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Pimary mirror attachment

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#1 deepspaceguy

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Posted 01 February 2013 - 11:20 PM

Hey all, my 12" Lightbridge mirror is being recoated. What is the best way to reattach it to the cell? I used silicone before. Is there a better way? Some have suggested using velcro. Could I just set the mirror in the cell and use some kind of edge support to keep it in position(like felt or cork)? Any help is apprecitated.

Steve

#2 Mirzam

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Posted 02 February 2013 - 07:15 AM

I would modify the cell as necessary to allow the mirror to sit there without any need for silicon. Then you control the support parameters and the mirror is easy to remove for cleaning.

Can you post a picture of the cell? Is the mirror thickness 1.5"?

JimC

#3 StarDusty

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Posted 02 February 2013 - 02:39 PM

I am not familiar with the Lightbridge cell details, but for a 12" primary I would stick with the silicon. I used silicon on a 13". It worked fine.

For larger primaries, say 16" and up you want to keep the primary free and floating on the cell support locations.

#4 deepspaceguy

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Posted 02 February 2013 - 05:12 PM

Thanks Allen. Don't know what else to do. I keep hearing about astigmatism but Meade mounted the mirror with double sided foam tape. I know larger scopes use slings for support; I was wondering if I could do something without gluing the mirror to the three support triangles.

Steve.

#5 deepspaceguy

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Posted 02 February 2013 - 05:18 PM

Thanks Allen. Don't know what else to do. I keep hearing about astigmatism but Meade mounted the mirror with double sided foam tape. I know larger scopes use slings for support; I was wondering if I could do something without gluing the mirror to the three support triangles.
Trying to figure out how to post a pic of the cell.
Steve.

#6 Mirzam

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Posted 02 February 2013 - 07:23 PM

Gluing the mirror to support triangles defeats the purpose of the triangles. The purpose is to allow the mirror to "float" on a planar surface formed by allowing the triangles to tilt in response to tiny forces that would otherwise stress and possibly distort the surface. If you glue to the triangles you are effectively supported at only 3 points and you don't know the locations of these points. The glue in effect eliminates or at least seriously restricts the movement that allows the mirror to be supported uniformly.

People often say "I used silicon and it words fine". This is what is known as anecdotal evidence. A properly designed floatation cell has the strength of numerical analysis to ensure that it does what we want it to do.

To post a photo first resize it to less than 200 Kb. Then choose "preview reply" beneath the text box at the bottom of the page. When you are in preview reply select "browse", which will give you an opportunity to attach the picture. Then click on Ok, submit.

JimC

#7 Pinbout

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Posted 02 February 2013 - 08:22 PM

Trying to figure out how to post a pic of the cell.



if you want to post more than one pic in a single post check out this long boring vid

posting pics

#8 StarDusty

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Posted 03 February 2013 - 02:40 PM

For small mirrors, I have been putting a small blob of silicon at each PLOP design contact point for my small telescope project for some time now. Plop is a great program, but only provides insight into mirror deflections for loads normal to the mirror's back. The action of the cell parts and their ability to flex and tilt is not really undone by the application of the silicon at the correct points.

I agree that for larger mirrors "gluing" the mirror down could restrain the mirror. It may not be to grow or contract freely in the radial direction when needed because of small thermal expansion coefficient differences between it and the cell supports. I was not suggesting “gluing” large mirror down.

I would be interested in learning more about the effects of in-plane loadings on small mirrors and the resulting deflections. Is there a way to account for typical lateral supports used to restrain a mirror from sliding around in its cell? Are there papers or programs out there to analyze supports like straps, side bumpers, etc?

To-date my experience and that of those who helped me become an ATMer is that the very small differences in lateral expansion/contraction of “small” mirrors just get absorbed by the flexibility of the silicon blobs and other parts of the cell that just “give” a little.

I agree that this practice is not based on the “strength” of numbers, as you put it.

Surely the number of successful telescopes in use today that included a blob of silicon here or there to support a small primary or secondary mirror counts for something?

#9 Mirzam

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Posted 03 February 2013 - 03:13 PM

As long as the mirror thickness ratio is not too extreme I would agree that small mirrors tend to be forgiving of mounting techniques. We don't know the thickness of the mirror in question, but I think that Lightbridge 12" mirrors are about 1.5" thick. This would give a thickness ratio of 8:1 and would probably tolerate the use of silicon for mounting purposes.

There is an excellent resource for comparing mirror edge supports that also talks about mirror gluing:

Mirror edge support calculator

JimC

#10 StarDusty

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Posted 03 February 2013 - 04:00 PM

Thanks for the reference to the edge support calculator. I will use it going forward.

#11 polaraligned

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Posted 04 February 2013 - 07:36 AM

My 14.5" Teleport has the cell triangles glued to the Zambuto mirror back, the cells then float on the collimation screws. It works for this scope.

#12 deepspaceguy

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Posted 04 February 2013 - 12:48 PM

I think I'm going back with silicon. Don't know what else to use.

Steve

#13 ZeroID

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Posted 04 February 2013 - 08:31 PM

Silicon if used correctly works well. You need to support the mirror above the touchdown points by a millimeter or so so the silicon forms a button of that thickness which allows the mirror to move the little bit it requires. Don't glue the mirror down hard on the support points so it touches. It needs to float above it slightly so the silicon can flex.
I've used the Velcro strip on my 10" and that works well too. The velcro hangs on like grim death ( Industrial strength ) yet it can move that little bit to allow mirror expansion without any strain. I have edge brackets just in case, they don't touch the sides. My mirror has never moved.






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