Measured free swing PRM-ITMY length was 230 nm RMS.
MI differential length was 85 nm RMS(elog #8248). This tells you that PR2, PR3 are not so noisy compared with usual suspensions.
Openloop transfer function:
OLTF of PRM-ITMY cavity lock using REFL55_Q_ERR as error signal and PRM as actuator is below.
UGF ~ 120 Hz, phase margin ~ 50 deg.
Somehow, phase delay was 460 usec, which is smaller than the empirical value 550 usec.
PRM-ITMY length spectra:
Below. Calibration was done using calibrated REFL55_Q_ERR and actuator response(elog #8255).