Quote:
Originally Posted by ausastronomer
... BK7 has a somewhat higher specific heat than pyrex (18% or 858 as opposed to 726). While it takes longer to cool from a given temperature, it also takes longer and requires more heat to heat up to that temperature. The practical effect of this, is that the temperature differential between the mirror temperature and ambient is usually slightly less for a BK7 mirror than it is for pyrex.
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That's not quite right either. The material with the lower specific heat will follow changes in ambient temperature more closely. The material with the higher specific heat will be more stable wrt fluctuations in ambient, i.e. not follow them so closely and therefore in general have greater difference in temperature from ambient.