The last couple of days Krishna and I have been working on sensor correction everywhere, but have been having troubles on HAM3. This is not exactly related to that effort, but we noticed that there is a very Q-y feature in the HAM3 ISI and SUS spectra. Jeff helped me dig into this and we are pretty sure this is not due to the suspensions, but it's been there the last couple of days, with and without sensor correction. It doesn't show in HEPI or the ground spectra, so I'm not sure where this comes from. When the IMC isn't being used I will try turning isolation loops off to see if the ISI is causing it. Attached spectra is of the MC2 suspension point(ISI GS-13's), but PR2, MC2 OSEMS and ISI CPS's see this, as well.
And by "pretty sure" it's not the SUS, Jim means "it's definitely not." The first HSTS modes are at 0.67 [Hz], and the recent design study (see G1401291) have revealed that even with crappy legacy damping filters and gains that are still in place, the Qs of all the modes are reduced well below what's seen here. Also of note -- The 0.6 [Hz], and 1.5, and 2.9 [Hz] features are seen in IMCL, PRCL, and the HAM Optical lever, so it's definitely the ISI, because it's common to both SUS in the chamber and their moving mass doesn't couple well enough to the ISI to move the thing in all DOFs like is shown here.
And now you don't!
I started looking for the .6hz whatsits and it's gone now. The local sensors didn't see it when I looked this morning, but looking back in time it disappeared some time between 16:00 (first plot) and 16:30 (second) yesterday. We are also seeing something similar on the ITMs' oplevs (third plot, you can see it in Krishna's post about Xarm sensor correction, too) now as well, so whoever fixed HAM3 should keep going.
