Takano, Komori

We estimated the calibration factor of OMC-LSC_ERR_IN1 [cnt/m] using two methods.
These two methods show inconsistent results, but we selected the second method, which seems more robust, and estimated the factor to be 7.8e9 [cnt/m].

The first method involves measuring the transfer function from OMC-PZT_HV1_OUT to OMC-LSC_ERR_IN1 and dividing it by the PZT actuator efficiency, as described in the parent post.
Considering the PZT driver transfer function shown in Fig. 1, the DC gain from OMC-PZT_HV1_OUT to OMC-LSC_ERR_IN1 can be calculated to be -81 dB + 63 dB = -18 dB, based on the measured gain above 100 Hz.
Since the PZT actuator efficiency is 1.3e-10 [m/cnt], the error signal calibration factor is estimated to be 9.7e8 [cnt/m].

However, this factor is not consistent with that estimated by the following method.
We compared the time series data of OMC-TRANS_DC_SUM_OUT and OMC-LSC_ERR_IN1 during the shaker injection in klog:30475.
Figure 2 shows clear linear oscillation in LSC_ERR and quadratic oscillation in TRANS_DC.
The OMC finesse, measured to be ~810 in klog:20764, allows us to estimate the oscillation amplitude to be 6.0e-12 m from TRANS_DC.
This corresponds to an amplitude of 0.047 cnt in LSC_ERR.
Therefore, the calibration factor should be 7.8e9 [cnt/m].

The second method seems more robust because we might not consider additional factors between OMC-PZT_HV1_OUT and OMC-LSC_ERR_IN1 in the first method.
We adopt the LSC_ERR calibration factor of 7.8e9 [cnt/m], but cross-checking would be highly welcomed.