Following up on the IMC alignment improvement described earlier, I took a look at the residual fluctuations in the IMC transmitted RIN. Indeed, the RIN is much better on average when the correct IMC alignment offsets are used, but there are still quite large fluctuations. These are very well correlated to the residual angular motion, as visible in the IMC ASC error signals.

To quantify this phenomenon, I computed the band limited RMS of the ISS signal in the 100-400 Hz band and computed my usual linear regression analysis using all the IMC WFS and QPD signals. The first attachment shows that all the residual RIN fluctuations are predictable based on angular motion. The most relevant signal is WFS_B_I_YAW (see second attachment). Looking at the spectrum of the signal, it is quite clear that the low frequencies are dominated by a wide and smooth shoulder, likely due to input beam jitter created by air currents in the PSL room.

If this hypothesis is correct, we could implement an additional control loop, that servos the input beam to WFS_B. Since the DC of the input beam is already servoed to MC2_TRANS and since the DC of WFS_B is already used in DOF1 and DOF2, this additional loop must be AC coupled.