View Single Post
Old 03-17-2017, 03:20 PM
beowulff beowulff is online now
Join Date: May 2001
Location: Scottsdale, more-or-less
Posts: 15,097
Originally Posted by Francis Vaughan View Post
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.
I know that this is a Zombie, but for anyone interested the Steward Mirror Lab gives tours, and you can see the mirrors in production. When I went there, they were polishing the dual-profile mirror for the Large Synoptic Survey Telescope.