
World's largest optical lens is 5.1 feet wide and took 5 years to make - bookofjoe
https://petapixel.com/2019/09/27/this-is-the-worlds-largest-optical-lens-its-5-feet-wide-and-took-5-years-to-make/
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unicornporn
Come on. Petapixel is a blogspam site. At the bottom of the posts you can find
the source. In this case it's "via Engadget":

[https://www.engadget.com/2019/09/23/moving-the-largest-
high-...](https://www.engadget.com/2019/09/23/moving-the-largest-high-
performance-lens-ever-built/)

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nwah1
Also helps when you need to burn some very large ants.

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woodandsteel
That's cool. But I am wondering how they keep that big a piece of glass from
sagging. I am asking because I have read that's a problem with big lenses, and
a key reason reflectors became the way to go for large telescopes.

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LordHeini
Because glass does not sagg.

It is a myth.

[https://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glas...](https://en.wikipedia.org/wiki/Glass#Behavior_of_antique_glass)

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gattr
You are mistaking two effects. Indeed, glass does not "flow" (it's not a
"very, very viscous liquid"); but most assuredly it does sag under gravity -
just as a heavy steel beam supported at the ends would.

This problem is much simpler to solve for mirrors than lenses, as mirrors can
be supported under their entire surface (see [0] for how it's done in larger
amateur telescopes), while lenses - only along the edge.

[0] [https://www.cloudynights.com/topic/562906-18-point-
flotation...](https://www.cloudynights.com/topic/562906-18-point-flotation-
mirror-cell-question/)

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propter_hoc
Article misses the big question: why did they put all this work into
developing a solid lens rather than a mirror or a Fresnel lens?

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woodandsteel
My guess is a mirror wouldn't fit into the design, and a Fresnel lens would
not have the needed accuracy for astronomical work.

