To measure the ITMY absorption I used the same methods as we did for O3: Looking at the Hartmann wavefront sensor spherical power, and comparing it to Aidan's comsol model of the defocus caused by the thermal lens caused by a 54mm beam depositing 1W of power. The scale factor needed to match the comsol model to the HWS data is the power absorbed by the optic.

The attachment shows the data from our lockloss last night, after an 11 hour lock at 47W requested input power. The absorbed power is used to scale the model is 96mW. The fit is pretty rough, and doesn't match the data at later times, which is why I've put "preliminary" in the title.

With 327 kW circulating in the Y arm, this gives an absorption of 290 ppb.