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If you weren't going down the observatory route you could go with something like a collapsible binewt setup. That way you could go with two primary's with a much shorter focal length than one large mirror. I have always wanted to experiment with different materials for the primary mirror. The original newtonians used a metal mirror I can only imagine that a 20" aluminium mirror would be much lighter than a glass one. If the mirror was large enough it could compensate for any reduction in reflectivity from a poorer surface finish, not to mention aluminium is much faster to grind and shape than glass. The last idea I had was that stuff they sell at bunnings called Rust-Oleum Mirror Effect spray. It basically turns any smooth surface into a mirror, could be fun to experiment with that.
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