Since we slowly stepped through the LOWNOISE ASC state yesterday, I was able to plot ASDs of the DHARD P error signal and determine what exactly creates the 1 Hz peak. My hypothesis was it was something from the damping loops, but I was wrong! The peak appears after we engage the low bandwidth DHARD P controller. I will take a look at the loop design and adjust to reduce this peak. My guess right now is some gain peaking is present at 1 Hz.
Attachment shows the sepctrum of the error signal during the state. If you are looking at the calibration lines in DHARD P and getting worried- this was before we fixed the A2L gains, and in full lock low noise there are no cal lines present in either DHARD P or Y.
We also have a 1 Hz ring up that sometimes appears after the lownoise ASC state. It has impacted our interferometer locking during this period, but "around the control room" we think that this has been a problem that appears time to time even before this vent recovery. It may show up during the ETMX transition states since it is a fairly slow ring up (occurs about 3-5 minutes after the control is changed).
I'm looking at the boosts we have engaged in DHARD to determine if we need them all, because they are stealing a lot of phase at 1 Hz and probably causing this problem.