I ran my brute force coherence script for the almost 2 hours lock of two days ago (the coherence for last night 2+ hours is running, you guys are locking too often!). Basically this script computes the coherence of the LSC-DARM_IN1 signal with *ALL* available channels. Some channels are supposed to be coherent (for example DARM_OUT), so I have a list of excluded channels. For the moment being I used the same list as for LLO, then I'll see if I need to fine tune it.

The full report is available here:

https://ldas-jobs.ligo.caltech.edu/~gabriele.vajente/bruco_1107680416/

Some preliminary comments:

• noise below ~40 Hz shows a lot of coherence with LSC-ASAIR_A_LF_OUT and other DC channels at AS. I guess this is normal, since these channels are sensing the main beam before the OMC, and the DARM signal is read after the OMC. It is just abuit strange that the power before and after the OMC is fluctuating in a coherent way, but this might simply mean that the low frequency DARM is very noisy and the TEM00 fluctuations dominate over the HOMs. This hypotesis seems to be confirmed by the coherence with signals like SUS-ETMX_L1_WIT_L and similar
• noise below 100 Hz is very coherent with SUS-ETMX_L2_OLDAMP_P_OUT. If I interpreter correctly this name, it should be an optical level damping on ETMX. This coherence is basically enough to explain all the noise up to 50 Hz and most of the noise up to 100 Hz
• In the low frequencies (below 40 Hz) there is coherence with many angular signals at AS port, but interestingly the coherences with those channels have a very similar shape of the coherence with LSC-POPAIR_A_LF. I can't be sure, but maybe this is an indication of large RIN at low frequency?
• Between 40 ans 200 Hz there is some coherence with the MICH control, for example LSC-MICH_IN1_DQ. In the same region (50-300 Hz) there is even more coherence with SRCL control: LSC-SRCL_OUT.
• Above 100 hz there is significant amount of coherence with ISS signals: PSL-ISS_SECONDLOOP_SUM14_REL_OUT

Basically, so far there is nothing new with respect to what Dan reported.

The attached plots are taken direclty from the report, and they show the coherence with some of the channels I discussed above. The top panel shows the coherence, in log scale for both x and y axis. The dashed red line in this panel corresponds to the level of coherence you expect for completely uncorrelated signals, given the resolution and the number of averages. Any coherence at this level or below is completely accidental. The bottom panel shows the spectrum of DARM signal and the noise projection for the auxisliary channel, based on the coherence. It gives a good indication of how significant the coherence is, but of course must be taken "cum grano salis".