The Great Canary Telescope is partially operational and in the midst of a year of testing. The testing involves twelve hexagonal mirror segments. When fully operational, the primary mirror will be composed of thirty-six segments, so it’s not quite there yet. It takes the title of the world’s largest optical telescope with a mirror 10.4 metres (34.1 feet) in diameter.
The telescope uses adaptive optics, which involves computer control of the mirror’s surface to correct for distortions in the atmosphere. With a mirror this big, active optics also corrects distortions in the mirror geometry caused by the pull of gravity from different angles depending on the mirror position. In the 1980s, telescopes were reaching the upper limit of what was technically possible because gravity distorted the large multi-tonne mirrors.
Computers came to the rescue and the mirrors have continued growing.
Telescope photo courtesy gtcdigital.net