After figuring out that the 6.18Hz was coming from the vertical loops of M0 (The left OSEM doesn't behave properly), spectra and transmissibility transfer functions were measured with the following configuration:

-  HEPI with position control (100mHz UGF)
-   Feedforward HEPI L4C to stage 1
-  Stage 1 blend

o   750mHz in Rx, Ry, Rz
o   100mHz with notch at 440mHz in X
o   40mHz with notch at 440mHz in X

-  Stage 2 blend

o   750mHz in Rx, Ry, Rz|
o   100mHz with notch at 440mHz in X and Y

-  Level 3 controllers

o   Stage 1 – 40Hz
o   Stage 2 32-35Hz

-  SUS damped except on M0 in Roll and vertical
-  No sensor correction

Spectra presented were measured at 1062915770 (start time) during 45 minutes. Spectra in all DOFs are presented in attachment. Performance is great.

Transmissibility transfer functions are measured while driving white noise on the HEPI actuators. Two transmissibility plots are presented. In the first one, the transmissibility is computed using the HEPI L4C as input.

In the second one, the transmissibility is computed using the HEPI excitation and the supposed input motion is calculated (alog https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=7692)

It is important to note that:
-  The isolation filters of the BSC-ISI were designed with the HEPI locked
-  These loops are stable and robust in time. There were closed few days.