March 27, 2022
Vibrations of the optical system are driven by wind loading. The terminology used here is to define 'dynamic pressure'. The total pressure on a surface is a combination of PV=nRT type motions due to thermal energy content in a parcel of gas that is not moving, plus a dynamic term P_d=(1/2) rho * v^2 which is the kinetic energy per unit volume of the material. This dynamic pressure exerts a force F=P_d*A on the projected surface A. That holds for incompressible fluids. For compressible gases P_d=(1/2)M^2 * gamma * P where M is the flow's dimensionless Mach number M=v/c with c being speed of sound, gamma =1.4 for air is the ratio of specific heats, and P is the static pressure in Pascals.
If the flow field u(x,y,z) was totally time-independent, then the dynamic pressure is steady in time and the mirror support system would compensate. Our problem arises from variations in the dynamic pressure at frequencies above the bandwidth of the active mirror support systems.
It's easier to measure wind speed than it is to measure pressure. Pitot tubes measure static vs. dynamic pressure, but as a quasistatic quantity.
Hot wire anemometers are a traditional way to measure turbulence and have good frequency response. This is effectively the use of a thermistor in a regime where self-heating is high, and the equilibration temperature is determined mainly by convective heat loss into the surrounding flowing air.
Bought this item on eBay:
So it's a Platinum wire of diameter 0.4 mils and length of about 0.125 inches (8/64 = 1/8 inch).
Resistance at 20C is 2.69 Ohms and lead resistance is 1.89 Ohms. Temperature coefficient of resistance at 20C is 0.39% per degree. Yes, the resistance increases with temperature. If we ran at constant current, since P=I^2R we'd get thermal runaway. Operating at constant voltage avoids this, since P=V^2/R as temperature goes up the resistance increases and power dissipation goes down.