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Old 10-21-2009, 05:13 AM
Francis Vaughan Francis Vaughan is offline
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Join Date: Sep 2009
Location: Adelaide, Australia
Posts: 4,550
I would be inclined to disqualify acrylic - it isn't a glass. It is a cast resin. It is only glass in so far as it is a transparent sheet.

There are a lot of single piece 8 metre diameter telescopes - they are spin cast by Roger Angel. This size is the technical limit of his process. The trick is that a spun liquid will form a parabaloid. So he fills a mould with glass pieces, starts it spinning around its axis, heats it up, lets the glass melt, form the right shape in the mould and then slowly (very slowly) cools it. Two advantages - the mirror's surface is already close to the right shape, so a lot less work is needed. But more importantly - you don't have huge changes to the internal stresses of the glass from removing all that extra glass. So making a single slab mirror that keeps its shape is possible. Bigger than this it gets pretty difficult.