Summary: tap tests from summer, along with GS13 coherence, suggest that the 878 peak in OMC spectra is from a tip tilt. Coherence between HAM6 geophones and DARM at LLO suggests that this acoustic noise is likely to be below the aLIGO floor, though not by a factor of ten.
I have been concerned that acoustic coupling at HAM6 will be a borderline noise source for aLIGO in the 400-500 and 700-1000 Hz bands (here). The ISI transfer function in these bands can approach 1. This was the source of acoustic coupling that remained at the end of eLIGO even after we suspended all of the optics that touched the beam. For this reason I have been watching for the appearance of these features.
I checked the peaks Koji and Dan noted in their recent OMC studies (here, here, and here) by looking at coherence between the HAM6 GS-13s and the DC PD. Figure 1 shows that a good fraction of the peaks are attributable to vibrations on the table in the 450 and 900 Hz bands, although a couple are not.
During the summer, Christina Daniel and I “tap” tested a number of parts of the HAM6 ISI and the ISI payload so that we could later identify peaks in this band. Of all the structures we tested, only TT2 had a resonance at 878 Hz, the frequency of the largest peak in the OMC spectrum (Figure 2). While this is not conclusive, we should probably look first at the TT structures if we need to damp this resonance. If the 878 Hz resonance indeed comes from a structure that interacts with the light, I would expect 878 to be overrepresented on the light, relative to other motions detected by the GS13s. This would explain why the 878 peak in the OMC spectrum is, relative to surrounding peaks, much larger than in the geophone spectra. For the tap test the situation was similar, the accelerometer was on the tip tilt so tip tilt resonances would be enhanced relative to other table motions detected by the GS13s.
I checked LLO’s DARM to see if there might be coherence between DARM and the geophones in these bands in order to get an idea of whether these peaks will be a problem. Figure 3 shows that I was able to see some coherence, at the 0.002 level, between the HAM6 geophones and DARM (notice that all six geophones show increased coherence at some peaks). So I would guess that the amplitude is roughly 1/20th of our current noise floor. At 1000 Hz we are currently about a factor of 6 away from our 195 Mpc sensitivity curve. Thus these features may not be a factor of ten below the sensitivity, as we would like, but they are unlikely to be prominent.
