Genevieve and Sam asked about HAM1 and HAM2 motion around 40 hz. I hadn't look in detail in this frequency band for a while, this is typically higher frequency than we are worried about with ISI motion. But it turns out HAM2 is moving a lot more than HAM3 generally over 25-55hz, and particularly around 39hz. It looks like it might be due to gain peaking from the isolation loops, but could also be from something bad in the HEPI to ISI feedforward. The extra motion is so broad I don't think it's just one loop has a little too much gain, so I'm not sure what is going on here.
First image are spectra comparing the motion of the HEPIs for those chambers (HAM2 HEPI is red and HAM3 is blue) and the ISIs (HAM2 ISI is green, HAM3 is brown). The HEPI motion is pretty similar, so I don't think it's a difference in input motion. HAM2 is moving something like 10x as much as HAM3 over 25-55hz. The sharp peak at 39 hz looks like gain peaking, but I'm not sure that explains all the difference.
Second plot shows the transfer functions from HEPI to ISI for each chamber. Red is HAM2, blue is HAM3. The 25-55hz tf for HAM3 is not very clean probably because HAM3 is well isolated. HAM2 tf is pretty clean, it makes me wonder if maybe something is messed up with feedforward on that chamber. Maybe that is something I could (carefully) fix while other troubleshooting for the detector is going on.
I looked at some of my design scripts and realized that the HAM3 FF X filter is probably a bit better fit, so I copied that into the HAM2 foton file, loaded and engaged it on HAM2. It improved the 50ish hz motion quite a bit, but HAM2 is moving more than HAM3 still. Probably some tuning that could still be done here.
Looked further into the peak at ~40Hz and found an improvement in coherence after the re-tuning Jim did. Image 1 shows the peak pre-tuning, and image 2 shows it post-tuning.