This is interesting. I suppose you are acting on the ETMY.
Can you construct the compensation filter with actuation on the MC length?
Also can you see how the X arm is stabilized?
This may stabilize or even unstabilize the MC length, but we don't care as the MC locking is easy.
If we can help to reduce the arm motion with the MCL feedforward trained with an arm sometime before,
this means the lock acquisition will become easier. And this may still be compatible with the ALS.
Why did you notched out the 16Hz peak? It is the dominant component for the RMS and we want to eliminate it.
I actuate on ETMY for YARM and ETMX for XARM. For now I did adaptive filtering for both arms at the same time. I used the same parameters for xarm as for yarm.
I've notched 16 Hz resonance because it has high Q and I need to think more how to subtract it using FIR filter or apply IIR.
I'll try MC stabilazation method.