Ajmoir,
It all depends on what level of accuracy you need, if you're looking for sub micron accuracy then you will probably have to use an interferometer (a phase shifting one will get great accuracy), there's a couple of sites I use which may help:
http://spie.org/ http://www.groupsrv.com/science/forum-22.html
Geof,
That surprises me! the 16 hexagon mirrors that make up each of the two Kecks are 2 metres in diameter (giving a 10 metre total dia for each telescope). To be able to diamond turn something as big as 2m is very impressive! The biggest I've done is off- axis parabolas about 1m dia.
Unless it's just some of the other optics that are turned.. I shall look into it.
Incidentaly, for those who don't know, the Keck telescopes are the largest in the world and will operate together (like a pair of binoculars) to get even more resolution and see things really far away.