What is a quaternary mirror and why does the E-ELT need one? 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:


What is a quaternary mirror
Why does the E-ELT need one
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:
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 &quot;mirror number four&quot;, 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