I collected a roll mode power spectrum and coherence at 0.01Hz BW for all optics, and I understand the results for ETMY, ITMY, and ITMX (red, green, brown), however I do not understand the results for ETMX (blue). Roll mode peaks in ETMY, ITMY, and ITMX show corresponding coherence peaks in DARM, but the ETMX roll mode peak at 13.8828Hz is oddly low in coherence, but it has a large coherence peak at 13.9375Hz. Plot attached.
Peaks in power spectrum: ETMY, 13.8203: ETMX, 13.8828: ITMY, 13.9297: ITMX, 13.9766
Cheryl and I looked at this, the plots here are of the monitor bandpass outputs, which are a filtered version of DARM control. Cheryl has noticed that ITMX roll mode doesn't get damped to the noise floor of DARM, even with her improvement to the damping phase. We looked at the spectra of the AS WFS signals (which are used to damp these modes), and see that AS 45 A yaw is just noise, both of the AS B 45 signals have a signal for ITMX roll, but with SNRs of only 3 or 4 (the roll mode is about a factor of 20 or 30 above DARM).
We could try to damp this better by switching to either AS B or DARM as an error signal for the damping loop.