We haven’t had an interferometer yet to evaluate the damping work that went on in HAM6 last month, so I looked directly at the peaks as seen by the GS13s. The HAM6 ISI suspension, specifically the blade springs and flexures, were damped in order to reduce vibration coupling to DARM. The HAM6 table top vibrates at its suspension resonances just as the test masses vibrate orders of magnitude more at their suspension violin modes than at surrounding frequencies. These table top vibrations likely couple to the light by moving the OMs or producing relative motion of OMC mirrors, perhaps enhanced by suspension resonances that overlap with the ISI suspension resonances. 

Factors of as little as two increase over ambient vibration levels at HAM6 in the 900 Hz region were shown to produce features in DARM. In addition, this coupling was particularly worrisome because it is the only site with strongly non-linear environmental coupling. These vibrations intermodulated with the OMC length dither, producing features in DARM at higher and lower frequencies. The GS13 pods were also damped as a precaution, though no features in DARM appear at pod resonances. 

The first figure shows the GS13 signals before and after damping. Below it is a DARM spectrum showing the peaks excited by pre-damping vibration injections at HAM6. The upper plot shows that the GS13 peaks at these frequencies are much smaller after damping then before, even though the ambient vibration level was slightly higher for the spectrum taken after damping. In places where the black after-damping spectrum is featureless, we are seeing the GS13 noise floor, and the difference between the red and black curves underestimate the improvement. Figure 2 shows the Y and Z axes.

Final evaluation will have to wait for measurements of vibration coupling to DARM, of course.