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Lens support

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#1 Lucullus

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Posted 26 October 2020 - 11:55 AM

What are tension-free lens cell designs, still allowing for centuring?



#2 Bob4BVM

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Posted 26 October 2020 - 12:30 PM

To me that means a cell thats just a bit oversized for the lens. The lens in the cell loosely enough to hear just the slightest bit of rattle when you shake it. So cell ID is around 30 mils larger that the lens OD, giving 15 mil spacing all around lens OD,

 

Centering ?

That should be taken care of by the concentric machining of the lens cavity to the tube mounting end of cell.

If you mean collimation, for that i use a 6-screw arrangement, 3-push, 3-pull on 120* spacing around front of cell

 

CS

Bob



#3 Benach

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Posted 26 October 2020 - 12:45 PM

Lucullus: what are you looking for exactly? There are a few standard solutions for lens cells such as the "three spring solution", Another famous one is the nested spring solution.



#4 Lucullus

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Posted 26 October 2020 - 12:53 PM

Lucullus: what are you looking for exactly? There are a few standard solutions for lens cells such as the "three spring solution", Another famous one is the nested spring solution.

Thank you Benach. I tried to google but found no helpful lens support setups with my keywords. Thanks for your suggestions!



#5 Benach

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Posted 26 October 2020 - 01:46 PM

Another famous one is the three O-ring solution. So yeah, plenty to choose from. But if you tell me your purpose, I can think what the best solution could be.

 

If you mean collimation, for that i use a 6-screw arrangement, 3-push, 3-pull on 120* spacing around front of cell

 

Ever considered to use kinematic solutions for that?



#6 davidc135

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Posted 26 October 2020 - 01:58 PM

Centering can also refer to the removal of wedge in the individual lenses by edge grinding before assembly. I guess this probably isn't what you have in mind but if there is unwanted wedge and thus prismatic error in an air-spaced doublet one simple solution would be to have the crown lens adjustable, side to side, by nylon grub screws. OK for small objectives.

 

The objective can always be de-centered to counter atmospheric dispersion if needed. Say on low planets.   David



#7 Lucullus

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Posted 26 October 2020 - 02:38 PM

Another famous one is the three O-ring solution. So yeah, plenty to choose from. But if you tell me your purpose, I can think what the best solution could be.

 

Ever considered to use kinematic solutions for that?

Thank you very much Benach. Last week I read quite a bit about mirror support options and found some fun in sketching some mirror cells with all kinds of beneficial aspects. Yesterday I wondered about refractor lenses and realised I have no idea how those collimation screws work and how astigmatism is avoided while still securely holding those doublets or triplets, maybe even hermetically sealing oil between them. An image appeared in my mind how one could apply the lateral mirror support with springs to lenses and thats just when you wrote your spring-solution suggestion :) .



#8 Benach

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Posted 26 October 2020 - 03:09 PM

Lucullus: Usually mass production refractor lenses are made slightly bigger and machined on a centering machine to the nominal diameter. These devices remove wedge error as much as possible as well. The lens cell is made within predetermined mechanical tolerances.

If you intend to make your own refractor objective, you'd better measure wedge error and thickness regularly. Lateral support can then be made with a simple three O-ring solution.

 

DavidC135: this can only be done if there are enough optical degrees of freedom left. In a catadoptric system this requires at least two mirrors and three lenses to have enough DOFs to make a ADC. Not sure but I think an ordinary triplet apochromat is too sensitive to shift errors to accomodate for an ADC.


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#9 Alrakis

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Posted 26 October 2020 - 09:49 PM

Check out this

 

http://wolfgangbusch...Seiten/hab.html

 

Information on a triplet and how the cell was designed (note that the link is in German) 

 

Chris 


Edited by Alrakis, 27 October 2020 - 01:32 AM.


#10 Benach

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Posted 27 October 2020 - 03:04 AM

Lucullus: here you have short video on lens centering: https://m.youtube.co...h?v=Yy4fbN7-uQY

I know the guy who made this video personally. I also know the telescope and ATM he refers to. So if you have questions, feel free to ask. Note BTW that no real springs are used for lens centering but often elastic flexures. These are highly elastic but have several advantages over traditional springs.


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