A common question seems to be: why not put such a telescope in Earth orbit? This is a spinning, liquid mirror telescope that uses centripetal acceleration to tu

A common question seems to be: why not put such a telescope in Earth orbit? This is a spinning, liquid mirror telescope that uses centripetal acceleration to turn a pool of liquid mercury into a parabolic mirror. Orbit does not provide the gravity to continuously 'cast' the mirror into a parabola. The parabolic shape is necessary to focus light to a point. A few prototypes have been carried out on Earth, notably one by the University of British Columbia. The main advantage is that a 100m (one hundred meters!) aperture with near optical perfection can be transported in a compact reservoir in liquid form. There are other ideas for large optical telescopes in space like 'painting' part of a balloon with a reflective material. The disadvantages are that these produce spherical rather than parabolic surfaces and would require complex re-imaging optics (like Arecibo). The other is that to observe optical and near-ultraviolet light the surface must be very accurate with deviations less than 15 times the wavelength of the light (so in the 20 nanometer regime). The technology has not been demonstrated for this with a space balloon, but there are plans to use this scheme for far infrared or microwaves where the surface accuracy constraints are less severe.