Similar to yesterday's plot but I've added MICH SRCL & PRCL
See the attached three panel for all the three building Pressures coherence to trhe IFO control signals.
Corner Station had coherence approaching 0.7 with Mich at ~20mHz, everything else is less.
Scanning through coherence plots from the LVEA pressure to individual platforms and DOFs finds some continuous coherence to the HPI BS RZ. See it on the upper right panel of the attached graph. The thin dark green is the current coherence and still not as bad as the pale thick lighter green reference line from before our controller tweeking back in Jan/Feb. However, there is a fair swath of non zero area from a few mHz to 100mHz. The same area where we see some coherence with MICH.
So why is the BS the most coherent. While the pump servo does get its signal from the pressure sensors at the BS, I'm more suspicious about the very large constant drive the Z DOF on the BS is currently applied by HEPI. When the Pump Stations are off, the BS HEPI sags so low that some of the vertical IPS are out of their linear range. This requires the DC BIAS to be almost 400um for the Z DOF. Maybe this gives us some Z to somethin-mich-is-sensitive-to coupling. It is on our wish list to lift this up with the HEPI Mechanical Springs.
