Here's a summary of the procedure and results. We then vented, removed the mirrors, and remeasured them today. The ringdown cavity is a 0.5 meter tube with the test mirrors clamped to either end. The cavity is placed in a vacuum chamber, and a roughly 45mW infrared laser beam is applied. 10 ppm as the precision of this cavity, so there is no change within the measurement's uncertainty.
Absorption is measured by sweeping a sideband through a range of frequencies to locate a resonance.
This will increate heating and thermal lensing of the mirrors, which shifts the resonance frequency. The magnitude of the shift will scale with the magnitude of absorption. Thus there is no measurable contamination of these mirrors over the course of 4 weeks exposure.