[RXA, KK]

As shown in the gigE camera image earlier in this thread, the MCO spot position is largely off in YAW (cm scale).

We observed the MCL control signal coupling from mirror YAW (PIT) motion (angle to length coupling) while changing the coil balance of YAW (PIT) from [H1 H2 H3 H4] = [1 1 -1 -1], [1+1dB, 1+1dB, 1-1dBm, 1-1dB] (let's call it 1 dB unbalance), ... to 3dB unbalance. When the spot position and the P/Y tilting center are matched on the mirror, no angular signal should be seen the MCL ctrl signal. We used the lockin osc matrix to apply the balances and actuations.

The result is attached. The x axis is the applied unbalance, which is equivalent to the beam spot distance from the center of ther mirror. The x axis should be calibrated to the spot position from the center, but I couldn't find the distance between the magnets. It will be updated later. The Y axis is the signal in MCL control therefore showing the coupling level from the angular motions. As shown in the plot, MCO YAW is the most mis-centered one.

Looking back the klog, MCO was repaired from the fallen-off magnet and re-installed in May 2018, then the in-air alignment and spot position check was done in June with Stefan. Also, as posted earlier, checked again in September 2018 (klog 6317). I am puzzled how this position change on MCO could have happened. Removal of the MZM path (May 2019) on the PSL table was very careful (checking the MC flash every time we removed a mirror) I don't think that could cause the big input beam misalignment.