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So, glassmakers were tasked with building parabolic mirrors for telescopes. I imagine that process is now entirely done in heavily automated factories.

How does one make a mirror and then carve it into a parabolic shape?

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    $\begingroup$ It's a bit of a grind. $\endgroup$ – Transistor Jul 1 '20 at 21:25
  • $\begingroup$ @Transistor Grinding is the solution to most problems. $\endgroup$ – Beliod Jul 1 '20 at 21:26
  • $\begingroup$ Have a look at stargazerslounge.com/topic/…. The truck is to grind a spherical mirror and then polish to parabolic. $\endgroup$ – Transistor Jul 1 '20 at 21:36
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Large telescope mirrors are made by spinning a disc of molten glass so its surface assumes a parabolic shape, and then slowly cooling it while spinning so it retains that shape after it solidifies. Mirror blanks up to ten meters in diameter are routinely cast this way. After the mirror blank is finished, it is placed under a four-axis polishing machine which perfects its curvature and removes any irregularities, leaving it optically "flat" and parabolically curved to an accuracy of better than a millonth of a meter.

Afterwards, the glass surface of the mirror is coated with a thin layer of aluminum to make it reflective, and then it is shipped to the location of the telescope under construction.

The ten-meter-limit is set by the clearances required to fit the mirror on a standard railroad car and pass it through tunnels on its way to the telescope.

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