Thermal expansion
For the 300 mm lens, we have to design a holder that does not warp the glass as the temperature changes.
RTV 566 (https://www.sciencedirect.com/science/article/pii/S0143749618302008): CTE is 400 e-6/deg C, but under 0.02 N tension (which I think is not relevant here), CTE is 220e-6/deg C at RT
Aluminum (https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html): CTE alpha is 21-24 10e-6 m/(m deg C)
BK7 (http://www.glassdynamicsllc.com/bk7.html): 71e-7 /(deg C)