M. Ball, S. Dwyer

During maintenance, we measured length drive to pitch and yaw transfer functions for our rerouted L2A control signal (LHO alog 51709). Earlier, we activated our L1 boost after rerouting the L2A signals and later added a 2Hz cutoff filter to the L2 length to L3 pitch decoupling filter, so we wanted to characterize this filter with the interferometer unlocked.

The first attached plot (Figure 1) shows the coherence between the drive signal in L2 and the pitch and yaw oplev signals in L3 before turning on our rerouted L2A filter (dashed lines), after rerouting (solid lines), and after loading a modified L2P filter (dotted lines). We see strong coherence between our drive signal and both pitch and yaw from 0.6 to 4Hz without our changes, and from 0.6 to nearly 5Hz with our changes.

The second attached plot (Figure 2) shows the transfer function magnitude and phase for L2 length to L3 pitch and yaw at the same three times. Again, dotted lines denote both rerouted L2A and the modified L2P filter, dashed lines denote no L2A filters, and solid lines denote just the rerouted L2A with no L2P cutoff. Between around 1.3 and 2.2Hz, the drive to pitch transfer function increases in magnitude after turning on the rerouted L2A then increases further after enabling the new L2P cutoff filter. However, above 3Hz, the transfer function with the new L2P cutoff filter drops off to below the rerouted case but above the original L2A case. This is the frequency range where we saw the impact on the ASC signals (LHO alog 51604). Between 0.6 and 1.5Hz, the drive to yaw transfer function also decreases after turning on the rerouted L2A and remains there after enabling the new L2P filter. Between 1.6 and 2.7Hz, the drive to yaw transfer function increases when turning on the rerouted L2A filter and remains there after turning on the new L2P filter.