[Physicist]

Why not use the moon as a platform for a 100 or more meters telescope

Because you can do far better than that on the moon, without building a behemoth. You can use a bunch of telescopes thousands of kilometers distant from each other, and use interferometry to make an optical telescope with an effective aperture that is thousands of kilometers wide. You can't do that on Earth because the uncorrelated atmospheric distortions destroy the relative phase information if the telescopes are too far apart. The limit for optical interferometry on Earth (I'm told) is a couple of hundred meters.

The two Keck telescopes (which were designed to work in concert this way) are optimally far apart, given the Earth's atmosphere. On the moon, the optimal solution would be to put the telescopes as far apart as possible.

[Prodigal Son]

to make an optical telescope with an effective aperture that is thousands of kilometers wide.

At this point, Tim Allen from Home Improvement makes the ape noise 'Augh, augh, augh!'.

Now that's a manly telescope ;-)

[longshadow]

You can't do that on Earth because the uncorrelated atmospheric distortions destroy the relative phase information if the telescopes are too far apart. The limit for optical interferometry on Earth (I'm told) is a couple of hundred meters.

Ah; that explains the mystery (to me) regarding why they don't just buy 25,000 Celestron C8's, link their GOTO controls to a common computer, digitize the output and timestamp it with an atomic reference clock, and ship the data off to some big-assed data base where a computer can leisurely number crunch to get everything in phase....

It would surely be cheaper than one of these behemoths....

[desertcry]

..... without building a behemoth. But the light gathering power of a telescope increases with the square of the diameter, does it not? Although using interferometry in conjunction with widely separated multiple telescopes does result in a great increase in resolution, it may not result in a large gain in the ability to observe extremely dim objects, like that of the infant universe or very small extra solar planets.