<p>The accepted answer is correct as far as it goes but to add a bit of why for the claim:</p>

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

<p>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).</p>

<p>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.</p>

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

<p>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.</p>

<p>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.  </p>

CC BY-SA 4.0
2019-11-16T22:42:34.870Z
Each small crystalline region ([crystallite](https://en.wikipedia.org/wiki/Crystallite)) will have a different orientation with respect to the surface being polished, and can therefore end up being polished at different rates.
uhoh
CC BY-SA 4.0
2019-11-16T18:26:52.467Z
2019-11-16T22:42:08.607Z
2019-11-16T22:42:08.607Z
tomc
<blockquote>
<p>Why are telescope mirrors made of glass?</p>
</blockquote>
<p>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.</p>
<p>From <a href="https://astronomy.stackexchange.com/a/31792/7982">this answer</a> to <em>How are space telescopes stabilised to a perfect standstill?</em>:</p>
<blockquote>
<p>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 <a href="https://space.stackexchange.com/a/18237/12102">answer</a> and in New Horizion's LORRI telescope <a href="https://astronomy.stackexchange.com/a/25276/7982">answer</a>.</p>
<p><a href="https://i.sstatic.net/MFmOF.jpg" rel="noreferrer"><img src="https://i.sstatic.net/MFmOF.jpg" alt="enter image description here" /></a></p>
<p><strong>above:</strong> Gaya's Silicon Carbide Optical Bench, with the two objective mirrors of it's twin telescopes pointing 106.5° apart. From <a href="http://spaceflight101.com/gaia/gaia-instrument-information/" rel="noreferrer">Spaceflight 101</a>, image credit: ESA/Astrium.</p>
<p><a href="https://i.sstatic.net/PKujo.png" rel="noreferrer"><img src="https://i.sstatic.net/PKujo.png" alt="enter image description here" /></a></p>
<p><strong>above:</strong> The LORRI telescope is described for example in the ArXiv preprint <a href="https://arxiv.org/abs/0709.4278" rel="noreferrer">Long-Range Reconnaissance Imager on New Horizons</a></p>
</blockquote>

CC BY-SA 4.0
2019-11-07T19:46:10.423Z
2020-01-21T05:13:33.023Z
2020-06-17T09:47:40.653Z
uhoh
<p>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.</p>

CC BY-SA 4.0
2019-11-11T21:45:15.643Z
2019-11-11T21:45:15.643Z
Uwe
<p>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.</p>

<p>Some useful references are found <a href="https://mirrorlab.arizona.edu/content/faq" rel="noreferrer">here</a> ,<a href="https://www.cmog.org/article/reflections-glass-telescope-mirrors" rel="noreferrer">here</a> and <a href="https://www.telescopenerd.com/answers/why-are-telescope-mirrors-made-of-glass.htm" rel="noreferrer">here</a>.</p>

CC BY-SA 4.0
2020-01-22T01:52:11.767Z
It grinds down nicely, evenly, without any nasty crystal planes to distort your 1/10th wave parabola. Lack of much crystal structure is *important*.
Wayfaring Stranger
CC BY-SA 4.0
2019-11-05T22:51:46.310Z
2019-11-05T22:58:28.433Z
2019-11-05T22:58:28.433Z
Natsfan
<p>Optical telescope mirrors are typically made from endlessly polished glass surfaces that have been <a href="https://en.wikipedia.org/wiki/Silvering" rel="noreferrer">aluminised</a> to provide a very smooth surface of reflective aluminium.</p>

<p><strong>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?</strong></p>

<p>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?</p>

<p>Or is it to do with the thermal expansive properties of the glass used? </p>

CC BY-SA 4.0
2019-11-06T13:11:02.940Z
Speculum metal (https://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.
Mark Olson
CC BY-SA 4.0
2019-11-06T19:43:38.113Z
@MarkOlson glass is a few zillion times cheaper than beryllium and much safer (to health) to machine :-) .
Carl Witthoft
CC BY-SA 4.0
2019-11-06T20:54:17.837Z
I imagine that's some of the reasons glass is better.
Mark Olson
CC BY-SA 4.0
2023-11-16T10:26:34.900Z
I hope a new answer will address non-glass, non-metal materials such as plastics
2080
<telescope><mirror><mirror-making>
Why are telescope mirrors made of glass?
Ingolifs
# Why are telescope mirrors made of glass?

(Asked by: username_0 on 2019-11-05)

Optical telescope mirrors are typically made from endlessly polished glass surfaces that have been [aluminised](https://en.wikipedia.org/wiki/Silvering) 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?

**Tags:** telescope, mirror, mirror-making

> **Comments:**
> **username_1**: Speculum metal (https://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.  
> **username_2**: @MarkOlson glass is a few zillion times cheaper than beryllium and much safer (to health) to machine :-) .  
> **username_1**: I imagine that's some of the reasons glass is better.  
> **username_3**: I hope a new answer will address non-glass, non-metal materials such as plastics  

---

## 4 answers

### Answer by username_7 on 2019-11-05. Score: 11 | Accepted answer

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](https://mirrorlab.arizona.edu/content/faq) ,[here](https://www.cmog.org/article/reflections-glass-telescope-mirrors) and [here](https://www.telescopenerd.com/answers/why-are-telescope-mirrors-made-of-glass.htm).

> **Comments:**
> **username_8**: It grinds down nicely, evenly, without any nasty crystal planes to distort your 1/10th wave parabola. Lack of much crystal structure is *important*.  

---

### Answer by username_4 on 2019-11-16. Score: 8

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.

> **Comments:**
> **username_5**: Each small crystalline region ([crystallite](https://en.wikipedia.org/wiki/Crystallite)) will have a different orientation with respect to the surface being polished, and can therefore end up being polished at different rates.  

---

### Answer by username_5 on 2019-11-07. Score: 5

> 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](https://astronomy.stackexchange.com/a/31792/7982) 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](https://space.stackexchange.com/a/18237/12102) and in New Horizion's LORRI telescope [answer](https://astronomy.stackexchange.com/a/25276/7982).
>
> [![enter image description here](https://i.sstatic.net/MFmOF.jpg)](https://i.sstatic.net/MFmOF.jpg)
>
> **above:** Gaya's Silicon Carbide Optical Bench, with the two objective mirrors of it's twin telescopes pointing 106.5° apart. From [Spaceflight 101](http://spaceflight101.com/gaia/gaia-instrument-information/), image credit: ESA/Astrium.
>
> [![enter image description here](https://i.sstatic.net/PKujo.png)](https://i.sstatic.net/PKujo.png)
>
> **above:** The LORRI telescope is described for example in the ArXiv preprint [Long-Range Reconnaissance Imager on New Horizons](https://arxiv.org/abs/0709.4278)


---

### Answer by username_6 on 2019-11-11. Score: -1

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.


---

