Measured and retuned the PRCL feedforward with a measurment with the IFO about 5 hours into a lock.
Good performance measured.
Everything is in guardian.
The interesting observation is that we are now limited by non-linear / non-stationary coupling of PRCL to DARM: while injecting PRCL noise with the new FF on, there is little coherence betwen DARM and PRCL, but DARM noise is increased broadband by ~10.
The residual PRCL noise ocupling to DARM is non-statioanry, as shown in the attached spectrogram and BLRMS plots.
One cal also look at the time with a PRCL noise injection and the PRCL FF on. A linear and stationary subtraction can improve DARM just a little, showing that the FF is already doing a good job. Allowing the subtraction to use ASC signals for coupling modulation allows us to subtract a lot of the residual PRCL coupling from DARM.
However, this is not necessary to clean DARM in normal condition: we had to inject PRLC noise to see the effect.
