I estimated the magnification of the ETMY HWS optical path. To do this, I took a measurement of the beam profile on the HWS CCD (without the Hartmann plate in place) and fit a Gaussian beam profile to it. This was then compared to the theoretical beam size at the ETM within the cavity. Given that the cavity scans show very little higher mode content it is a safe assumption that the beam size at the ETM is approximately the cavity beam size (although I don't yet have an estimate on the error/uncertainty in this approximation).

Measured 1/e^2 beam radius at the HWS = 1.51mm

Nominal 1/e^2 beam radius at the ETM = 31.49mm

Magnification (HWS to ETM) = 20.83x