The ISS second loop photodiodes are very well coherent with the IMC WFS B yaw signal, as visible in the first plot.

This WFS is used for both DOF1 and DOF2 of the IMC alignment. I added an offset to the DOF1 yaw error signal. This improved significantly the intensity noise at almost all frequencies, see the second plot.

The thrid plot is a spectogram of the intensity noise as a function of time. The bottom trace shows the IMC DOF1 error signal. It is clear that a mean value of 250 counts gives lower intensity noise. It is also clear that intensity noise is highly non stationary, even with a good IMC offset.

The fourth plot shows the same story, but this time intensity noise is characterized with a trace showing the band-limited RMS between 100 and 200 Hz. Using the best IMC alignment offset reduces the noise in the band by a factor ~10. The last plot is a zoom into the last minuts of the test. It is clear that the residual fluctuations of intensity noise are related to the residual angular motion of the IMC.

In summary, we can improve a lot the intensity noise at the output of the IMC by choosing a better alignment position, and by improving the accuracy of the angular control loops.