K. Venkateswara

After the changes to the way BRS is used at ETMX, detailed in 15497, tilt-subtraction shows significant improvement. Attached files show ASD plots from 40k seconds of data from last night, when winds were barely a few mph at ETMX.

The first plot is in angle units.  The blue line shows the ground seismometer output, the green shows the raw BRS output and the red curve shows the new tilt-subtracted super-sensor output. The roll-off at 5 mHz is due to the high-pass filter but most of the other difference is from the new tilt-subtraction. This is also clear in the coherence plot shown below - the coherence between the super-sensor and the T240 is less than the coherence between the T240 and BRS. The BRS ref is shown in cyan.

The tilt-subtracted super-sensor is tilt-free and reliable till ~30 mHz or so. Why is this important? For one thing, it can be used to do sensor-correction to Stage 1 of the BSC-ISI, while keeping tilt-reinjection at a minimum. This has the effect of reducing the microseism, which is a big problem at LLO and will be a problem here as well. This was shown recently in 15498.