[alocky (Andrew lockwood)]

Hi all,
I'm doing some due diligence before buying another scope, and am trying to decide what benefit ultra low expansion substrates offer in practice. Ignoring considerations about stability during figuring, since that won't be my problem, at least, what sort of conditions 'require' the use of better substrates than pyrex? I'm looking at about 2800mm focal length, so I expect the main limiting factors will be thermal equilibrium and local seeing.
I'd welcome any informed opinion on the topic!
cheers,
Andrew.