<p>Front-surface glass mirrors weren't developed until the 1850s.
Before that, the most common material was <a href="https://en.wikipedia.org/wiki/Speculum_metal" rel="noreferrer">speculum metal</a>, a high-tin bronze.
Notable examples include Isaac Newton's 1668 prototype and William Herschel's late 18th century telescopes.
Since speculum metal tarnished easily, it was common to have a spare mirror to continue observations while the other was being reconditioned.
Reflectivity and thermal characteristics were not great either.</p>

CC BY-SA 4.0
2020-01-21T00:49:02.893Z
2020-01-21T00:49:02.893Z
Mike G
<blockquote>
  <p>Can a material other than glass be used for making telescope mirrors?</p>
</blockquote>

<p><strong><em>Yes!</em></strong></p>

<p>From <a href="https://astronomy.stackexchange.com/a/33867/7982">this answer</a> to <em>Why are telescope mirrors made of glass?</em></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="nofollow 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="nofollow noreferrer">Spaceflight 101</a>, image credit: ESA/Astrium.</p>
    
    <p><a href="https://i.sstatic.net/PKujo.png" rel="nofollow 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="nofollow noreferrer">Long-Range Reconnaissance Imager on New Horizons</a></p>
  </blockquote>
</blockquote>

CC BY-SA 4.0
2020-01-21T05:14:57.017Z
2020-01-21T05:14:57.017Z
uhoh
<p>Good, fast , cheap -- pick any two.   And here you need to add things like "machinable," "stable,"  "nontoxic."   </p>

<p>So, there are plenty of metal mirrors but they are much more prone to deformation than a nice rigid glass structure.  There are mirrors of SiC (with overcoat, just like glass) for strength and thermal properties.  There have been primaries consisting of a tub of liquid Hg, spun at just the right speed to get the desired curvature.   </p>

<p>Now, if you're looking to make relatively small primaries for use in non-stressing environments, you could consider carbon-fiber or even 3D-printer resin-based approaches.  These methods will produce a good "blank" but will certainly need some final fine polishing to get the surface roughness down under a fraction of a wavelength.  </p>

CC BY-SA 4.0
2020-01-20T19:05:07.797Z
2020-01-20T19:05:07.797Z
Carl Witthoft
<p>Although I do know of a few instances where professional or observatory mirrors have been made by alternate materials, it was primarily for targeting specific nuances, like weight reduction and low thermal expansion etc. </p>

<p>I was more keen on finding a material whose primary unique selling point is low cost ;P , keeping in mind a budding amateur astronomer. </p>

CC BY-SA 4.0
2020-01-20T14:34:11.090Z
What does USP mean?
usernumber
CC BY-SA 4.0
2020-01-20T14:39:18.800Z
Oh, my bad. Somewhat like it's marketable quality.
Maverick139
CC BY-SA 4.0
2020-01-20T14:58:59.660Z
I guess anything that you can make very very smooth, in a parabolic shape and that you can apply a reflective coating to could work ?
usernumber
CC BY-SA 4.0
2020-01-20T15:20:59.133Z
Yep, that's the idea =). Wanted some examples to start from, and possibly the problems that come up with them
Maverick139
CC BY-SA 4.0
2020-01-20T15:44:45.510Z
Glass is pretty cheap
Steve Linton
CC BY-SA 4.0
2024-09-19T13:04:49.023Z
@SteveLinton But very heavy and (time) costly to process
2080
<observational-astronomy><amateur-observing><telescope-making>
Can a material other than glass be used for making telescope mirrors?
Maverick139
# Can a material other than glass be used for making telescope mirrors?

(Asked by: username_0 on 2020-01-20)

Although I do know of a few instances where professional or observatory mirrors have been made by alternate materials, it was primarily for targeting specific nuances, like weight reduction and low thermal expansion etc.

I was more keen on finding a material whose primary unique selling point is low cost ;P , keeping in mind a budding amateur astronomer.

**Tags:** observational-astronomy, amateur-observing, telescope-making

> **Comments:**
> **username_1**: What does USP mean?  
> **username_0**: Oh, my bad. Somewhat like it's marketable quality.  
> **username_1**: I guess anything that you can make very very smooth, in a parabolic shape and that you can apply a reflective coating to could work ?  
> **username_0**: Yep, that's the idea =). Wanted some examples to start from, and possibly the problems that come up with them  
> **username_2**: Glass is pretty cheap  
> **username_3**: @username_2 But very heavy and (time) costly to process  

---

## 3 answers

### Answer by username_6 on 2020-01-20. Score: 4 | Accepted answer

Good, fast , cheap -- pick any two. And here you need to add things like "machinable," "stable," "nontoxic."

So, there are plenty of metal mirrors but they are much more prone to deformation than a nice rigid glass structure. There are mirrors of SiC (with overcoat, just like glass) for strength and thermal properties. There have been primaries consisting of a tub of liquid Hg, spun at just the right speed to get the desired curvature.

Now, if you're looking to make relatively small primaries for use in non-stressing environments, you could consider carbon-fiber or even 3D-printer resin-based approaches. These methods will produce a good "blank" but will certainly need some final fine polishing to get the surface roughness down under a fraction of a wavelength.


---

### Answer by username_4 on 2020-01-21. Score: 5

Front-surface glass mirrors weren't developed until the 1850s.
Before that, the most common material was [speculum metal](https://en.wikipedia.org/wiki/Speculum_metal), a high-tin bronze.
Notable examples include Isaac Newton's 1668 prototype and William Herschel's late 18th century telescopes.
Since speculum metal tarnished easily, it was common to have a spare mirror to continue observations while the other was being reconditioned.
Reflectivity and thermal characteristics were not great either.


---

### Answer by username_5 on 2020-01-21. Score: 3

> Can a material other than glass be used for making telescope mirrors?

***Yes!***

From [this answer](https://astronomy.stackexchange.com/a/33867/7982) to *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)


---

