Since yesterday we are observing an anomalous behaviour in some corrections signals: looking at an unlock, many of them are similar to the one in Figure 1, where the ITF simply drifts away. However, a weird thing noticeable is a big difference in the corrections of WE/NE, when the former has much higher values at low frequency (mode at ~0.7 Hz?).
Looking at the spectra in Figure 2 the difference is visible; it is also visible the opposite behaviour of the MAR corrections (NI noisier than WI), although in a higher frequency range.
Comparing the spectra with the ones from the other night (after the change in behaviour around midnight), we can see the corrections being much noisier below 10 Hz, I don't know if this can be attributed to the weather conditions alone. Some of the spectra show structures similar to scattered light, and we can see in the 6-10 Hz region the dithering lines being completely covered.
Looking at DARM itself, where those signals are demodulated from, in Figure 4 there are three subsets of data of the same 2 minutes I used for the previous plots: the noise at low frequency is very non stationary, and going towards the unlock it becomes much higher, spoiling most probably the dithering signals used for the alignment of the soft modes.
This was observed also yesterday, when unrealistically high signals were observed before an unlock; already yesterday Paolo put in place a clipping for the signals used by the alignment loops, but this cannot fix the quality of the signals.
