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Optical telescope mirrors are typically made from endlessly polished glass surfaces that have been aluminised to provide a very smooth surface of reflective aluminium.

But why the need for glass at all? Why can't mirrors be made of pure metal, or of metal deposited onto some other surface?

Is there something special about the glass grinding process that means no other surface can be made as smooth and still have aluminium deposited on the surface?

Or is it to do with the thermal expansive properties of the glass used?

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    $\begingroup$ Speculum metal (en.wikipedia.org/wiki/Speculum_metal) was the material of choice for a long time. And there is at least one mirror made of mercury (liquid mercury, spun slowly to form a parabolic surface) and used for very special purposes. And some large mirrors have been made of beryllium for satellites. In general, glass is better, though. $\endgroup$
    – Mark Olson
    Commented Nov 6, 2019 at 13:11
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    $\begingroup$ @MarkOlson glass is a few zillion times cheaper than beryllium and much safer (to health) to machine :-) . $\endgroup$
    – Carl Witthoft
    Commented Nov 6, 2019 at 19:43
  • $\begingroup$ I imagine that's some of the reasons glass is better. $\endgroup$
    – Mark Olson
    Commented Nov 6, 2019 at 20:54
  • $\begingroup$ I hope a new answer will address non-glass, non-metal materials such as plastics $\endgroup$
    – 2080
    Commented Nov 16, 2023 at 10:26

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you are partially correct. Glass is used for several reasons. It is a very stable material and will hold its shape well for thousands of years. Glass can also be polished to a high degree of accuracy without having defects. Another major reason is the expansion properties are very favorable as well. Glass doesn't corrode and is easier to mold into shape than metal mirrors. Glass is relatively cheap and readily available.

Some useful references are found here ,here and here.

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    $\begingroup$ It grinds down nicely, evenly, without any nasty crystal planes to distort your 1/10th wave parabola. Lack of much crystal structure is important. $\endgroup$
    – Wayfaring Stranger
    Commented Jan 22, 2020 at 1:52
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The accepted answer is correct as far as it goes but to add a bit of why for the claim:

jmh> "Glass can also be polished to a high degree of accuracy without having defects"

Comes down to glass being amorphous instead of polycrystalline as metal is. When a material is polycrystalline its atoms have preferred (stronger) connections with some of its neighbors and weaker connections with others. It is these chains of preferred connections (conga lines) that metallurgists tweak to give the material trade off in properties of tensile strength, ductility, hardness. Amorphous material on the other hand has no preference and it is every atom for itself (mosh pit).

This means that when polishing/figuring, as atoms are removed from from a polycrystalline material they leave at an uneven rate resulting in pits where there were stronger connections and lumps where there were weaker atomic connections.

Whereas single atoms can be removed or jostled into lower energy configurations without prejudice in amorphous material resulting in a (nigh) atomically smooth surface.

At this point it is good to remember that the glass is not typically the mirror as glass no matter how smooth is not highly reflective, even worse it is transparent to the visible light we may be interested in. Instead the mirror is just the few atoms of thickness of highly reflective metal that coats the glass.

If we could convince a single atom thick sheet of metal atoms to adopt the same shape without the glass it would be a great day for astronomy.

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    $\begingroup$ Each small crystalline region (crystallite) will have a different orientation with respect to the surface being polished, and can therefore end up being polished at different rates. $\endgroup$
    – uhoh
    Commented Nov 16, 2019 at 22:42
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Why are telescope mirrors made of glass?

They are not always made of glass. In situations where mass counts and thermal variations can be large, optical telescope mirrors are sometimes made out of silicon carbide instead.

From this answer to How are space telescopes stabilised to a perfect standstill?:

Silicon carbide is a very popular material in newer space telescopes and is found in the optical system of too man of them for me to remember, but here is mention of it in GAIA's optical bench answer and in New Horizion's LORRI telescope answer.

enter image description here

above: Gaya's Silicon Carbide Optical Bench, with the two objective mirrors of it's twin telescopes pointing 106.5° apart. From Spaceflight 101, image credit: ESA/Astrium.

enter image description here

above: The LORRI telescope is described for example in the ArXiv preprint Long-Range Reconnaissance Imager on New Horizons

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When we look at the thermal expansion, Zerodur is much better than glas. It was used for GTC, Keck I, Keck II, and SOFIA mirrors.

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