Re: [ATM] new type of floating cell

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

> rubber for the mat. The spring constant of the mat must be smaller (order
> 10^5) than taht of the mirror. If this criterion is met and co-planarity
is
> in the order of 0.5 millimeter, deformation can be held within the order
of
> 5 nanometer.


Youngs' modulus for glass is 65 GPa, Aluminium is 70GPa and that of the
softest rubber is about 0.01 GPa (when deformed only a little): a ratio of
less than 10^4

The spring constant is also determined by some geometrical dimension. The
10^5 rule then means that this geometrical parameter must have a 10 times
larger value than that of the glass(e.g. the thickness). What did the author
define as spring constant?

Spring constant is proportional to Young modulus E and (original) cross
section area A, and inversely proportional to (original) thickness or length
L:

    k = E*A/L

So using a thick foam rubber mat with lots of holes puched in it will result
in a spring with very low k. The holes also facilitate better cooling.
As an alternative you can use a floor polishing pad, which consists of
millions of tiny plastic fibers, which are all little springs. The pad will
breathe better than foam.

A drawback: lower k also means worse definition of mirror position. The
question therefore is: where is the optimum?

Arjan te Marvelde
(AtM since 1961)

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