Michael, Jim, Krishna
We had 20-40 mph winds for most of the day yesterday. In the evening we tested a couple of different ISI configurations at the Corner Station (ITMX, ITMY) and the End-Stations. We tested the following two configurations:
1. 90 mHz Blends on ST1 and no sensor correction (SC) on all test-mass chambers - The standard configuration used for most of O1 which works well in low microseism.
2. 250 mHz Blend and BRS SC on ST1 at the End-Stations and 90 mHz blends and no SC on ITMX, ITMY - The idea is to use the tilt-subtracted ground seismomter in feedforward more and rely less on the tilt-contaminated ST1 seismometer.
Quick Answer: The effect on some of the Interferometer channels we looked at was a small but uniform improvement of a factor of ~1.5 or so over the 90 mHz blend configuration. Near 0.1 Hz we were limited by ITMY chamber motion.
Details: The first attached pdf shows some of the Interferometer channels in Config 1 (Dashed) versus those in Config 2 (Solid) in units of ADC counts. The red line is equivalent to H1:DARM_CTRL_OUT_WHITEN_DQ which for some reason was not available today. The other three lines are some of the Angular Control Signals, all of which show small improvements. The second page shows the ASD of the wind-speed in both Configs showing that wind-speeds were comparable during the two measurements.
The second pdf shows data from some of the ISI local sensors. The first plot shows the ground motion near each chamber - note that the ITMY seismometer sits in the 'bier garten' which is far away from the walls of the building. Also shown are the tilt-subtracted super-sensor signals (dashed lines). It is worth noting that the secondary microseism becomes visible after tilt-subtraction and is consistent with what the ITMY seismometer measures. The next plot shows the CPS signals which at low frequencies, are representative of the motion of ST1. The combination (ETMY-ITMY)-(ETMX-ITMX) should roughly correlate with DARM_CTRL at low frequencies, since the translational ground motion is small below the microseism. The third plot shows the ST1 seismometer(T240) motion. In all these plots, notice that despite the apparently quieter ground motion, ITMY moves nearly as much as ETMY. This is also seen in the fourth plot - which shows the coherence of the DARM_CTRL with the various CPS sensors. ITMY shows significant coherence between 60 to 200 mHz.
I think this suggests that the ITMY seismometer may not be a good measure of the tilt of the ITMY chamber, which is closer to the walls of the building. Thus our assessment that the Corner tilts significantly less than the End Station may not be valid. But if ITMY seismometer acts as a nearly tilt-free seisometer we can use it for sensor correction just as we do at the End-Stations and gain another small factor of ~1.5-2 in low frequency DARM_CTRL.
