Keita, Sheila, Georgia, Jenne, Craig

We noticed that with the ground motion a little high, our X arm green transmissions sometimes dip low enough to cause ALS to have trouble locking, because the drive to the UIM was causing the test mass to yaw.  The first attachment shows a time series, the dips in the transmitted green power seem to correlate well with excursions of the optical lever in yaw, which are very well correlated with the length drive to the UIM.  

We looked back a little at the history of this stage's coil balancing and decoupling since the vent: Two of the magnets were found to be flipped in June 42723 The coils were balanced in July 42740, measurements were made for the  frequency dependent decoupling at the end of July 43150 and at that time Hang did the fitting for the pitch measurements and installed a L2P filter, but the yaw measurements weren't fit.  

Based on the first attachment, the L2P decoupling is working well.  We noticed while doing driven measurements that the pitch mode of the suspension rings with any transient for a long time, but things seem to be OK when we are just looking at the ambient ground motion. 

Since we are concerned with motions that are at 40 mHz or so today, we tuned the DC gain of the L2Y filter without engaging any frequency dependent filter.  We reduced the coupling seen by the optical lever by a factor of 4 or more by setting the DC gain to -0.00075