J. Kissel

While gathering data of the ground motion from Thursday's windy afternoon, I found that the calibration filter used to turn the ground inertial sensor into (asymptoting to) 1 [nm/s] (at high-frequency) for the new, temporary, T240 at End X (see LHO aLOGs 10594, 9758, and D1400077) was incorrect. The "Cal" filters in FM1 of the H1:ISI-ETMX_ST1_GNDSTSINF_[A,B,C]_[X,Y,Z] banks were ON and still calibrating for an STS2 read-out chain, i.e.
1 / (1500 [V / (m/s)] * 40 [V/V] * 2^16/40 [ct/V] * 1e-9 [(m/s) / (nm/s)]) = 10.1725 [(m/s) / ct]
These filters were most likely installed and turned on by some script that was preparing for the permanent solution; not surprising.

I've installed a new filter in FM2 of all nine banks, called "T240Cal" with the following gain:
1 / (1200 [V / (m/s)] * 2 [V/V] * 2^16/40 [ct/V] * 1e-9 [(m/s) / (nm/s)]) = 254.3132 [(m/s) / ct]
where I've assumed that the Trillium Interface Chassis is configured in its low-gain mode, with a gain of 2.0. 

I attach a spectra of data in the past, comparing all four ground inertial sensors, with the ETMX GND sensor corrected for the factor of 
(254.3132 [T240] / 10.1725 [STS]) = 25.0 [T240/STS]
difference in gain between the STS and T240 calibration. Where we expect the ground motion to be coherent, namely between 0.1 and 0.5 [Hz], the ETMX sensor matches all three corner station's sensors exquisitely, and the micro seismic peak is roughly 1e-6 (m/s)/rtHz as expected.

As of now, until the T240 is replaced with an STS2 as designed, the ETMX_ST1_GNDSTSINF_[A,B,C]_[X,Y,Z] filter banks should have FM2 ON and FM1 OFF. Once the T240 is replaced by an STS2, we can switch back to using FM1, and "T240Cal" should be removed from FM2 to avoid confusion.

All calibration values were obtained from D1001575.