I know Arnaud has tried this long ago, but I've been meaning to try reducing SRCL motion by sending signal from ground STS2 Z to the HAM4 Y sensor correction path.  I tried this in June of 2017, but had limited time and the microseism was really low. Yesterday, while the IFO was locked and the commissioners were at a meeting, I was able to collect the tfs I needed to design the filter and install it. I found that it worked, but I did some more tweaks today and I have something I think we should try.

First attached plot is one of my design plots. The solid blue trace is the ratio of two transfer functions, SRCL/GND_Z and SRCL/HAM4_DRIVE. The solid gold and red traces are a couple of attempts to fit it. The dashed lines are estimates of the suppression for each filter (i.e 1 + fit filter / meas tf ratio).

My second plot is the before/after SRCL to GND Z transfer function, red and blue are with the sensor correction on, green and brown are with it off, you can see between .1-.3 there is less SRCL length for GND Z motion.

Third plot are the asds for the different measurements, blue, red and pink are the SRM M3 drive, CAL SRCL and ITMY GND Z with the sensor correction off, brown, green and light blue are with the sensor correction on. Most of the improvement is still between .1-.3hz, the SRM drive is reduced by almost a factor of 10, I think most of the difference below .1hz is due to less tilt at the chamber. 

I would like to leave this on, but if people are worried this is screwing stuff up, ramping H1:ISI-HAM4_SENSCOR_GND_STS_Y_WNR_GAIN to 0 will shut it off, i.e. typing in a terminal:

caput H1:ISI-HAM4_SENSCOR_GND_STS_Y_WNR_GAIN 0

will turn it off.