2. They don't need to be. Most telescopes aren't trying to image the corona of the sun or another star.
3. Space-based telescopes are really expensive and have significant limitations on size, shape, mass, etc.
Sorry I meant instead of having one big piece of metal which would have restriction in terms of length, having 2 pieces of lenses seperated by just vaccum? You would have lesser material, easier launch and higher magnification because of larger focal length etc?
Mostly because they don’t need to be, i would guess.