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There have been many radio telescope arrays which are capable of resolving stars significantly fainter than can be resolved with a single "small" radio telescope.

Why don't they do the same with light telescopes?

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The Keck telescopes are an array of two with an interferometer connecting them. – Mark Adler Aug 23 '13 at 1:36
up vote 10 down vote accepted

The optical equivalent of radio telescope arrays has been done. The Wikipedia article on optical interferometry gives a good summary.

The problem is that although interferometry gives improved angular resolution it doesn't necessarily give you any improvements in the ability to collect light. In many (most?) applications of optical telescopes the limiting factor is how much light you can collect i.e. how faint the objects you can see are. Interferometry doesn't help with this - you want a biiiiiig mirror!

There are applications where the limited light capturing isn't a problem. For example the CHARA array has been studying details on the surfaces of stars, and the Very Large Telescope has been studying extrasolar planets.

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+1, but just a footnote: it's still more difficult (read: expensive) to do optical interferometry, given that you have to be much more precise in preserving phases, and redirecting the path of visible light tends to lead to more losses/signal degradation. – Chris White Aug 22 '13 at 18:01

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