[Jennie, Sheila, Gabriele]

Low frequency OMC alignement lines were injected between 20:19 UTC and 21:16 UTC, following a similar method to what described in 76462.

Using the code attached, I looked at how the noise in DARM and the amplitude of the 410.x Hz calibration line changes with respect to the OMC QPD signals.

One can see from a spectrogram that the noise in GDS-CALIB_STRAIN_NOLINES is strongly modulated by the OMC angular lines. The spectrogram shows GDS-CALIB_STRAIN_NOLINES normalized to the median over time.

Then one can compute BLRMS as several bands of GDS-CALIB_STRAIN_NOLINES to see how the noise changes, and of OMC-DCPD_SUM at 410-411 Hz to monitor the amplitude of the DARM calibration line, that is a proxy to optical gain. Time series are shown in the attached figure, where the OMC-DCPD_SUM BLRMS time series are comapred with the OMC QPD signals.

Finally, I plotted the 410-411 Hz BLRMS of OMC-DCPD_SUM (optical gain) in a scatter plot vs all four QPD. This allows me to select a value of the QPD signals that maximize the optical gain. The negative of those values is the offset that should be added to the OMC QPD on top of the existing offset.

CAVEAT: However, looking at the noise in GDS-CALIB_STRAIN_NOLINES between 800 and 900 Hz (that should be shot noise) one see that it is minimum for the current OMC offsets. Maybe this is an indication that the SQZ is aligned with the current OMC, but the current OMC is not well aligned to the IFO. I suspect that using the offsets above will increase the optical gain, but make SQZ worse