Small but important: The Daniel K. Inoyue Solar Telescope's heat-stop. The heat-stop is a liquid-cooled aperture which reduces the light from the 4m primary mirror to a very narrow beam to protect the secondary mirror and instruments from excessive heat. Failure of the cooling system automatically shuts down the telescope by deploying covers over the primary and secondary mirrors, and closing the dome.
(photo by NSO/NSF/AURA)