While increasing the cavity feedback gain on H1:SUS-MC2_M2_LOCK_L from 0.3 to 0.12 I noticed a 40.91 Hz (measured in DTT with a BW of 0.01) line slowly ringing up. Modeling of the IMC locking control suggested that the cavity should have robust stability, even at the 4 times higher gain at M2. The MSTS model predicts a roll bounce mode at 40.37 Hz, so a good bet is that is what this is. There were similar problems with cavity feedback at LASTI.

I installed a 20 dB, Q 20 notch filter at 40.91 Hz to reduce the ring up in filter module 5 of H1:SUS-MC2_M2_LOCK_L. The name of the filter is notch41Hz. At a gain of 0.12 the cavity now seems much happier. I committed the updated H1SUSMC2.txt file to the svn.