Tagging CAL. Also DetChar so they're aware the change was made and what collateral damage occurs after potentially fixing zee glitching.

This change can potentially affect the calibration of the IFO at low frequency, since we use ETMY during nominal low-noise control. In order to quantify this, we're going to take an L2/PUM stage actuation strength sweep once we get the IFO back up and running. Then, tomorrow during maintenance, we'll re-measure the chassis alone to check if all poles and zeros of the filtering need to be re-compensated as we'd done back in May 2016 (see LHO aLOG 27180 and 21232).

If there is a significant change in the frequency dependence then we must update the PUM stage compensation filters (and no change will be necessary to the calibration). If there is a significant change to the actuation strength, then we'll need to change the actuation strength of L2 stage in the front-end CAL-CS model.  

Further, the coil balancing for each stage will potentially be different as well. So we may need to resurrect re-balance the coils a. la. LHO aLOG 11392. I sincerely hope we don't have to change any of the finely tuned L2P or L2Y filters that are in place in the L2_DRIVEALIGN_L2P and L2_DRIVEALIGN_L2Y banks. I can't find the original design aLOG, but Sheila found that the L2 L2Y filter doesn't do anything back in March of 2015 LHO aLOG 17322, so I suspect only L2 L2P may need to be adjusted. Hopefully some remembers how that filter was designed.