We found that the TX and TZ dampers of suspension filter 7 are injecting control (sensing) noise  on DARM between 2 and 6 Hz. This is evident only since DARM has been improved in that region due to the mitigation of PR_MAR_TX control noise. The largest contribution comes from WI. The marker of that is the presence of two narrow peaks around 3 Hz in WI_F7 sensing, which are well visible in DARM and disappear (together with some broadband noise) when the dampers are turned off. DARM improves more when the dampers of all the suspensions are turned off (Fig. 1). A precise noise budget of each d.o.f. has not been done.

We tested a lock acquisition with all the dampers switched off at the beginning, but some oscillation started to grow up when the dark fringe was reached. We moved the disabling of the dampers just before the end of the locking procedure, and we saw no bad effec for a few hours. But later in the night the NE payload got excited, mainly at 0.4 Hz along the transversal axis, and the effect was visible in the sensitivity at low frequency. It would be interesting to make some specific test in order to understand the coupling mechanism, and if it is present on all the suspension or only on NE.

Tonight I'm going to leave the ITF with all the dampers disabled except NE, in order to see if some other slow growing of resonances shows up.