newbie question

[solidstate]

Why dont we use aluminium for mirror instead of just the coating glass/pyrex I understand there must a reason I was just wondering what it was, does it polish well enough or are the thermal properties lacking.

[jjjnettie (Jeanette)]

Good question.
I'm looking forward to seeing what the answer is.

[leon]

With aluminium there would be to much flexing due to temperature changes, and pyrex one the other hand is very stable and has a very low expansion rate.
Focusing would be difficult to hold with aluminium.

leon

[janoskiss (Steve H)]

Solid state, you've answered your own question. metals have high thermal expansion coefficient (or whatever it's called) - ie expand a lot more with increasing temperature than glass.

[okiscopey (Mike)]

Probably all about thermal expansion, which need to be as low as possible to preserve the shape and thus focal length (etc.) of the mirror.

Compared with Pyrex, aluminium has about seven times higher 'thermal coefficient of expansion' (if I got my calculations wrong, it might be 70 times!), and ordinary glass two to three times. Pyrex also has a low 'thermal resistance' ... presumably this is a good thing.

Speculum metal was used in telescopes until the mid 1800's but tarnished easily: http://en.wikipedia.org/wiki/Speculum_metal

Also I seem to have heard that mirrors have been created by rotating large pools of mercury which then form the desired paraboloid shape (under hi-tech computer control I suppose).

That's about all I can contribute about metallic mirrors!

[astro_nutt]

I was thinking of casting an aluminium plate with copper tubing coiled in it so as to maintain an even temperature via pumping glycol coolant through it..this should maintain a steady temperature and therefore minimize thermal expansion...theorectically...just like a cars cooling system!!...

[solidstate]

the only real advantage to aluminium is ease of manufacture particualy with larger mirrors. You might be able to improve the thermal characteristics of the cell with heat sinks fluid etc maybe thin plate fluid base but the added cost would offset the gains. Having said that even a half working system would be interesting after all if we just wanted pictures of stars we could just download them.

[solidstate]

still would expand though unless you cooled it then you get condensation

[bojan]

There is also a problem with figuring and polishing: easy to achieve with glass but not so easy with metals.
BTW, first reflectors were made of speculum alloy - brittle material suitable for making mirrors, but it is quite heavy.

[bojan]

Originally Posted by okiscopey

Probably all about thermal expansion, which need to be as low as possible to preserve the shape and thus focal length (etc.) of the mirror.

Compared with Pyrex, aluminium has about seven times higher 'thermal coefficient of expansion' (if I got my calculations wrong, it might be 70 times!), and ordinary glass two to three times. Pyrex also has a low 'thermal resistance' ... presumably this is a good thing.

Speculum metal was used in telescopes until the mid 1800's but tarnished easily: http://en.wikipedia.org/wiki/Speculum_metal

Also I seem to have heard that mirrors have been created by rotating large pools of mercury which then form the desired paraboloid shape (under hi-tech computer control I suppose).

That's about all I can contribute about metallic mirrors!

I just saw your answer, Okiscopey, so my previous note is quite unnecessary

[bojan]

Long time ago I had an idea of a mirror, made of thin foil covering a metal plate with shallow cavity (Mylar was not available then), and the parabolic surface would have been formed by lower pressure of air between the metal plate and mylar foil. I even contemplated to patent the idea :-) I even made the small prototype and it even worked somehow ...
Another method of parabolizing of such mirror could be achieved by applying the high voltage beteeen the two ..
In both cases such mirror would have been adjustable in terms of focal length..
But the mere fact that the mirrors are today made of pyrex for so long now it telling us something: that glass is THE way to go.
Other methods are just to fussy and /or expensive or not good or stable enough.
B

[astro_nutt]

Thanks for the info guys!!...back to the drawing board

[okiscopey (Mike)]

Originally Posted by astro_nutt

Thanks for the info guys!!...back to the drawing board

I bet you'll find the drawing board is made of wood, just as astro mirrors are made of glass!

[astro_nutt]

Oh HA..HA!!!