I have measured very, very carefully the transfer function from pushing on MC2 to the Yarm ALS beatnote. In the newest loop diagram in http://nodus.ligo.caltech.edu:8080/40m/11030, this is pushing at point 10 and sensing at point 4.
Since it's a bunch of different transfer functions (to get the high coherence that we need for good cancellation to be possible), I attach my Matlab figure that includes only the useful data points. I put a coherence cutoff of 0.99, so that (assuming the fit were perfect, which it won't be), we would be able to get a maximum cancellation of a factor of 100.
This plot also includes the vectfit to the data, which you can see is pretty good, although I need to separately plot the residuals (since the magnitude data is so small, the residuals for the mag don't show up in the auto plot that vectfit gives).
If you recall from http://nodus.ligo.caltech.edu:8080/40m/11020, we are expecting this transfer function to consist of the suspension actuator (pendulum with complex pole pair around 1Hz), the ALS plant (single pole at 80kHz) and the ALS sensor shape (the phase tracker is an integrator, with a boost consisting of a zero at 666Hz and a pole at 55Hz). That expected transfer function does not multiply up to give me this wonky shape. Brain power is needed here.
Just in case you were wondering if this depends on the actuator used (ETM vs MC2), or IFO configuration (single arm vs. PRFPMI), it doesn't. The only discrepancy between these transfer functions is the expected sign flip between the MC2 and ETMY actuators. So, they're all pretty consistent.
Before locking the PRFPMI, I copied the boost filter (666:55) from the YARM ALS over to Xarm ALS, so now both arms have the same boost.
YARM_actTF_compareActuators.pdf
Things to do for ALSfool:
 Put fitted TF into the MC_CTRL_FF filter bank, and try to measure the expected cancellation, a la http://nodus.ligo.caltech.edu:8080/40m/11009
 Quick test with single arm, ALS locked using full loop (high gain, all boosts), since the previous versions were with ALS very loosely locked.
 Does this measured transfer function actually give us good cancellation?
 Think. Why should the transfer function look like this??
 Try it on the full PRFPMI
