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: