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Question from a total noob: How modular are these segmented mirrors? Ie, if the budget runs to 'only' 39m now, is it possible to upgrade it to 42m later by adding segments to what's already there?

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The segments themselves are 100% modular and you could tile an US state with them. The main problem is engineering: the whole thing needs to behave like a single object, turning 360 degrees and moving between horizon and zenith (horizontal and vertical), with a minimum of mechanical flexure. And also resisting wind load. In essence, it's a lightweight building that has to move freely, and this one won't get bigger.

They are modular but not very interchangable (there are 133 different designs of the segments in the primary mirror).

Making a much larger mirror would also face another obstacle: the focal length is designed to be practical for a ~40m mirror.

I have no special expertise on the subject but by reading and watching about it I think that’s more complex than you might think because there’s a lot a extra systems to make the mirror work with that set up. So it’s not a matter of just bolting in a new ring of hexagonal mirrors and I suspect the majority of the cost comes not from the actual mirrors themselves.

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