I was taking some measurements on the BSCs earlier today, and I noticed a feature at about .73 hz in the OAF CARM and DARM suspoint channels that didn't show up in the OAF MICH suspoint spectra. I then looked at the GS13s on ETMY and there is a pretty sharp feature that seemed to have appeared after recovery this morning. A quick cruise through the Summary pages, though, showed that there is some intermittent feature that has been there off and on since at least the end of June. I thought it might be the RX/RY loops on ST2 I've been trying, but turning those off had no effect. I then took the ISI St2 to damped, but the feature was still there. Jeff said that the TMS has a resonance about .73 hz, so I turned the damping off and that made the feature much worse, but it also rang up a bunch of other features. Whatever this is, it has to be very intermittent since I hadn't noticed it before today, and I look at ETMY pretty frequently, and I've never noticed it before.
Attached plot is the ST2 RX GS13, which showed this the strongest, though RY and Y also saw it. The CPS also saw this in all configurations, even damped. The TMS sees this, though not as big, and the quad sees even less and only in pitch. ST1 of the ISI does not see this.
For future reference, recall that H1's TMSY is the only "first article" TMTS, slightly different from every other "production" TMTS. The mode that is close to this frequency is modeled to be at 0.734 Hz. *If* this resonance is the source of the feature seen in the ISI, I suspect the coupling to ST2 rotational DOFs is because the TMS is mounted far from the center of the optical table. I've taken a damped and undamped transfer function this morning to confirm that this mode is "well" damped when damping loops are on. RED is current damped BLUE is current undamped and BLACK is some previous test in June of 2015. Indeed, the undamped resonance is at 0.73 Hz (and there's actually a cluster of three resonances from various other modes cross-coupling into the measurement), but as the damped measurement confirmed, the Q is a low ~15-30, as designed.
