WEBVTT

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How do you build a mirror thirty-nine meters
across?

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The European Southern Observatory designed its
Extremely Large Telescope around hundreds of
polished hexagons.

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Seven hundred and ninety-eight segments will
form its primary mirror.

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Each one has to work with its neighbors.

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If neighboring segments shift, the optical
surface falls out of alignment.

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Sensors along their edges measure tiny changes
in relative position.

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Those measurements tell the control system where
corrections are needed.

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Three positioning actuators on each segment
assembly adjust its height and tilt.

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Small corrections help the segments behave like
a single giant mirror.

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The challenge is keeping hundreds of separate
pieces aligned while the telescope observes.