K. Venkateswara

This morning I noticed that the BRS was rung up to large amplitudes (500 microrad), so I looked at the data from yesterday and noticed two main incidents/causes: Borja and Sheila were near the BRS at ~ 3:30 PM but that disturbance was small and did not drive it up the resonance much. The ASD and time series of the first disturbance is shown in the first attached file.

The second disturbance at ~5 PM was much bigger and really drove up the resonane of the bar from 50 nrad to 500 nrad as shown in the second file (the y-axis scale in the time series should be in nrad not rad). I'm not sure who this was yet. The gravity gradient from a person standing next to the balance is very large and can easily drive up (or down) the resonance amplitude to microrads. As the decay time is ~two days due to the high Q, it will take a long time for the bar to return to normal amplitudes. Furthermore, due to the non-linearities of the autocollimator, the harmonics of the fundamental become visible and destroy coherence with the seismometer as visible in the third file.

This is a known problem due to the open loop operation of the BRS and we hope to address it very soon with some simple feedback/damping loop. In the meantime, I would like to ask that people stay away from the BRS unless necessary. Gravity gradient torque falls off as distance^3, so even an extra  distance of 2-3 meters is sufficient to keep the influence small. Grvaity gradient torques are also filtered by the resonance of the balance, so if you move by it fast, that is generally okay.

When it does get rung up, it is kind of fun to drive it back down using your gravity :)