We just had a repeat of my alog 32847, so Detchar may want to look more closely at the BRS-X damping state during lock stretches. The first attached trend shows the BRS velocity, angle, damp bit (which indicates when the damper switches on and off) and IMC-F. As before, I saw on the wall traces that IMC-F and an arm signal (ETMX L1 DRIVEALIGN L?) showed a sudden 120 sec period oscillation, I immediately suspected the BRS-X damping. When I ramped the ETMX sensor correction off the IMC and arms signals immediately settled down some, though you can see that IMC-F is noisier with sensor correction off at the end of the plot (from increased microseismic motion). I think that we may want to adjust some thresholds for BRS-X, but that carries some risks of its own. What is weird is that around 19:15 BRS-X showed a sudden increase in motion (second plot). I'm not sure what happened there, maybe some HVAC kicked off?
There are a couple of 'easy' options that may fix this - 1) Have a notch in the sensor correction at the BRS-X resonance or 2) have the operator check BRS-X amplitude periodically and damp BRS-X (by adjusting thresholds) when not in 'Observe'. In 'Observe' mode we could keep the damping off. BRS-X is unlikely to ring up to large amplitudes over a short stretch unless it gets super windy.
Another fix would be to reduce the autocollimator non-linearity in the BRS-X, which will allow much larger amplitudes before damping is needed.