Comparisng two stretches of lock that Dan pointed to me (one from Friday, good stable  sensitivity, and one from last night, worse sensitivity), I find a clear non-stattonary behavior of the sensitivity, see the first two plots, which are spectrograms of ten minutes of data during each lock.

Analyzing last night's lock, I computed the band-limited RMS between 100 and 300 Hz and it indeed shows an almost regular oscillation at about 120-140 mHz. I compared this oscillation with few auxiliary channels, and found out good correlation with the SRM angular motion (third plot), and expecially with AS_RF36 WFS signals (fourth plot).

The last two plots show my usual least square fit of the BLRMS using a set of auxiliary channels:

• 5th plot uses the SRM pitch and yaw witness sensors, and we can reconstruct quite well the period of the oscillation
• 6th plot uses the AS RF36 WFS signals, and we can see that this is quite good in reconstructing the BLRMS variation

In summary, I think we need to commission an angular control for the SRM, and it seems we have good signals from AS_RF36. The main motion is in pitch.