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>"that final polishing process ... takes about 18 months"

That's a lot faster than it used to be. The 200 inch mirror at Mt Palomar took 13 years to grind and polish. I guess they have greatly improved the technology.

https://en.wikipedia.org/wiki/Hale_Telescope

I'm curious if its actually polishing for 18 months, or if its a few days of polishing, followed by a few days inspection, on and off.* I can't imagine they simply turn it on and let it run for a year and a half non-stop.

* Or some other tempo of alternation.

There were probably even longer timespans involved in planning and getting it approved by everyone.