Jenne, Jim, Georgia, Sheila, Craig, Elenna, OPS, Daniel, Fil, everyone in the control room

We haven't been able to lock since Friday (though we only started trying on Tuesday evening). Some of the symptoms are listed in alog 63407.

Looking at the test mass top mass damping loops showed ETMX had extra low frequency noise and a peak at ~6Hz (first attachment, all traces taken with the interferometer in down/ready). This noise appeared around 11pm local on Monday night. The references in the first attachment are from before that time. Fil and Craig went to ETMX to power cycle the M0 IO chassis, the noise was still there when the ETM came back online.

We tried combinations of damping loops on and off, ISI fully isolated and damped, and R0 tracking on and off, with confusing results (see second attachment which shows a couple of the things we tried). At one point the resonance shifted down to ~5Hz.

We suspected rubbing, and so put offsets in the test filters for ETMX M0.

• We tried increasingly large offsets in V (vertical) and this improved the noise. Going from 40000 cts offset to 100000 cts did not improve things so we've parked it at 40000 cts. This corresponds to 13um according to the OSEM inmons.
• We also put a 20000 ct offset in T (transverse), which is 7.3 um. This combination of offsets brought the noise back to nominal (before Monday night, see third attachment).
• We tried removing the Vertical offset but the 6Hz peak and noise returned.

We aren't totally sure why we needed these offsets in the first place - it didn't look like a temperature drift was bringing the OSEMS far away. I've accepted the offsets in the SDF for now since we are finally back at Low Noise.