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To silicone or not to silicone?

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14 replies to this topic

#1 calypsob

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Posted 31 July 2020 - 03:01 PM

I have a 12" F4 Newtonian which has an unusual cell, there are 3 pivot points spaced 120 degrees around the cell with 2 pads and additionally beneath the mirror clips are threaded nylon screws that are designed to keep the mirror centered.

 

I am not a fan of mirror clips because they make additional diffraction spikes.   I would like to remove the plastic/rubber pads that are currently resting on the pivot points and replace them with silicone by shimming the mirror at an elevated height with a 2mm piece of metal while the silicone dries.

 

Ideally this will let me remove the mirror clips. I will come up with some way to secure the primary during transport so it is not relying on silicone alone.  I also hope that with a silicone secured primary there will be no shift of primary at all during my imaging sessions. 

 

With that said I my question is how should I space the silicone blobs out so they do not introduce astigmatism? 

 

In the image below, the allen head screw can see above the central triangle is where the pivot point is located. there are 3 points in total each with two 2mm thin plastic/rubber pads maybe .5" in diameter  

 

thumbnail_IMG_8761.jpg


Edited by calypsob, 31 July 2020 - 03:02 PM.


#2 mac57

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Posted 31 July 2020 - 03:21 PM

I can tell you that silicone will hold very well.  I re-mounted my secondary mirror with three dots and couldn't pull it apart. Use flat toothpicks as spacers. Mark


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#3 calypsob

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Posted 31 July 2020 - 05:38 PM

I can tell you that silicone will hold very well.  I re-mounted my secondary mirror with three dots and couldn't pull it apart. Use flat toothpicks as spacers. Mark

Yea, I mounted my smaller 6" f4 with silicone. It worked well. I know its possible to introduce stig on a larger thinner mirror so Im hoping someone can chime in. I have been reading about the PLOP method a bit and wonder if I should try to optimize the design of my cell with some 3d printer discs that can hold the silicone like you see on aurora cells 



#4 calypsob

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Posted 01 August 2020 - 01:42 PM

anyone else care to chime in? 



#5 coinboy1

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Posted 01 August 2020 - 02:44 PM

Silicone could cause astigmatism and pinched optics in a primary mirror. I wouldn't do it. You could always make a circular retaining ring/mask to cover the mirror clips so you don't get the diffraction patterns.


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#6 SteveV

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Posted 01 August 2020 - 05:30 PM

Your cell isn't all that unusual.  It looks derived more or less from a typical 6 point PLOP cell from the bottom.  The edge supports as is or what you propose are not going to pass muster with the thinking in the current 'edge support' thread.  

 

But if you like it as is, why not trying gluing?  What do you have to lose?  Some time and effort.  Glue it up put it to work.  If it works you're good.  If not you learned something.


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#7 lphilpot

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Posted 01 August 2020 - 05:51 PM

Will you wash the mirror with the entire cell attached (by removing cell and all from the scope)? Disassemble the cell and remove just the attached parts? Or just cut the silicon and re-glue each time? I personally don't care for glued-on mirrors, for that very reason.



#8 calypsob

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Posted 01 August 2020 - 06:00 PM

Silicone could cause astigmatism and pinched optics in a primary mirror. I wouldn't do it. You could always make a circular retaining ring/mask to cover the mirror clips so you don't get the diffraction patterns.

So are you saying that no matter what, if I shim and let the silicone dry, remove the shims, or duplicate the aurora silicone holding pads, there is no way to avoid astigmatism? 

 

Your cell isn't all that unusual.  It looks derived more or less from a typical 6 point PLOP cell from the bottom.  The edge supports as is or what you propose are not going to pass muster with the thinking in the current 'edge support' thread.  

 

But if you like it as is, why not trying gluing?  What do you have to lose?  Some time and effort.  Glue it up put it to work.  If it works you're good.  If not you learned something.

I am going to do it, but its such a pain to remove I want to have some sort of guidance before I do it.  

 

Will you wash the mirror with the entire cell attached (by removing cell and all from the scope)? Disassemble the cell and remove just the attached parts? Or just cut the silicon and re-glue each time? I personally don't care for glued-on mirrors, for that very reason.

Yes, I really dont wash it alot, it has a shroud.  The cell comes off easily, it is a truss setup. 



#9 MitchAlsup

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Posted 01 August 2020 - 06:19 PM

I made my mirror cell in such a way that there are 4 screws pushing on ball transfers at CoM in order that the mirror is held on the optical axis. The first picture is using dial indicators to limit mirror movement to 0.004:

 

assembly01.JPG

 

The next figure shows me attaching the front clips such that the mirror will not (at any angle, including pointing straight down) allow the mirror to leave the cell. For the most part, I got the clips with the same 0.004".

 

assembly02.JPG

 

At this spacing, the mirror clips hold on the bevel of the glass and do not interfere with the parabolic section.

 

The reason for choosing 0.004" is that this is how much differential contraction between the mild steel cell and frame and the supermax mirror from 100ºF to 0ºF the telescope might see. With 0.004, the mirror will never be pinched, and will never be able to leave cell, and will still not cause any additional diffraction effects.


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#10 calypsob

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Posted Yesterday, 06:32 AM

Wow that is a very precise procedure. How does she perform?

I want to avoid clips all together for imaging purposes. A mask will slow down my aperture which I also want to avoid. when I drive to my dark site, the mirror has a tendency to want to lean forwards and rest against the clips while the Newtonian sits in my car, which is one reason the silicone option is appealing 


Edited by calypsob, Yesterday, 06:32 AM.


#11 Pinbout

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Posted Yesterday, 08:32 AM

I would run some beads... let them dry and see how much they deform from the weight of your mirror. then make sure you use spacers at least that thick while letting the sillycone dry while the mirror is resting on the cell.

 

I use toothpicks for spacers while gluing my 2ndry's to the stalks.


Edited by Pinbout, Yesterday, 08:33 AM.

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#12 MitchAlsup

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Posted Yesterday, 10:51 AM

Wow that is a very precise procedure. How does she perform?

 

The scope performs great.


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#13 calypsob

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Posted Yesterday, 05:34 PM

I would run some beads... let them dry and see how much they deform from the weight of your mirror. then make sure you use spacers at least that thick while letting the sillycone dry while the mirror is resting on the cell.

 

I use toothpicks for spacers while gluing my 2ndry's to the stalks.

So in your opinion is a thinner shim better than a thick one?  I like the idea of running the beads, where on the cell would you put them? 



#14 Pinbout

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Posted Yesterday, 05:51 PM

No. not thinner the better.  Too thin is trouble, too thick you need proper edge support. You have to find that thickness for your mirror by testing. My opinion 


Edited by Pinbout, Yesterday, 05:51 PM.

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#15 calypsob

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Posted Yesterday, 06:06 PM

No. not thinner the better.  Too thin is trouble, too thick you need proper edge support. You have to find that thickness for your mirror by testing. My opinion 

sounds like a plan !




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