About Parabolic Telescope Mirrors

[Saladsamurai]

I have been looking through websites with the intent to build a telescope. I have been looking at mirrors today and there are many to choose from including prefabricated ones which I will use for my first scope.

But my question is with regards to the the other non prefabricated glass...

Is it necessary to use glass that needs to be polished down? Why not curve some other material like aluminum and then coat it with some reflective plating?

It seems time and cost effective to do it the latter way, but I am sure there is some reason why it is not used

Someone please enlighten me,
Casey

 

[chroot]

Metal has a very large coefficient of thermal expansion as compared to glass. If your mirror expands or contracts with temperature, its surface changes shape. This is, of course, undesirable for a precision mirror designed to focus light.

Most mirrors are made of Pyrex (borosilicate glass), specifically because it has such a low coefficient of thermal expansion.

- Warren

 

[Saladsamurai]

Ahhh...what an terribly obvious yet elusive point. Thank you Chroot.

What about using a fan or thermal junction of sorts...does the amount of heat that needs to be removed make that impractical?

 

[russ_watters]

Yes, if you want to be able to use the telescope in a wide range of conditions.

 

[chroot]

What you want is for your mirror to equalize with the temperature of the outside air, and to equalize throughout its volume, as rapidly as possible. Fans are actually essential for large mirrors that would otherwise take hours to equalize.

- Warren

 

[Saladsamurai]

I see, like the shaving mirror in my shower:tongue2:

 

[Chronos]

Metallic mirrors are permanently affected by thermal distortion. That's the long and the short of it.

 

[mgb_phys]

Metal mirros are more difficult to polish using traditional grinding techniques becasue they deform elastically ie. they spring back when you stop pushing on them.
They are used in a few applications where you need lightweight such as actively controlled mirrors or extremely steeply curved or convex surfaces. Usually you use Berylium ( expensive and difficult to work ) and diamond turn the mirror (expensive )