Found it Geof,
Apparently the individual hexagon mirrors are traditionaly ground and polished, but they use an ion polishing process to do the final figuring- I've heard of this ion- polishing technique years ago, but didn't know it was in production.
The material it's made of is Zerodur which is a glass ceramic with virtualy zero co-efficient of expansion- I've worked this material ages ago using traditional hand grinding and polishing techniques- it's a lovely material to work- very very stable and you don't need to let it cool down after polishing, only trouble is it's a kind of amber/ yellow color and so no good for transmission optics, but it is perfect for mirrors after being coated (which I do as well
You cannot Diamond Turn zerodur and get an optical finish... beleive me, I've tried lol. Would be nice if I could... I'd make a fortune hehe, but if I remember correctly ion polishing involved removing material at the atomic level by bombarding the surface with Argon(?) so in a sense the mirrors were machined
we engineers are good!
Here's a short extract and the link:
"Each of the 36 mirror segments (plus spares) was cast in a relatively easy process, ground to its proper shape in a special process under tension, and then finished using an ion-polishing process."
http://scikits.com/KFacts.html