Complementary filters are used on the HAM-ISI, to blend the signals coming from both the CPSs (Capacitive Position Sensors) and the GS13s (Innertial Sensors). A new set of those Blend Filters was recently created  at LLO, for the ISI involved in the IMC commissioning: HAM2 and HAM3. These filters are more sophisticated than the ones we used in the past. The goal of the update is to maximize the use of the GS13s in low frequency, while still limiting the differential motion injected into the mode cleaner by the ISIs. Those filters are now installed on LHO's HAM2 and HAM3 ISIs, and they were tested today.

Spectra were taken on the cavity length control signals under two configurations (Fig. 1): 
1) ISIs Damped
2) ISIs Damped + Isolation loops Level 2 (UGF=25Hz) applied to the output of the new blend filters mentionned above.
Thanks to the recent progress, and help, of Giacomo and Kiwamu, these plots are calibrated. 

(Fig.2): The motion of the ISIs, along the cavity length(X-axis), was monitored during both phases of the test. Transfer functions were calculated. Amplitude and Phase are respectively close to 1dB and 0deg, on most of the frequency range of the analysis. This is an indication of low differiencial motion injection by the ISIs. Improvements could be made below 100mHz, where the phase goes towards 180deg (opposite motion of the ISIs). However the frequency resolution is not very high there and new measurements should be made for observatins to be more reliable.

(Fig.3): Ground motion was monitored in the LVEA with a STS2 seismometer throughout this test. No relevant shift could be noticed at the input of the seismic isolation chain (ground). This last observation tends to confirm that the changes seen on the IMC legth control signal do not come from changes in the spectral content of the ground motion, but from improvements of the blend filters.

Note: The gain for the IMC lock servo was set to 10dB during this test.