(Gordon D. Pusch) wrote in message ...
(dave schneider) writes:

(Henry Spencer) wrote:

A factor of 100 improvement would bring it down within reach of reason,
but a 1km mirror is beyond what's reasonably practical in the near future.
Eventually, yes.

Is there any indication of how big a bubble could be blown in
microgravity to create an Al or Au sphere, that could be sectioned to
provide several spherical mirrors (yes, I know, there's another conic
section that is better for focussing; ignore the man behind the
curtain for the moment) ?

For a sufficiently large focal length, you don't even need spherical sections;
the individual sections can be optically _flat_, and still not deviate from
the ideal figure by more than a fraction of a wavelength. (IIRC, a 10 km
focal length is sufficient for this to be true.) The primarily problem
then becomes one of _aligning_ the array of mirrors --- not machining.

An idea that keeps going through my head is to make a virtual mirror,
similar to a diffraction grating. A series of free-floating panels of
reflective material in orbit could hold their position with
microthrusters to form the surface of the mirror. The virtual mirror
could be thousands of kilometers across, while each piece is only a
few meters in size.