In a recent post (87010) Roland C mentions that Linwood's Celestron C11 optics is made with "normal" glass.
"Do you find the C11 to need a lot of focusing with temperature change? It would seem to me that would be the case since the tube is aluminum and the optics are normal glass, not quartz or zero expansion materials. Maybe where you are in Florida the temps are pretty stable?"
While I think I understand the concept of low or no thermal expansion glass as used for mirrors (fused quartz, zerodur, etc) reduces surface distortions during temperature changes, I noticed that AP's 254mm MC uses crown glass for the corrector plate (and fused quartz for the mirror). Would not crown glass have an optically significant coefficient of thermal expansion, ie introduce some distortions during cool down? For that matter, would not any of our refractors do the same to some degree? Would lenses or correctors made of low thermal expansion optical material like quartz glass (?) not be better in this regards? Or is it a matter of trade-offs again? I do know that a Maksutov corrector is a very large piece of deeply curved glass, so proably difficult to make. I'm just curious
Thanks
C
