A BruCo scan for the period listed in 45013 is available here. There are a lot of interesting coherences in the low frequency region (10-30 Hz), some of them already identified and corrected (see 45036). None of them is near one, but they are all not neglegible and very likely enough to explain most of the noise when summed:
- SRCL (figure 1)
- SRC2_P (figure 2)
- CHARD_Y (figure 3)
- MICH (figure 4)
A couple of channels are intriguing, and I don't have a good explanation for them
- ASC-Y_TR_A_NSUM (figure 5)
- POP_A_LF_OUT (figure 6): shows a straight line contribution which seems very close to 1/f^4, and it runs parallel to the measured sensitivity... It would be interesting to check if this coherence is constant over time or non-stationary
- SUS-IM1_M1_VOLTMON_LL_OUT_DQ: (figure 7) between 20 and 70 Hz this channels shows a coherence which puts is very close to the sensitivity. What is this? Some input beam jitter maybe? Interesting, only LL shows coherence, while LR, UR and UL do not show any coherence at all (figure 8 is an example). I wonder if those channels are even working...
Follow up on IM1 coherence.
Looking at the IM1 VOLTMON signals ('H1:SUS-IM1_M1_VOLTMON_LR_OUT_DQ', 'H1:SUS-IM1_M1_VOLTMON_UR_OUT_DQ', 'H1:SUS-IM1_M1_VOLTMON_LL_OUT_DQ', 'H1:SUS-IM1_M1_VOLTMON_UL_OUT_DQ'), the four channels appear very different. In particular, LL has large glitches, and LR has somewhat smaller glitches, while UL and UR looks stationary. The plot below shows the spectra and time series of the four channels. Solid lines in the top panel are spectra computed with the entire 600s period, while the dashed lines are spectra computed only selecting the shaded time range in the bottom plot, where there are no big LL glitches (but probably still some small or medium size glitches).
The coherence with DARM is there only if we include the glitches. So it's worth investigating this channel to se what's going on there. Maybe it doesn't explain the noise in DARM (and the coherence is an over estimate), or maybe it can explain some of the DARM glitches.
Looking more closely to the LL glitches, here's a spectrogram of DARM and IM1 LL. There are a few glitches in both channels, but only one DARM glitch seems to line up with a IM1 glitch. However, if you zoom in, it looks like the IM1 glitch happened a second before the DARM glitch. So the coherence might just be a coincidence. I think it's still worth investigating the IM1 coil drivers.
Thanks Gabriele.
The coherence with POP LF was gone last in last night's lock, attached is a coherecnce taken at 15:33 UTC Nov 6th. This could have coupled through the one of the ASC loops (CHARD Y or SRC2P) that we rolled off better yesterday.
