved that the collimation is ok in term of lens cell /tube alignment. Plus if there is heavy mechanical misalignment, the star image will look like astigmatism, not coma. A slight mechanical misalignment will have almost no impact on the star image. Been there, done that.

Your best bet is to send to Takahashi USA to adjust the lens within the lens cell.

Lei

Edited by Star_Shooter, 30 June 2020 - 07:13 AM.


#3 Star_Shooter

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Posted 30 June 2020 - 07:25 AM

I forgot to say, there is another possibility. The lens could be grounded to have coma, if you look at the interferometer graph,one side of the lens could be higher/thicker than the other side. But being a Takahashi lens, that’s probably not the case.

Lei

#4 John Turley

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Posted 30 June 2020 - 07:31 AM

 

This makes me think of possible miscollimation of the scope. Unfortunately, these scopes do not have collimation screws and I don't know what to do...

 

Your best bet is to send to Takahashi USA to adjust the lens within the lens cell.

Do all TAK Refractors have non-collimatable lens cells, returning the scope to Takahashi will be very expensive (unless covered by warranty), at least the much cheaper Esprits have fully collimatable lens cells.

 

John 


Edited by John Turley, 30 June 2020 - 11:13 AM.


#5 emilslomi

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Posted 30 June 2020 - 07:57 AM

A brightness difference in the diffraction rings without any obvious asymmetry of the ring pattern or in/out of focus pattern sounds like there is some coma, as Lei put it, "ground" into the lens. If everything else is fine and you usually enjoy your scope, I would not worry.

 

Emil

 

PS Welcome!


Edited by emilslomi, 30 June 2020 - 07:58 AM.


#6 RichardZ

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Posted 30 June 2020 - 08:02 AM

A brightness difference in the diffraction rings without any obvious asymmetry of the ring pattern or in/out of focus pattern sounds like there is some coma, as Lei put it, "ground" into the lens. If everything else is fine and you usually enjoy your scope, I would not worry.

 

Emil

 

PS Welcome!

Ultimately the in focus diffraction pattern look asymmetric to me... on one side of the star you clearly see the rings, on the other side you struggle to see them, maybe because they are not even there...



#7 Bill Barlow

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Posted 30 June 2020 - 09:55 AM

Sounds like the collimation is off and needs to be adjusted.  I once owned a Tak FC100 DC that also exhibited similar characteristics of a defocused star so I sent it in to Texas Nautical/Takahashi America and it was out of collimation.  I believe the cost was minimal to fix it.

 

Bill



#8 gjanke

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Posted 30 June 2020 - 10:24 AM

Do all TAK Refractors have none collimatable  lens cells, returning the scope to Takahashi will be very expensive, at least the much cheaper Esprits  have fully collimatable lens cells.

 

John 

 

No, some are can be collimated.



#9 Kunama

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Posted 30 June 2020 - 03:46 PM

Before you send the scope off to TNR might be a good idea to check that your focuser is aligned correctly.

Use a 2” laser directly in the visual back and make sure the laser beam hits the centre of the objective...



#10 Rutilus

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Posted 30 June 2020 - 04:37 PM

Sounds like the collimation is off and needs to be adjusted.  I once owned a Tak FC100 DC that also exhibited similar characteristics of a defocused star so I sent it in to Texas Nautical/Takahashi America and it was out of collimation.  I believe the cost was minimal to fix it.

 

 

Will they touch/repair a triplet?



#11 payner

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Posted 30 June 2020 - 07:32 PM

Yes, at least that is what I've read on these fora. Also, they check, and if needed tweak, every TOA before it leaves their store. That is based on direct conversation.




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