made another step from 26.6degC to 30.6degC (4 Kelvin). Checked beat frequency to be ~72.192MHz @26.6degC, so we can re-use the iLIGO VCO when operating at high enough temperatures.

The initial step of 100mK was only a test to see if everything works. Local temp fluctuations have still a lot of influence to the precision of the reading, so the following numbers are rough estimates. The larger step will hopefully result in a better value.
The Marconi feedback signal changed by ~1.2V with a tuning coeff. of ~284kHz/V, so this gave a change of 341kHz total, so about 3.41MHz/K.

The original sensitivity can be estimated as

a_SiO2 = 5.5e-7  , L_spacer = 203.2mm  -> dL_spacer = 111.76nm/K

with FSR = 738MHz

-> df = 155MHz/K