J. Kissel

Since we're still whistling in the dark regarding understanding some of the noise in the mid frequency band, and because we haven't done these tests since O2 (see LHO aLOGs 38132, 38122, and 37752), I gathered a round of broadband injections driving the BSC-ISIs' Stage 2 (ST2) in Pitch (RX and/or RY) and Yaw (RZ), which *used* to have caused coupling with the IFO via what we think was charge. This time, I also measured the BS in addition to the Test Masses, just in case this might be the source of the nasty 48 Hz feature. For RX/RY, my drive (between 10 and 100 Hz) increased the motion of ST2 by a factor of ~100. For RZ, the ST2 motion was increased by a factor of ~1000 during excitation.

I plan to do a more complete (allowing for non-linear) analysis, but from comparing the ASD of -- and coherence between excitation and -- DELTAL EXTERNAL during the excitation against ambient, the only injections that were even remotely coherent (at the 0.3-0.4 kind of level at worst, in select ~few Hz wide, likely QUAD Cage frequencies around 70 and 90 Hz) were excitations that drive the stage a factor of ~1000 above ambient noise floor. In other words, *any* coupling from this kind of motion to DELTAL_EXTERNAL is at least a factor of 100 away from the noise floor.

Data result templates live and have been committed here:
/ligo/svncommon/SeiSVN/seismic/BSC-ISI/H1/Common/Data/
    2019-07-22_BSCISI_ST2_BB_RX_Injections_BS.xml
    2019-07-22_BSCISI_ST2_BB_RY_Injections_BS.xml
    2019-07-22_BSCISI_ST2_BB_RZ_Injections_BS.xml

    2019-07-22_BSCISI_ST2_BB_RXRY_Injections_ETMX.xml
    2019-07-22_BSCISI_ST2_BB_RXRY_Injections_ETMY.xml
    2019-07-22_BSCISI_ST2_BB_RXRY_Injections_ITMX.xml
    2019-07-22_BSCISI_ST2_BB_RXRY_Injections_ITMY.xml

    2019-07-22_BSCISI_ST2_BB_RZ_Injections_ETMX.xml
    2019-07-22_BSCISI_ST2_BB_RZ_Injections_ETMY.xml
    2019-07-22_BSCISI_ST2_BB_RZ_Injections_ITMX.xml
    2019-07-22_BSCISI_ST2_BB_RZ_Injections_ITMY.xml

Attached is the kind of "worst" case coupling I saw, from RZ injections of the ITMs. On the ETMs, during RZ injections, there is a sharp feature at ~95 Hz with a coherence of 0.5. Again, this coherence is with excitations *far* above ambient noise levels, so ambient coupling will be very small, and in most cases, we will only be able to provide an upper limit.

More analysis to come.