Andy, Jess, Josh, TJ

Following up alog 30790 and comments (range drop may be related to excess RM motion), we've found some very strange motion with apparently quantized jumps in frequency. This motion is seen in accelerometers near HAM1, in the nearby HAM HEPI L4Cs, in the RM mirror motion, and in some REFL and POP DC signals. Some of this motion seems to show up in DARM as well.

The first plot is a zoom in frequency on the kind of motion that we see in these channels. It looks like several lines which sometimes jump suddenly in frequency, then jump to another frequency, then another. It looks like a MIDI music file (or a music box). We've found this in many channels, but the clearest so far is in the HAM1 floor accelerometer. It's also in the ISCT1 accelerometer, and the RM OSEMs see it in some degrees of freedom (which is how we first noticed it). The HAM1 and HAM2 HEPI L4Cs see some of it, see second and third plots. Even the HAM6 accelerometer sees some of it, so it's not local just to HAM1/2 - but we haven't checked exactly how widespread it it. We've also checked at least in the HEPI that this was there at an earlier time in the day, and also two weeks prior.

The last plot is DARM, showing that this seems to couple at least in the 10 to 20 Hz region. That could be through the RMs somehow, or maybe through scatter from ISCT1. Since the beam diverter was closed (alog 30835), the next locks can check if this is through ISCT1.

What's causing this motion? It looks really peculiar. It's hard to pick it out in just a few minutes of data, because the lines are narrow and don't wander - they jump suddenly. So it may have gone un-noticed before, but it would be nice to understand it even if the coupling to DARM is easily fixed.