Re: ATM shear in silicone pads

[Aart Olsen <aolsen@prairienet.org>]

I'm now making my mirror cells out of maple, at least the parts that
have to mate with the mirror.  Most hardwoods have a longitudinal CTE
that's virtually the same as Pyrex, and fine maple has excellent
dimensional stability.  For a small mirror I make a triangle frame out
of 1.5 by .75 inch stock, putting six silicone dabs (positioned by PLOP
analysis) between the mirror and the triangle.  The collimation screws
are at the corners of the triangle and are in line with these silicone
dabs.  This means that effectively the triangle legs have only length
(i.e. dimensional changes in width and thickness, already small in
maple, don't matter), and this length changes with temperature exactly
as much as pyrex.  The collimation mechanism necessarily keeps the
triangular frame in a plane, so there's no tendency to warp.

Aart Olsen

"Arjan te Marvelde (ELN)" wrote:
> 
> Connecting the mirror cell to the mirror by means of a dab of silicone glue is, as i understand, the de-facto standard way of doing.
> However, if you look at cell and mirror materials, the differential expansion coefficient can be up to 20 um/m.K. If you assume a temperature range of +/- 15K and a typical dab distance of 10cm, the dabs will undergo a shear of +/- 30um.
> 
> Is there anybody who can say something about what the resulting force on the mirror is, as a function of dab area and thickness. I only get as far as: smaller and thicker dab means lower force, but i cannot stick a number on it. Maybe the effect is so small that you can just ignore it?
> 
>  ... Arjan te Marvelde

-- 
Aart Olsen
aolsen@prairienet.org