Summary: In switching QOSEM whitening gains from the 1st to 2nd modification, there has been a slight increase in noise in the P and Y DOFs, all others uneffected. Whitening gain should be increased again by a factor of 2-3x.

This is one in a series of posts investigating the excess noise seen on the BBSS M1 QOSEMs. In short, at both LHO and LLO, the noisefloor of the QOSEMs appears to be around 8pm/rtHz at 100Hz, quickly increasing to 50pm/rtHz by 1Hz; where we expect the noisefloor to be flat at ~5pm/rtHz across this bandwidth. See more details in LHO alog 91466. 

Below in Figure 1, we plot QOSEM readout spectrums when the sat amp was configured with the 1st and 2nd modifications of whitening gain, as described in LHO:91998. The 1st modification dataset was taken from T0:1470725000, during which BBSS M1 damping was on. The 2nd modification dataset was taken from T0:1474137198, during which BBSS M1 damping was off, explaining the additional peaks seen in 2nd modification dataset.

This shows that the reduced whitening gain has not significantly impacted the QOSEM noisefloor, and further rules it out as the source of the excess noise. There is a slight increase in noisefloor in the pitch and yaw DOFs in the 2nd modification whitening, so there is a case that whitening is insufficient and should be increasing by a small margin again, perhaps a factor of 2-3x or so.