We had an episode of really large beam motion at about 0.6Hz on OMC QPDs as well as OMCR QPDs but not on ASC_AS_C nor AS WFSs. It was very very slow to damp, and at some point it didn't damp further. It was clear that the OMC or OM3 was moving, but the motion didn't show up on none of OMC and OM3 OSEMs. But the OMC SD and RT OSEMs were super noisy.
On inspecting the spectrum of OSEMs, it was clear that the SD and RT OSEMs of OMC had a large broad band noise, the signature of the satellite amp oscillation. However, the difference is that we couldn't see the oscillation in kHz range even using the 16kHz channel (sorry no screen shot). Maybe the oscillation frequency got higher?
Anyway, apparently it has been like that since power outage (attached). Though it's not shown in the spectrum, we confirmed that the broadband noise was there every day after the power outage.
We disabled the OMC damping and the beam motion on OMC QPD immediately got quieter.
We went to the floor and power cycled the satellite amp for RT/SD of OMC (they're on the same box, everything else is on another one), and the noise went away.
Turned on the damping again and it worked fine, no excessive beam motion.
Apparently this happens even though all satellite amplifiers are "fixed". It's worth checking all OSEMs for all optics.
I made a PDF with spectra of all of the OSEMs. The green trace is after the power outage, the black is a reference time before. I used the same times as in Jenne's alog. There's a turn-up in some quadrants of ETMX L2 at high frequency. IM3_M1_LL has a big broadband noise increase. ITMX L2 has a new peak at 3 Hz in all quadrants (it might be worth checking with faster channels in case this is aliasing). ITMY_L2_LR has a turn-up at high frequency. MC1_M3_UL has a small broadband noise increase. Obviously OM1, OM2, and OMC look terrible. PRM_M2_UR has a broadband noise increase. SR2_M1 LF and T1 look like they might have started glitching (bouncy shape in the spectrum). A bunch of channels also have excesses at low frequency, but I don't know if that's a change in physical motion, or maybe it's a result of other things being unstable.
