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A comment on The $11B Webb telescope aims to probe the early universe. Parent comment

Re:The $11B Webb telescope aims to probe the early universe by a9h74j on Wednesday December 08, 2021 @03:51PM (#29490324)
That's huge. Everyday profile accuracy can be spoken of in "quarter wavelength"-like terms.
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Re:The $11B Webb telescope aims to probe the early universe by xxpor on Wednesday December 08, 2021 @07:42PM (#29492510)
Was that true in 1979?
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Re:The $11B Webb telescope aims to probe the early universe by HPsquared on Thursday December 09, 2021 @04:42AM (#29495498)
Accuracy less than a wavelength has been possible for a long time:

https://en.wikipedia.org/wiki/Foucault_knife-edge_test

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Re:The $11B Webb telescope aims to probe the early universe by xxpor on Thursday December 09, 2021 @11:53AM (#29499745)
Huh wow, that's brilliant! I was expecting something super complicated with extremely high tech sensors.
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Re:The $11B Webb telescope aims to probe the early universe by a9h74j on Sunday December 12, 2021 @01:07AM (#29527533)
Spherical-against-spherical and flat-against-flat grinding can be self-truing, so in effect centuries-old clever results required clever testing to verify.

High tech in those days? Rumor I heard in the mid-1980s was that LLNL had [vertical?] lathes which could mechanically cut mirrors to optical-profile accuracy. Supposedly 100gpm liquid flows were required to keep the part temperatures uniform.

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