I tried more tests to investigate the 1.1Hz HAM3-ISI line, following up on Jim and Hugh's effort. I am keeping track of the tests done so far in this document. One of the ways to remove the peak is to turn off the Ry isolation loop, at the expense of low-frequency horizontal motion, see the X motion before vs after, or Ry motion before vs after.
With low enough useism, the ifo was locked in this configuration to test if this would change the 1Hz ASC behavior. The two problems look independent since we found ASC to be still marginally stable around 1Hz but with no peak present on the ISI, see attached screenshot.
Will keep investigating.
This morning, I was looking at some other stuff on HAM3 and trying to clean up SDF, when I must have misclicked and turned the gain back to 1 on the RY loop. Because Arnaud had disengaged the boost, the ISI stayed isolated, but it glitched some of the IFO signals. It also made an immediate improvement in the PR recycling gain, making it much quieter, but this brought back the 1.1hz feature on the ISI. The take away is that leaving the RY loop off is good for PRCL, bad for the PR gain (when the soft loops are off), but this may only be true because the microseism is low right now.
First attached image are timeseries of PRCL, POPAIR_B_LF, the HAM3 RY loop gain and LSC_PR_GAIN. The glitch in the middle of each window is me accidentally turning on the RY loop, PRCL immediately gets noisy, but POPAIR_B gets quiet.
Second plot is asds comparing before and after. The dashed lines are the HAM3 RY gs13s, blue and pink are POPAIR_B, green and light blue are PRCL. With the RY loop off, POPAIR_B is dominated by the extra .05-.3hz extra motion in HAM3 RY. With the loop on, this extra motion goes away, but PRCL is dominated by the 1.1hz peak.
