Input jitter noise is more coherent with DARM now than it was immediately after the replacement of ITMY, even in bands where the jitter itself is lower (Figure 1). Figure 2 compares predicted levels of jitter noise in DARM from then and now; the jitter noise in DARM is currently predicted to be right at the current noise floor in the 100-200 Hz, and several other regions. Figure 3 shows the May 2021 and current coupling functions, which are independent of the drive (the vibration level in the PSL). The current coupling function is about 5 times worse than that of May, 2021, which was when we highlighted the factor of about ten reduction from O3 levels associated with replacement of the test mass (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=58976 ).

To help reduce jitter coupling, I suggest that we inject a  comb onto H1:IMC-PZT_YAW_EXC when we are tuning things that might affect jitter coupling, like TCS settings and beamspot positions, so that the amplitude of the injected lines can act as an additional figure of merit.  A 35 Hz comb hits the worst coupling regions – I used awggui with a 35 Hz ramp waveform and an amplitude of 100 (GUI screenshot on second page of Fig. 3).