It's been a rough one so far. ETMX ISI was rung up when I came in. I've tried a bunch of things, I'm not sure what or IF I've fixed it. Currently, the Z sensor correction is running on the ISI and the RY loop is running on St2. I've switched everything back to nominal, I'll keep watching the ISI, but for now everything looks OK. Shifting the Z sensor correction was what I thought earlier was the fix, but when I got almost to NLN, EX ISI rang up and blew the lock. I just gotten to NLN I've now turned on the St2 RY isolation loop, and the ISI may have settled down, but I've been here before. The current St2 RY blend is a 750mhz blend that injects a lot of GS13 noise at low frequency and CPS noise at high frequency, so it's not ideal, and I have a hard time believing that this configuration is really necessary for the ISI's stability but it's difficult to argue with qualified success.

Attached are a few spectra of the EX ISI St2 sensors. First plot is the RX (blue), RY(brown) GS13s, with GS13 noise curve(black). You can see turning on the loop makes the microseism better by a little, but worse at the blend cross-over and at a couple hz. Maybe the high microseism is whats giving us problems? Next plot are the CPSs red is RX, green is RY, I kept GS13 noise in for scale. Again, microseism is suppressed, but GS13 noise is making the low frequency worse by almost an order of magnitude, below 70mhz.

Turning on the St2 RY loop looked like it settled the ISI down, but I've backed everything out, and I think I can go back to observing. The EX ISI still needs monitoring.