While TJ and Camilla finished initial alignment I looked at the coherence between intensity noise (seen by ISS array and IM4 trans) with the ISS on and off. The 1st attachment plot shows that there is not coherence between IM4 trans and HAM2 GS13s until the second loop is on, meaning that the clipping problem is in the path to the ISS array.
We went to 30W input power, waiting for the IMC WFS to settle (the beam moved on the ISS 2nd loop QPD while this was happening) and then I pico'd using PM1 while the ISS was open. I was able to reduce the peaks until I could no longer see them in the ISS array spectra this way, but when I closed the ISS 2nd loop the peaks became visible again (and the coherence between the GS13 and the ISS out of loop signal was near 1). I continued to pico in the yaw direction until the ISS QPD reached -0.66 Y, and the coherence and peaks were again reduced. The third attachment shows the change to the pico motors and the ISS 2nd loop QPD. It looks suprisingly similar to the move that Jenne made, 52815.
The 4th attachment shows the coherence between HAM2 and ISS and DARM in last night's lock, which is lower than the coherence before Jenne's pico'ing 52815 but still pretty high.
Edit: the coherence between DARM and the ISS at 380Hz seems to be gone in this lock, last attachment here, reference is in last night's lock, live traces are after pico'ing.
This 400Hz-ish thing is mostly the yaw coupling. Look at coherence between 2nd loop QPD YAW HAM2 GS13 (1st plot, this is right now in NLN).
Since we don't see this in DARM probably this doesn't matter that much, but with the current input alignment, inner loop is more sensitive to the YAW beam motion than the outer loop. Look at the 2nd attachment where Sheila was pico-ing things. Inner- and outer-loop sum are normalized at an arbitrary time at around t=-7000 to make the plot easier to understand. In the right is the plot of the 2nd loop sensor sum VS 2nd loop QPD YAW. It seems like the inner loop is about a factor of 1.7 or so more sensitive to YAW motion than outer sensor.
We might be able to slightly improve the YAW to intensity coupling using the outer loop sensor.
