[m4rtink]

Aren't the terrestrial mirrors so thick due to the need to survive all the tilting under the strong terrestrial gravity ?

I would assume a micro gravity only mirror could be much thinner & thus easier to cool down. Or possible alternative techniques could be used to get the needed reflective surface geometry if it does not need to take gravity and atmosphere into account.

[HorstG]

Yes, but stability against vibration and thermally induced warping is also important. So you could make the mirror thinner than on the ground, but not really thin.

[petschge]

You also need a minimum thickness and stability if you ever want to turn the mirror to point elsewhere.