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Originally Posted by IanW
T Mirrors and lenses expand when it's hot and contract as the temperature cools. The amount of expansion and contraction varies with the matieral lenses and mirrors are made of. The rate of expansion per °C is known as the thermal co-efficient of expansion. Plate glass for example expands many times more than Pyrex which in turn is worse than Zerodur/Duradur or Aerosital which have co-efficients that are for all purposes essentially zero for most optical purposes.
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Keep in mind too that it is only an internal heat differntial in the glass that warps a mirrors figure. If the temp is even through , a mirror will perform identically at +20 or - 20 degrees C. Zero expansion materials like Zerodur have poor emmisivity, so while the mirror may not bend with a differntial temp, it will still be shedding image destroying currents onto the light path for much longer than a Pyrex mirror. I doubt there is any practical advantage in it except for very large mirrors. Optics are best to be kept thin if from Pyrex , at 40mm thick at 20" diameter and below is optimal.
Mark