Michael, Krishna, Jim, Hugh

Here's a brief look at the performance of the BRSY sensor correction during Wednesday's ~20-40 mph winds. We were using 250 mHz blends and Warn_SC_2 sensor correction filters for Y axis (the beam axis). The interferometer was able to lock well as noted in 27658.

The first plot in the attached pdf shows an ASD of the ground seismometer Y, the BRS rX, the tilt-subtracted seismometer Y and the ST1 seismometer Y signals. The tilt-subtraction worked well and kept ST1-Y motion small even as isolation was provided above 0.1 Hz from the sensor correction filter. Note that ST1 still sees the ground tilt and the seismometer output is contaminated by tilt, hence the effect of the isolation is not clear in the ST1 Y channel but we expect it to be there. Ideally one would like to measure the tilt going into ST1 from the ground and cancel that (rX sensor correction), but as Robert showed in his alog a few weeks ago, ground tilt varies significantly on the LVEA floor and one would need multiple angle sensors to map it well or one (good) angle sensor on Stage 1.

The second plot shows the coherence between the ground seismometer Y and BRSY rX and that between the tilt-subtracted Y and BRSY rX. The high coherence in the first is good, but the high coherence in the second indicates that the tilt-subtraction is not optimal yet and improvements of ~2 could be had at low frequencies.

The third plot shows has the same lines from the first, along with the buried seismometer Y for comparison. One can see that the tilt-subtraction is more effective and is also more useful above 0.1 Hz.

The fourth plot has two additional lines - ST1 CPS and it's RMS. The RMS motion was kept below 1 micron but had an odd bump at 8 mHz. Jim and I tried to add a notch to the sensor correction at 8 mHz but were unsuccessful in suppressing it well.

This morning I modified the high pass filters for the tilt-subtracted seismometer channels as shown in the second attachment. The new (red) filters suppress signals below ~20 mHz with some gain-peaking at 40 mHz (which is acceptable) and decent phase above ~60 mHz. These seem to work well and keep the CPS signals small. They are installed at both ETMX and ETMY. The third attachment shows the sensor correction with this new filter . The wind-speeds were only ~10 mph but the CPS signal is smaller and doesn't show the ~8 mHz peak.