Coherences

The summary of all coherences can be found here:

https://ldas-jobs.ligo.caltech.edu/~gabriele.vajente/bruco_1111224016

Here is my personal summary: still a lot of coherence with MICH/SRLC/PRCL, angular motion at low frequency, PSL periscope

• low frequency power fluctuations (OMC flicker) are coherent with the power before the OMC, so they are likely generated inside the IFO and not by some OMC control. Moreover, there is high coherence with both PIT and YAW RF45 signals at the AS port, but this might be simply due to power fluctuations coupling to angular signals due to miscentering on the WFS. Coherence is good with REFL WFS too.
• Noise between 5 Hz and 20 Hz shows some coherence with HAM1 signals: HPI-HAM1_BLND_L4C_Z_IN1. There is a similar amount of coherence with LVEA seismometer signal: PEM-CS_ACC_LVEAFLOOR_HAM1_Z. HAM1 shows some coherence too.
• Between 1 Hz and 20+ Hz, many WFS signals are coherent with DARM: mainly AS_RF45, but also REFL_RF9. I can imagine two possible explanations
  1. DARM correction leaks into the angular signal due to miscentered WFS or misbalanced actuation
  2. Angular control pollutes DARM due to misbalanced actuation
• There is coherence with ITMY control, which I believe is due to the MICH feedforward. Indeed, the coherence of DARM with ITMY is the same as DARM with MICH. This means that the MICh feedforward can still be improved.
• In the 30-200 Hz region coherence with PRCL/SRCL is quite large. Maybe we need some more feedforward paths.
• 57 Hz and 74.8 Hz are coherent with magnetometers: PEM-EX_MAG_EBAY_SUSRACK_Z_DQ and SEIRACK. The harmonics are there too.
• The noise bumps at ~75-83 Hz and ~98 Hz are coherent with ASC-Y_TR_B_PIT 
• Peaks from the PSL periscope are highly coherent in the region between 180 and 380 Hz. See for example PEM-CS_ACC_PSL_PERISCOPE_X, PEM-CS_ACC_PSL_TABLE1_Z and all IMC signals
• The region between 460 and 520 Hz is completely coherent with ITMX L2 control
• At high frequency (>100 Hz) there is still broadband coherence with a lot of RF signals in REFL and POP. This is likely related to the IMC longitudinal offset issue (to be confirmed and possibly fixed)
• There is some coherence above 100 Hz with intensity noise. The projection is still a factor of few below the DARM noise floor, but not too far away. It should be easy to improve intensity noise by tuning the IMC alignment
• Around 970 Hz there is large coherence with PRCL

In general, there are many frequency bands where there is coherence with signals, so it's worth scanning though the main table to see if yopu favorite bump is coherent with something. The list would be too long to be included here.