Michael, Krishna

Right on cue, winds picked up today and we got a good chance to test out both BRS-2 at EY and BRS-1 at EX. The wind speeds were in the 20-30 mph range during the following measurements.

EY: The first pdf shows the ASD of Ground seismometer (STS-2) and BRS-2 data and the online tilt-subtracted Y super-sensor data. The magnitude of Y ground motion at ~ 50 mHz is 10 microns/rt(Hz) while the tilt-subtracted Y channel is at ~1 micron/rt(Hz). The second page shows three lines - the first is the coherence between the ground seismometer and BRS-2 showing ~99% coherence in 10-100 mHz region. The second is the coherence between BRS-2 and Stage 1 Y and the third is between Stage 1 rX and BRS-2, showing significant coherences between them.

With BRS-2, we modified the damping scheme to have the damping set at Q of ~50 in the quiet state and Q of ~5 if driven up to a very large amplitude state (>5000 counts). We expect that we won't reach this large amplitude state under wind-speeds of < ~60 mph. The Q=50 state seems to not add any additional noise so far. We will do a more detailed analysis on this soon.

EX: The second pdf is for BRS-1, the ground STS-2 and a similar tilt-subtracted X super-sensor. The magnitude of Y ground motion at ~ 50 mHz is ~4 microns/rt(Hz) while the tilt-subtracted X channel is at ~0.7 micron/rt(Hz). On the second page,  the coherence between the ground seismometer and BRS-2 is ~96% in 10-100 mHz region. The coherences between BRS-2 and Stage 1 X and Stage 1 rY is smaller, especially that between rY and BRS-1 near the microseism. This last part is worrisome and I suspect that the rY sensor may have some problems - (I've noted this in the past - see 14426). Jim has promised to help look into this more by designing some high-frequency CPS-only blends for Stage 1. The idea is that by locking the platform as rigidly as possible to the ground, we can calibrate  the on-board rY sensor by comparing it to the ground (match T240 scale-factors). This may also prove useful for all other chambers which can be roughly calibrated by ensuring that the rY signal is small under quiet conditions.

The second minor issue with the EX data is that the coherence of BRS-1 with the ground seismometer data, and hence the subtraction is not as good as it was with the T240. Jeff tells me that this was the least reliable seismometer they had and will be replaced with a T240 very soon. A third issue is that the ~35 mph wind-gusts occasionally triggered the damper at EX. We will increase the large amplitude threshold for this damper, soon.

Summary: The tilt-subtraction at EX and EY is working reasonably well.