Out of curiosity, I looked at coherences of channels listed above from 24Oct2016 (Stefan's data above), and 24Jan2016 (a time when Gabriele ran a BruCo, so was probably a good lock time). 

In the attached screenshot, the right half is coherences, and the left half is spectra of those channels.  On the coherences half, top right is SRCL vs. IMC-MC2_YAW (ASC control signal), bottom right is SRCL vs. WFS_B_Pit (ASC error signal), top left is PRCL vs. POP_PIT and bottom left is PRCL vs. POP_SUM.  On the spectra half of the screenshot, the top row is the length DOF signals, and the lower 2/3 of the plots are in the same locations as the coherences plots.  In all plots, green is now-ish and blue is O1 config. 

• First, we see that the coherences are much higher now (green traces) than they were in January (blue traces). 
• However, looking at the PRCL and POP spectra, I do not think that the excess PRCL noise is jitter-driven. 
  • The PRCL signal certainly has more noise now, but the POP_A_PIT signal has less noise now than it did in January. 
  • Note that the POP_SUM spectra are slightly different because we changed what channels get saved, and so their normalizations must be a bit different. 
• If we look at the SRCL and IMC ASC spectra, it looks like above ~9Hz we are seeing noise in SRCL due to noise in the IMC WFS. 
  • Below 9Hz or so, SRCL is dominated by other noises - this is consistent with the coherence only starting to be significant around 8 or 9 Hz.
  • The IMC-MC2_YAW signal is the control signal we're sending to MC2 - it's clear that our control loop shape has changed, and we're not rolling the WFS signals off as sharply as we used to be.  We should try lowpassing the WFS loops more aggressively to see if that helps the auxiliary loop signals.