Re: ATM Has anyone worked with membrane mirrors?

[astillma@nit.AirTouch.COM (Anthony Stillman)]

Rick Nelson wrote:

Anthony,

Thanks for being so optomistic!  :)  I too have read "Unusual
Telescopes" so I figure
the idea is almost practical - maybe with a good idea..  I have also
thought of spinning a   
slow-curing medium to form a mirror.  I thought the figure would be
hyperboloid for
the membrane type and paraboloid for the spinning type, but I guess I am
mistaken.

A problem I see with using a liquid medium to reduce the pressure under
a membrane
A problem I see with using a liquid medium for reduction of pressure and
a thin film
is that tilting the mirror will force some liquid to drain to the lowest
part of the outer 
edge of the mirror creating lots of distortion even if the liquid's
container is made to be
very thin itself.   I thought of trying this with some medium that could
be "frozen" -
if one could find a medium that froze without too much distortion
created by either 
expansion or contraction during freezing.

Thanks again,

Rick

First I would like to keep the initial discussion about this on the list so
others can add there comments.  Later when we pick up some more suckers, I mean
interested parties, we can go off line then post the highlights.

Yes I thought about the displacement issue also.  This is why I was considering
working over pressure.  Beside, the technology would then be transferable to
space.

I played with spinning mirrors for a while, a couple of decades ago.  The curve
to first approximation is parabolic.  By that I mean you must discount the
curvature of the Earth.  Which is good enough if the mirror less than a couple of
meters.  The biggest problem here is vibration induced by the bearings.  The best
surfaces I've seen were only good enough for IR work.  Clearly some work has been
done in this area (all the new mercury pool mirrors).

In Unusually Telescopes a reference is made to keeping the contact ring uniform.
I've thought about this, but have not yet come to a solution.  This could be the 
killer for really large vacumn forms.

Liquid medium can be extracted quite easily.  This is the perfered method for 
making a vacumn chuck to make Schmidt correctors, when the glass blows, sorry
if the glass blows there is less scatter.  Also oil will actually help seal the 
ring.

Distortion due to gravity is also a problem with gas.  Another good reason to keep
the pressure differential large compaired to the weight of the mediums.

I'm glad to find some interest in this idea.

Anthony