<p>You may know that a standard Newtonian telescope has two mirrors, they are called the primary and secondary mirror.</p>
<p>The E-ELT has five mirrors: The quaternary mirror is simply &quot;mirror number four&quot;, counting in the direction the light enters the scope.</p>
<p>It's complex because that's where the adaptive optics sits:</p>
<blockquote>
<p>The quaternary mirror has an approximate diameter of 2.4-m (2380x2340mm). It is a flat adaptive mirror, with up to 8000 actuators, thereby allowing the surface to be readjusted at very high time frequencies. [...]</p>
<p>This mirror will correct in real time for high order wavefront errors (e.g. atmosphere, wind shake, low spatial frequency telescope errors) and small amplitude residual tip-tilt corrections.</p>
</blockquote>
<p>Source: The European Southern Observatory's <a href="http://www.eso.org/sci/facilities/eelt/telescope/mirrors/" rel="noreferrer">page on the E-ELT optics</a></p>

CC BY-SA 3.0
2016-05-26T04:32:03.287Z
OK - exactly what I wanted to know, thank you! I'd say that certainly qualifies as "very complex"!
uhoh
CC BY-SA 3.0
2016-05-26T17:11:54.470Z
another [related question](http://astronomy.stackexchange.com/q/15006/7982)
uhoh
CC BY-SA 3.0
2016-05-26T20:41:01.663Z
@uhoh I don't think I can add much to [this answer](http://astronomy.stackexchange.com/questions/14795/why-actually-arent-ground-based-observatories-using-adaptive-optics-for-visib) and the ESO page on the [challenges with adaptive optics](http://www.eso.org/sci/facilities/develop/ao/what_ao.html). Looks like "no" to me. Also, there seems to be too few actuators to correct the visual band for a 40 m primary. But that's just my semi-educated guess.
j-g-faustus
CC BY-SA 3.0
2016-05-27T00:45:07.200Z
Thanks! - if that's a comment on the related question, do you think you could *also add it there* so people can see it when they read that question? The idea is to maximize the usefulness of information for future readers.
uhoh
CC BY-SA 3.0
2016-05-26T03:42:22.317Z
2016-05-26T03:42:22.317Z
2020-06-17T09:47:40.653Z
j-g-faustus
<p>In this recent <a href="http://www.bbc.com/news/science-environment-36381581" rel="noreferrer">BBC article</a> I read the one-sentence paragraph:</p>

<blockquote>
  <p>"Development on the mirrors - in particular, a very complex quaternary mirror - continues apace."</p>
</blockquote>

<p>and became interested in the following questions:</p>

<ol>
<li><em>What is</em> a quaternary mirror</li>
<li>Why does the E-ELT need one</li>
<li>What is it about this mirror that is "very complex"?</li>
</ol>

<p><em>note:</em> the E-ELT is the European Extremely Large Telescope nearing final design, and to be installed in Chile's Atacama Desert (host to many other telescopes).</p>

<p>A drawing of the E-ELT from from <a href="https://en.wikipedia.org/wiki/European_Extremely_Large_Telescope" rel="noreferrer">Wikipedia</a> (note scale of humans):</p>

<p><a href="https://i.sstatic.net/KjoAL.jpg" rel="noreferrer"><img src="https://i.sstatic.net/KjoAL.jpg" alt="E-ELT"></a></p>

<p>An image from the BBC article:</p>

<p><a href="https://i.sstatic.net/GWAl2.jpg" rel="noreferrer"><img src="https://i.sstatic.net/GWAl2.jpg" alt="BBC image of E-ELT vs Big Ben"></a></p>

<telescope><observatory><e-elt>
What is a quaternary mirror and why does the E-ELT need one?
uhoh
# What is a quaternary mirror and why does the E-ELT need one?

(Asked by: username_0 on 2016-05-25)

In this recent [BBC article](http://www.bbc.com/news/science-environment-36381581) I read the one-sentence paragraph:

> "Development on the mirrors - in particular, a very complex quaternary mirror - continues apace."

and became interested in the following questions:

1. *What is* a quaternary mirror
2. Why does the E-ELT need one
3. What is it about this mirror that is "very complex"?

*note:* the E-ELT is the European Extremely Large Telescope nearing final design, and to be installed in Chile's Atacama Desert (host to many other telescopes).

A drawing of the E-ELT from from [Wikipedia](https://en.wikipedia.org/wiki/European_Extremely_Large_Telescope) (note scale of humans):

[![E-ELT](https://i.sstatic.net/KjoAL.jpg)](https://i.sstatic.net/KjoAL.jpg)

An image from the BBC article:

[![BBC image of E-ELT vs Big Ben](https://i.sstatic.net/GWAl2.jpg)](https://i.sstatic.net/GWAl2.jpg)

**Tags:** telescope, observatory, e-elt

---

## 1 answers

### Answer by username_1 on 2016-05-26. Score: 11 | Accepted answer

You may know that a standard Newtonian telescope has two mirrors, they are called the primary and secondary mirror.

The E-ELT has five mirrors: The quaternary mirror is simply "mirror number four", counting in the direction the light enters the scope.

It's complex because that's where the adaptive optics sits:

> The quaternary mirror has an approximate diameter of 2.4-m (2380x2340mm). It is a flat adaptive mirror, with up to 8000 actuators, thereby allowing the surface to be readjusted at very high time frequencies. [...]
>
> This mirror will correct in real time for high order wavefront errors (e.g. atmosphere, wind shake, low spatial frequency telescope errors) and small amplitude residual tip-tilt corrections.

Source: The European Southern Observatory's [page on the E-ELT optics](http://www.eso.org/sci/facilities/eelt/telescope/mirrors/)

> **Comments:**
> **username_0**: OK - exactly what I wanted to know, thank you! I'd say that certainly qualifies as "very complex"!  
> **username_0**: another [related question](http://astronomy.stackexchange.com/q/15006/7982)  
> **username_1**: @uhoh I don't think I can add much to [this answer](http://astronomy.stackexchange.com/questions/14795/why-actually-arent-ground-based-observatories-using-adaptive-optics-for-visib) and the ESO page on the [challenges with adaptive optics](http://www.eso.org/sci/facilities/develop/ao/what_ao.html). Looks like "no" to me. Also, there seems to be too few actuators to correct the visual band for a 40 m primary. But that's just my semi-educated guess.  
> **username_0**: Thanks! - if that's a comment on the related question, do you think you could *also add it there* so people can see it when they read that question? The idea is to maximize the usefulness of information for future readers.  

---

