Re: [ATM] Aluminum mirror?

["Vladimir Galogaza" <vladimir.galogaza1@zg.t-com.hr>]

>A few days ago I found a telescope stuff vendor that offers blanks and kits 
>for making
>aluminum mirrors. Is there anyone in the group with experience with this 
>material? How
 >about thermal distortion?

I have no experience with aluminum mirrors but nevertheless this might help.
Thermal distortion apart from expansion or contraction is consequence of
temperature gradients in  material. Glass, being pure thermal conductor is 
prone to such
distortions. Therefore those rare ATMs that  use their scopes for observing
are patiently waiting in the evenings for mirror to achieve thermal 
equilibrium.
Aluminum mirrors will  be better due to its  higher thermal conductivity
which will bring them to thermal equilibrium (absence of thermal gradients) 
more quickly.
Thermal conductivity of aluminum is 200 times greater than same for glass.
So much about thermal distortion.
For one other possible distortion, aluminum mirror has also great advantage 
over glass
mirror. This is mechanical distortion, which happens when  mirror moves 
under
 influence of  universal gravity and in the process hits the ground. More 
often than not
 resulting mechanical distortion  is recognized as destruction (of glass and 
self-confidence).
This is very stressful but has nothing to do with the internal stress in the 
glass unless
 the glass was tampered. In this particular case it is better if this 
happens sooner than later.


Regards
Vladimir.

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