Edgard, Conor and Brian.

We are investigating the feasibility of implementing feedforward control from the suspension point longitudinal motion (as seen by the GS13s) to longitudinal drive at the Top Mass. The evidence suggests that it could reduce the coupling to DARM in the band from 0.1 to 0.3 Hz.

However, to do this, we need to understand and compensate for the DC cross coupling to pitch at the top mass.

Details can be found here: dcc.ligo.org/LIGO-G1800467


As a way to track the variability of the cross coupling we took a set of measured transfer functions from 2017 (thanks for putting these together!). A summary of results here:

1) The figure attached shows the top mass Length to top mass Pitch transfer function at different times during 2017. The two outliers (light blue and green traces) correspond to measurements taken in air. 

2) The discrepancy of the measurements could not be traced back to a simple (linear) relation with the vertical position of the top mass. 

3) We don't understand why the cross coupling is so much smaller in air than in vacuum, need to check for other suspensions.

The complete version of this log post can be found here: SEI alog 1316