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A few things of interest about the fabrication and polishing of these 8.4 meter mirrors.
The parabolic curve is spun cast in the surface good to a mm or so.
Then the diamond milling with spinning cutter is so precise that thay only need then to lap the surface with 1 grinding powder , 12 micron abrasive befor they go straight to polish.
The large polishing tools contain hundreds of computer controlled motors which constantly bend the polishing tool as it passes over the surface and keep the polisher in total contact with the strongly aspheric shape of the mirror.
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