During the last stretches of full lock,  powers have been remarkably stable, but still showing some residual fluctuation. Here I'm analyzing the correlation of those power fluctuation with residual angular motion. I compute the best linear combination of all the ASC error signals (WFS and QPD) and their squared value, to predict the power fluctuation. The results fit very well the power time variation. One general comment is that the largest contributions to the power fluctuations come from pitch residual motions. The MATLAB code is attached.

AS RF90

The first plot shows the real AS_RF90 power (in blue) and the best fit obtained from the ASC signals (red trace). The second plot is a zoom in time. Using the same algorithm that I developed in the non stationary noise studies, I obtain a ranking of the most relevant ASC signals. The full result in shown in the third figure. The most important channels are AS_B_RF36_I_PIT, AS_A_RF45_I_PIT and REFL_B_DC_PIT.  The first signal is used for SRM, the second one is the other quadrature of the DETM signal (maybe the demodulation phase is not well tuned for DETM?) . One important remark is that most of the dependencies are with the linear channels and not with the squared values, meaning that the actual alignment working point is not the optimal one (in that case one would expect only quadratic dependencies).

AS LF

As above, the plots 4 and 5 show the fit to the power fluctuation. Plot 6 gives the channel ranking. In this case the largest contributions come from: AS_A_RF45_I_PIT^2, AS_A_RF45_Q_PIT and AS_B_RF36_I_PIT.  In this case we have mostly a quadratic contribution, meaning that the alignment is better for this particular port.

POP LF

As above,  plot 7 shows the fit to the power fluctuation. Plot 8 gives the channel ranking. In this case the largest contributions come from POP_A_PIT and AS_B_RF36_PIT. The first channel is highly correlated with the PRM motion, and the second one is used for SRM.

XARM

As above, plot 9 shows the fit to the power fluctuation. Plot 10 gives the channel ranking. The largest contribution is the same as for POP LF: POP_A_PIT. This seems to indicate that most of the power fluctuations in the arms comes from fluctuations in the PRC.