Gabriele, Jim, Arnaud

We have been looking for the 1.23Hz line seen largely in DARM and other loops (73622). We think this is coming from the ETMX ISI, which shows a strong line at the same frequency mostly in the Y and RZ dofs. The summary pages from today show the evidence of this problem, as well as the ndscope attached in fig1. Trending back the signal, this issue started around October 01 at 05am PDT, see bottom plot on fig2. Spectrum of all individual sensors (in loop) before and after are attached. Some outliers in the stage 1 sensors (H1 L4C, Y1, Y2 T240 ...) but hard to tell what's the problem without taking the ISI down. More to follow at next opportunity (maintenance, or ifo down).