Sheila, Evan, Gabriele

We retuned the MICH feed-forward path. We injected noise in MICH and measured the transfer function to DARM: TF_MICH_DARM. Then we injected noise at the feed-forward input, with the FF shaping filters off, but with all the stop-band filters on and with the invBS filter on. We measured the transfer function from MICHFF_IN2 to DARM: TF_MICHFF_DARM. The optimal feed-forward filter should then be -TF_MICH_DARM/TF_MICH_FF. The first plot shows the measurement and the fit. We decided not to fit the sharp features due to the BS bounce and roll filters.

We implemented the new filter in the bank 'newFF' and engaged it. The performance improved, as shown by the improved DARM noise and reduced coherence. See the second plot: green no feed-forward; blue old feed-forward, red new filter. The last plot compares the old (red) and new (blue) filters. With the new filter, we also switched on an AC coupling (double zero, double pole at 0.1 Hz).