Not a huge deal but I changed OFI OSEM input/output matrix for L and increased L damping gain in FM6 of H1:SUS-OFI_M1_DAMP_L from 20 to 4000 because they didn't make sense (and because IFO was down due to EQ).

What was wrong?

OFI OSEM input/output matrix for L were nonsensical, L mostly sensed and actuated on T.  See the first attachment where L was mostly just about 4*T. L and T were perfectly coherent even at noise-dominated frequency.

SD OSEM is sensitive to L as well as a bit of Y(=LF-RT), thus L should be linear combination of SD and LF-RT. In reality L was configured to be mostly LF+RT(=T) with much smaller SD mixed in as shown in the second attachment.

These matrices were set in Jan 2018 (alog 40204).

What was done?

SD sensitivity to YAW is proportional to the position offset of SD OSEM from the longitudinal (i.e. Y-arm direction) center line of the bread board, which seems to be 2.18" according to D0900136.

Similarly LF and RT offset from the X-direction center line of the bread board seem to be 4.38". Due to OSEM locations, the sign of Y sensitivity for SD and LF  are the same.

SD=-L+2.18"*Y

LF=T+4.38"*Y

RT=T-4.38"*Y

L=-SD +(LF-RT)* 2.18/4.38/2 = -SD + 0.249 LF - 0.249 RT.

Actuation math should be the same (if I ignore potential difference between COG and the center of the bread board). 

I changed both of the matrices accordingly. Since there's no good reason for L damping gain (was 200) to be much smaller than T damping (4000) any more, I increased the FM6 ("gain") filter bank in H1:SUS-OFI_M1_DAMP_L from 200 to 4000. In the third attachment, right plot is before increasing the damping gain, left is after, both with a new set of matrices and both during the earthquake.

It seems like Y is mixing into L a bit (look at 1Hz Y resonance), I won't bother to fix it for now.