About Parabolic Telescope Mirrors

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  • Thread starter Thread starter Saladsamurai
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Discussion Overview

The discussion revolves around the considerations for constructing parabolic telescope mirrors, specifically comparing the use of glass versus metal materials. Participants explore the implications of material choice on performance, particularly in relation to thermal expansion and the practicality of polishing techniques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Casey questions the necessity of using glass for telescope mirrors and suggests that curving aluminum and applying reflective plating could be a more cost-effective alternative.
  • Warren explains that metal has a large coefficient of thermal expansion, which can lead to undesirable changes in the mirror's shape with temperature fluctuations.
  • Casey acknowledges Warren's point and inquires about the feasibility of using fans or thermal junctions to manage heat in metal mirrors.
  • Warren responds that rapid temperature equalization is essential for mirror performance, particularly for large mirrors, and that fans are crucial to achieve this.
  • Another participant asserts that metallic mirrors are permanently affected by thermal distortion, reinforcing the challenges associated with using metal.
  • It is noted that polishing metallic mirrors is more complex due to their elastic deformation, and while they are used in specific applications, they often require expensive materials and techniques.

Areas of Agreement / Disagreement

Participants express differing views on the viability of using metal for telescope mirrors, with some supporting the use of glass due to its thermal properties while others explore the potential of metal despite its challenges. The discussion remains unresolved regarding the practicality of alternative materials.

Contextual Notes

Participants highlight limitations related to thermal management and the complexities of polishing metal mirrors, but do not resolve these issues. The discussion reflects a range of assumptions about material properties and their implications for telescope performance.

Saladsamurai
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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:confused:

Someone please enlighten me,
Casey
 
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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
 
Last edited:
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?
 
Last edited:
Yes, if you want to be able to use the telescope in a wide range of conditions.
 
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
 
I see, like the shaving mirror in my shower:-p
 
Metallic mirrors are permanently affected by thermal distortion. That's the long and the short of it.
 
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 )
 

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