Photography : Astronomy : Amazing Mirrors
A zoomed-in view of the massive 33 foot (10 meter) wide segmented primary mirror of the Keck II telescope. The mirror is made of segments because it is so large. If it were a single mirror (typically called "monolithic" mirror), it would have to have an increasingly complicated support structure (all those struts behind the mirror) to prevent it from collapsing on itself just from its own weight. Also, the larger the mirror, the thicker it has to be so it doesn't sag due to gravity. Telescope mirrors are shaped to perfectly reflect and focus light to a single spot; sagging distorts this shape and reduces the quality of the images. More support and a thicker mirror means lots more money. An innovative solution to this problem is splitting the mirror into segments, but it is more technically challenging to ensure the mirrors are all aligned. New techniques were developed to grind the mirrors, support them, and prevent sagging. As the telescope is operating, the 36 segments of the mirror have 108 motors behind them that move the mirrors twice a second to achieve proper alignment.