This is a follow up on LHO:LHO:74113

Indeed there is a high Q feature in LSC_MICHFF in FM5 (10-12-23-EY) right at 17.7Hz that is coupling into DARM and conflicting with the LINE3 SUS line at 17.6Hz (see attached). 

It can also be seen in the LSC FF posted in LHO:73428. The resonant peak is about an order of magnitude higher than before the filter changes on October 12.

Options forward include: 
1. revert the MICH FF until the 17.7Hz can be removed.
2. increase the L3 SUS line even more to accommodate.



Addon: Camilla asked me what physically is causing the peak. From talking to JoeB, Vlad, and Gabriele: 
It's caused by beamsplitter bounce mode modeV3. It's listed in https://awiki.ligo-wa.caltech.edu/wiki/Resonances/LHO. Oddly, it's listed as being at 17.522 Hz but the alog the records points to (via a wrong link), LHO:49643, pegged it right at 17.79Hz(!)

Joe & Vlad: We should notch the bounce mode in MICH to avoid driving it during the excitation, e.g. when retuning the MICH FF.