Today I tried to add in new CHARD P and DHARD Y loop cutoffs. We have seen some coherence from both of these loops with DARM, 70758.

The CHARD P loop failed because as soon as I engaged it, I saw CSOFT Y start to ring up. The DHARD Y loop failed because I designed it hoping to reduce noise from 10-30 Hz, but with a sacrifice of more noise at 40 Hz. I was hoping that wouldn't matter, but I did see more noise at 40 Hz and very little improvement at 20 Hz. Gabriele had designed a much more dramatic DHARD Y cutoff that I tried a few days ago, but it rung up quickly at 3.8 Hz. I think DHARD Y as currently designed cannot afford much more of a cutoff. I don't understand why CHARD P causes a problem for CSOFT Y.

This might be the best we can do without a bigger overhaul of the loop design. However, we just overhauled the loop design pre-O4 to improve mircoseism suppression and reduce coupling above 10 Hz. I am not confident we can win much more right now.