In this recent BBC article 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 (note scale of humans):


An image from the BBC article:

BBC image of E-ELT vs Big Ben


1 Answer 1


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

  • $\begingroup$ OK - exactly what I wanted to know, thank you! I'd say that certainly qualifies as "very complex"! $\endgroup$
    – uhoh
    May 26, 2016 at 4:32
  • $\begingroup$ another related question $\endgroup$
    – uhoh
    May 26, 2016 at 17:11
  • 1
    $\begingroup$ @uhoh I don't think I can add much to this answer and the ESO page on the challenges with adaptive optics. 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. $\endgroup$ May 26, 2016 at 20:41
  • $\begingroup$ 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. $\endgroup$
    – uhoh
    May 27, 2016 at 0:45

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