As a follow-up to Hartmut's question about the DARM rms, here are spectra of the in- and out-of-loop DARM sensors before and after transitioning to DC readout (at 2 W input power). The spectra are calibrated into RIN.

The left-hand plots show the sensors when DARM is controlled via AS45Q. Above 10 Hz or so, we see the sensing noise of AS45Q impressed onto the DARM loop. From 2 Hz to 8 Hz or so, and below 0.5 Hz, both sensors appear to see some DARM displacement noise. Between 0.5 Hz and 2 Hz, the sensors are not seeing the same signal. If the AS45Q signal in this region is indeed DARM displacement noise (rather than some kind of loop-suppressed sensing noise), this would seem to indicate that the sensing noise of the DC readout chain is higher than the intrinsic DARM displacement noise by a factor of a few in this region.

The right-hand plots show the sensors when DARM is controlled via DCPD sum. During the transition, an additional boost is engaged (two complex poles at 2 Hz, two complex zeros at 8 Hz), so this makes it tricky to directly compare these plots with the left-hand plots. Probably it is best to try to get an equivalent set of plots for the intermediate case (DARM controlled by AS45Q, boost engaged).