After yesterday and today recentering and tweaking, the ISS second loop is back to good working condition. The first attached plot compares some signal with the second loop open and closed. Dashed line, loop open, solid lines, loop closed.
The ISS second loop signals (SUM14 and SUM58) are calibrated in volts and the whitening filter is compensated. Using the slow DC channels, I could compute the total signal for each combination, which is about 11-12 V. I used those values to calibrate the signals in terms of RIN as in the figure. This should roughly correspond to about 20 mW total power hitting the array.
The out-of-loop signal is still showing some excess noise with respect to a flat level. Assuming that there are 10 mW impinging on the four out-of-loop diodes in total, the shot-noise-limited dP/P should be sqrt(2*nu*hplanck/P) = 6e-9 /rHz. Counting both the in-loop and out-of-loop shot noise, we get an additional sqrt(2). Therefore we expect the out-of-loop signal to be at the level of 8.6e-9 /rHz, not too far from the 1e-8 /rHz that we see at about 100 Hz.
The structures between 10 and 50 Hz are variable, but I was not able to change them significantly moving the IMC alignment or the picomotors in front of the ISS array. Notably, for most of the day there was an additional bump between 10 and 20 Hz, which disappeared some time at about 13.05 local time, without any conscious intervention on my side. See second plot. No idea what the origin of this thing is, but it was there also yesterday.
One more peculiar thing. We can use IM4_TRA_SUM to measure the RIN in transmission of the IMC, as an additional out-of-loop sensor. This is included in the attached plots. When the second loop is open, the RIN measured by IM4_TRANS agrees quite well with what is measured by the ISS second loop diodes. However, when the second loop is closed, IM4_TRA measures a significantly larger noise. This might be due to the sensor electronic or dark noise (I haven't checked it yet). However, at least at 10-30 Hz it measures the same structures visible in the ISS diodes, but a factor 4 larger. The shape is also slightly different. Surprinsingly, there is no coherence between IM4_TRANS and SUM58. This might tell us something about the origin of those peaks, but what, it's not clear.
The peaks between 15 and 25 Hz are visible, with different amplitudes and shapes, in all the eight photodiodes, but there is little coherence among them. See third plot. These peaks are not visible in the ISS QPD signals, even though there is a wider bump at similar frequencies. They may be due to scattered light. The most prominent structure is a 18 Hz peak.
