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Andrew,
assuming the substrate is (reasonably) stress free,
the impact on the resolving power (of a 16" DK) for a temperature differential between the primary mirror and the air will be dominated by thermal convection for any substrate that you are likely to encounter.
Focus stability is a separate issue that can be addressed through automation and the appropriate equipment.
I think it is more cost effective to address the issues associated with temperature gradients at a fundamental level. Remove them (through active cooling) rather than compensating for one (minor) symptom using exotic materials.
c
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