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 Incoming links: Mirror segment phasing

Three complementary approaches replace the simple thick solid disk:

A thin meniscus on an active optics support distributes and adjusts forces to preserve the figure despite changing gravity and temperature. The Very Large Telescope has four 8.2-metre unit telescopes of this type; the ESO active-optics account describes their controlled mirror supports.
A lightweight telescope mirror uses a honeycomb or ribbed backing: a deep structure retains stiffness without a massive solid interior and cools more readily. The Large Binocular Telescope uses two 8.4-metre primaries; its official history describes their honeycomb construction.
A segmented mirror constructs the aperture from manageable pieces. Support actuators and mirror segment phasing control figure, piston and tilt. Each ten-metre Keck telescope uses 36 hexagonal segments, as recorded in the observatory’s telescope description.

“Operational” and “planned” here refer to the 2018 examination date. The next generation included the 39-metre Extremely Large Telescope with 798 segments, the thirty-metre Thirty Meter Telescope with 492 segments, and the Giant Magellan Telescope using seven 8.4-metre primary mirrors. These designs extend segmentation or lightweight casting with active control, rather than simply making a thicker slab. The specifications are documented by the 2017 ELT announcement, TMT optics design and GMT primary-mirror description. Adaptive optics is also needed to exploit the large apertures against atmospheric turbulence; it corrects faster disturbances than active optics.

Thin actively supported menisci, lightweight honeycomb mirrors, and phased segmented primaries enable larger apertures.

A segmented mirror assembles smaller mirrors into a large aperture. Sensors and actuators maintain position and figure; coherent imaging additionally requires mirror segment phasing.