After buzzing the table with a probe at 500 Hz, the source of the 500 Hz resonance is the first steering mirror after the PMC. The PMC itself also exhibits a smaller 500 Hz resonance, unclear how much of that is actually the PMC mount vs. coupling through the PMC to the first steering mirror.
If you look at our beatnote ASD you can see a broad dirty hump at 500 Hz.
awade played a pure 500 Hz tone through our lab speakers, and you could see the resonance peak being driven. Blue are the driven spectra, orange are the non-driven spectra.
If we turn up the speakers it causes our North path to lose lock.
Some sort of mechanical resonance in the North path is causing this. Buzzing is underway.