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BK7 (http://www.glassdynamicsllc.com/bk7.html): 71e-7 /(deg C)
So for a 300 mm diameter mirror, and a 40 degree temperature swing at the observatory, that's 300e-3 * 71 e-7 * 40 = 85 um.
For a 30 mm chunk of aluminum, that's 30e-3 * 23e-6 * 40 = 28 um.
Say we have 30 mm on each side, and that's 28*2 + 85 = 141 um or just under 6 thousandths of an inch. I think we're going to leave more space than that...