<p>As shown in <a href="https://www.eso.org/public/images/eso9711d/" rel="nofollow noreferrer">an ESO image</a>, the mirrors are coated in a giant <a href="https://en.wikipedia.org/wiki/Sputter_deposition" rel="nofollow noreferrer">plasma sputter</a> system, where ions produced in a (presumably argon) plasma are accelerated and strike the pure aluminum, freeing neutral aluminum atoms which then reach the miror surface and (usually) stick.</p>
<p>It has a diameter of more than 9 meters and a volume of 122,000 liters, pumped with 8 large cryopumps. During deposition the mirror is spun at a stately 5 revolutions per hour under the sputter gun array.</p>

CC BY-SA 4.0
2023-10-14T15:49:28.947Z
2023-10-14T21:54:28.173Z
2023-10-14T21:54:28.173Z
Jon Custer
<p>In Tim Scott's October 2, 2023 video &quot;<em>The largest telescope that will ever be built<sup></em></sup>*&quot; (note the asterisk, link below) there is a discussion of the &quot;re-silvering&quot; of the four main mirrors of the <a href="https://en.wikipedia.org/wiki/Very_Large_Telescope" rel="nofollow noreferrer">Very Large Telescope</a> (VLT) every 2-3 years when despite regular <span class="math-container">$\require{mhchem}\ce{CO_2}$</span> cleaning they accumulate too much dust/dirt:</p>
<blockquote>
<p>They atomise aluminium to create the mirror layer. And when I say atomise, I don't mean like perfume sprayer atomiser. <strong>I mean literally, they use plasma to make individual atoms of aluminium float around in a vacuum</strong>, and then they just let it softly rain down onto the mirror until they have a coating about 1,000 atoms thick.</p>
</blockquote>
<p>One common way to deposit thin aluminum films is thermal evaporation - a ceramic alumina (<span class="math-container">$\require{mhchem}\ce{Al_2O_3}$</span>) crucible containing aluminum metal is heated to a point where the vapor pressure of aluminum results in a flux of atomic aluminum vapor that is emitted at thermal velocity and generally then stick to to whichever surface they hit.</p>
<p>But in this case, at least according to Scott, a plasma deposition process is used.</p>
<p><strong>Question:</strong> Why is plasma used to deposit aluminum on the VLT's four large mirrors?</p>
<p>I've in fact never heard of plasma deposition of aluminum. I've done a lot of <a href="https://en.wikipedia.org/wiki/Plasma-enhanced_chemical_vapor_deposition" rel="nofollow noreferrer">Plasma-enhanced chemical vapor deposition</a> for dielectrics (e.g. <span class="math-container">$\ce{Si_3N_4}$</span>, <span class="math-container">$\ce{SiO_2}$</span>) and PECVD <a href="https://en.wikipedia.org/wiki/Diamond-like_carbon" rel="nofollow noreferrer">diamond-like carbon</a> or DLC is quite popular, but plasma deposition of layers of pure metal is new to me.</p>
<p>So any details about the mechanism will be appreciated!</p>
<hr />
<p>cued at 11:07, just before the block quote begins:</p>
<p><div class="youtube-embed"><div>
                <iframe width="640px" height="395px" src="https://www.youtube.com/embed/QqRREz0iBes?start=672"></iframe>
            </div></div></p>
<hr />
<p>Somewhat related:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/16583/7982">When did &quot;resilvering&quot; large telescope mirrors actually refer to aluminization, and why was it necessary?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/16547/7982">Was the use of silver on &#39;optical&#39; telescope mirrors more common in the past? Why?</a></li>
</ul>

CC BY-SA 4.0
2023-10-13T23:59:42.573Z
Sputtering with argon is a good guess. Standard semiconductor processing step.
Jon Custer
CC BY-SA 4.0
2023-10-14T00:07:46.680Z
@JonCuster Ah, (e.g. argon) plasma sputtering of aluminum, not aluminum plasma, ya I think you've nailed it. I'll bet googling *something like* "sputtering VLT" will get you the supporting source for an answer.
uhoh
<observational-astronomy><observatory><instruments><mirror><mirror-making>
Why is plasma used to deposit aluminum on the VLT's four large mirrors?
uhoh
# Why is plasma used to deposit aluminum on the VLT's four large mirrors?

(Asked by: username_0 on 2023-10-13)

In Tim Scott's October 2, 2023 video "*The largest telescope that will ever be built*\*" (note the asterisk, link below) there is a discussion of the "re-silvering" of the four main mirrors of the [Very Large Telescope](https://en.wikipedia.org/wiki/Very_Large_Telescope) (VLT) every 2-3 years when despite regular $\require{mhchem}\ce{CO\_2}$ cleaning they accumulate too much dust/dirt:

> They atomise aluminium to create the mirror layer. And when I say atomise, I don't mean like perfume sprayer atomiser. **I mean literally, they use plasma to make individual atoms of aluminium float around in a vacuum**, and then they just let it softly rain down onto the mirror until they have a coating about 1,000 atoms thick.

One common way to deposit thin aluminum films is thermal evaporation - a ceramic alumina ($\require{mhchem}\ce{Al\_2O\_3}$) crucible containing aluminum metal is heated to a point where the vapor pressure of aluminum results in a flux of atomic aluminum vapor that is emitted at thermal velocity and generally then stick to to whichever surface they hit.

But in this case, at least according to Scott, a plasma deposition process is used.

**Question:** Why is plasma used to deposit aluminum on the VLT's four large mirrors?

I've in fact never heard of plasma deposition of aluminum. I've done a lot of [Plasma-enhanced chemical vapor deposition](https://en.wikipedia.org/wiki/Plasma-enhanced_chemical_vapor_deposition) for dielectrics (e.g. $\ce{Si\_3N\_4}$, $\ce{SiO\_2}$) and PECVD [diamond-like carbon](https://en.wikipedia.org/wiki/Diamond-like_carbon) or DLC is quite popular, but plasma deposition of layers of pure metal is new to me.

So any details about the mechanism will be appreciated!

---

cued at 11:07, just before the block quote begins:

---

Somewhat related:

* [When did "resilvering" large telescope mirrors actually refer to aluminization, and why was it necessary?](https://astronomy.stackexchange.com/q/16583/7982)
* [Was the use of silver on 'optical' telescope mirrors more common in the past? Why?](https://astronomy.stackexchange.com/q/16547/7982)

**Tags:** observational-astronomy, observatory, instruments, mirror, mirror-making
> **Comments:**
> **username_1**: Sputtering with argon is a good guess. Standard semiconductor processing step.  
> **username_0**: @JonCuster Ah, (e.g. argon) plasma sputtering of aluminum, not aluminum plasma, ya I think you've nailed it. I'll bet googling *something like* "sputtering VLT" will get you the supporting source for an answer.  

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## 1 answers

### Answer by username_1 on 2023-10-14. Score: 3

As shown in [an ESO image](https://www.eso.org/public/images/eso9711d/), the mirrors are coated in a giant [plasma sputter](https://en.wikipedia.org/wiki/Sputter_deposition) system, where ions produced in a (presumably argon) plasma are accelerated and strike the pure aluminum, freeing neutral aluminum atoms which then reach the miror surface and (usually) stick.

It has a diameter of more than 9 meters and a volume of 122,000 liters, pumped with 8 large cryopumps. During deposition the mirror is spun at a stately 5 revolutions per hour under the sputter gun array.


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