Today I tried to measure the spectrum of the beam motion on the optical end bench, after the end mirror. The main problem is that the beam shape is pretty irregular and very elongated in the orizontal direction (almost comparable with the sensor size). In the attached plot is shown the spectrum I found for the vertical motion (both with open and closed PR and BS loops). It is just a very qualitative measurements and I'm not sure about how much it makes sense.

Some remarks

1)  I used a calibration measured here for a PSD of the same kind, with red light. I'm not sure if and how it scales with the wavelenght (to be measured..)

2) The total rms (using this calibration) seems underestimating the real motion of the beam which by eye should be of the order of few millimiters.

3) The peaks are clearly coming from the PR and BS resonances. In particular the one at 10 Hz desappears when the BS pitch loop is closed as observed on BS spectrum

4) The PSD was not covered and the measurement seems affected by acustic noise.

I'll try do more quantitative measurement (better calibration, smaller beam, box on PSD) as soon as possible.