Thanks guys - some really good discussion here! I'm probably going to go with pyrex, and use the cost difference to put active cooling in the observatory as per Clive's suggestion. I've settled on this OTA as a compromise between deep sky and planetary, and will be trying to do both from within the light pollution of Perth.
The OTA is a mixture of CF and aluminium, so I will definitely report back on focus stability. However, since the first floor of the house is still being built, and the observatory is on the second floor, it will be a while before anything actually happens. Don't watch this space for at least 9 months!
cheers,
Andrew.
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