[alocky (Andrew lockwood)]

Thanks Ray, I had seen those before. I guess more specifically, I'm considering a 16" f7 Dall-Kirkham for imaging, and was wondering at what sort of temperature gradients the additional thermal stability of the exotics might make the rather significant investment worthwhile. Perth has a fairly moderate climate, so I think I've answered my own question to some degree. Certainly when you look at the results from Damien Peach, Anthony Wesley etc, there's no mention of exotic substrates in their OTAs. The primary advantage would seem to be reducing the cooldown time, but a small airconditioner in the observatory will be significantly cheaper than zerodur!
interested to hear from anyone who's had the chance to compare the two side by side!
regards,
Andrew.

Originally Posted by Shiraz

http://www.rfroyce.com/pyrex.htm

http://www.skyandtelescope.com/astro...lescope-myths/

probably worth a read Andrew, if you haven't already come across them.

also note that the Riccardi Honders design is based on crown glass optics (eg BK7) and seems to be successful.