Out of pure curiosity, I'm wondering a few things about the Extremely Large Telescope, the 39.3m reflector that the Europeans are building, and a little bit more generally about the new wave of, well, extremely large telescopes (e.g. the Thirty Meter Telescope in Hawaii).
First: there seems to be a pretty decent amount of scaffolding in the light path, but part of that may just be visual bias. I wonder roughly what fraction of the mirror becomes obstructed, and how that would compare to amateur reflectors.
Second: at this scale, do you think the mirrors would be aluminized, silvered, or given some more exotic coating? I'd imagine they'd want to capture as much light as possible, even given a stable mount permitting long exposures.
Finally: the published resolution of 0.005 arcseconds is quite close to the visual-spectrum Rayleigh resolution of 0.0026-0.0044 arcseconds (depending on wavelength). Does that mostly just keep on going with adaptive optics, or do new issues crop up for these superscaled telescopes?