J. Kissel, K. Venkateswara

The output filter for the BRS included two zeroes at 8.8 mHz and two poles at 1 mHz to compensate for the real pole of the beam-balance and a zero due to the gravitational spring (proportional to d, distance between CoM and pivot). Based on the data during windy periods (see 13563 and 14422), it looks like d is roughly -35 +/- 5 microns, which corresponds to an imaginary zero at ~7.3 mHz. Since Foton doesn't allow imaginary poles, I put in a complex pole with Q of 3 as an approximation but this means that the output is going to be incorrect between 5-10 mHz.

With this new output filter in place, the tilt-subtraction is working well above ~ 20 mHz. The attached pdf shows the ASD for the ground seismometer (blue), the tilt-subtracted super-sensor (red) and the tilt-correction output (green). Wind-speeds were in the range of 20-30 mph during this measurement. Note that while the super-sensor is lowered by a factor of ~5 at 50 mHz, the subtraction near 10 mHz is limited by the approximation I made above. If we could add the imaginary pole in Foton, the subtraction would be better.