Updated lens holder design

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)

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...

Sent the following to the machine shop: