The Green Team has been been complaining of excess ~0.5 [Hz] pitch motion of the QUADs for the past few days. We had first chalked this up to the fact that the ISI were not yet at full performance. However, after Sebastien and company got both chambers up to a reasonable performance, and even going so far as to tailor the blend filters to get as much performance at 0.4-0.5 [Hz], the cavity pitch motion was deemed still too high. 

I finally just took a comparison ASD of both chambers and their optics' optical levers, and discovered a giant spike in ISI ITMX at 0.5 [Hz], and a subsequent comb cascading up to higher frequency. The 0.5 [Hz] (and its harmonics -- note that 0.5 [Hz] is *not* the fundamental) spike appears obviously in both the ISI ITMX ST2 GS-13s. Only the 0.5 [Hz] harmomic shows up in the SUS ITMX optical levers -- but it happens to perfectly coincide with the QUAD's first "pitch" mode at 0.5 [Hz]. Awesome.

Given that we've seen similar combs at LLO (and I believe DetChar has confirmed their presence at LHO), which were confirmed to be a result of the BSC-ISI's capacitive position sensor master oscillators beating against each other, I immediately suspect that this is our problem.

In order to confirm, we're going to run the corner BSCs in various configurations of OFF to assess whether the comb disappears, if not at least changes. 

Note: since this is systematic problem seen at both sites, a permanent fix to the problem is in the works (i.e. syncing all chambers to a single, GPS synced, master oscillator). It's just not ready yet.