Re: Why 20"?
1) 10 cm = 4 inches :)
2) The Hubble would not work if pointed at Earth. It's extremely sensitive to infrared radiation. Plus differential heating would ruin the performance.
3) 20in is a *big* telescope. I'm assuming military satellites use Maksutov-type closed-end optics. Putting up, say, a 40in telescope would be an enormous undertaking on the level of the Hubble scope. I doubt a corrector plate that big, that did not go geometrically apeshit when moving in and out of the Earth's shadow, could be made.
