Re: [ATM] Does glass retain moisture?

[Woodchuck <djv@bedford.net>]

On Thu, 18 Mar 2004, Mel Bartels wrote:

>> changes a tiny bit after the mirror as a whole equilibrates to a different
> >temperature. Given that, why do some people with mirrors of very high
>> quality claim that the star test is different at warm temperatures than at
>> cold temperatures?? (Of course, I'm talking about AFTER the mirror has
>> equilibrated to the ambient temp. We are fully aware that the star test is
>> horrible AS the mirror equilibrates.)
>
> You can see this effect in average to poor quality mirrors.  It's simply due
> to the unequal cooling of the glass, that is, the front of the glass cools
> at a different rate than the back portion of the glass.  Plate glass has it
> worse than pyrex.
>
> Mel Bartels

But once equilibrated, that would still result in a paraboloid shape.
I think we need to have regions of the mirror that have different coefficients
of thermal expansion, so that a paraboloid polished at one temperature
would be a paraboloid at only that temperature, in fact would be a surface
of revolution only at that temperature.  This should result in the
surface exhibiting a sort of "dog biscuit"-like lumpiness at any other
temperature, regardless of how well equilibrated.

It is probably less mysterious how such regions of different expansion
come into being than trying to explain how a big hunk of amorphous glass
could have any physical property uniform throughout its extent.

Dave

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