Lisa, Dan, Sheila, Evan, Kiwamu
We checked the evolution of the flickers in OMC TRANS over time. It appears that the features between 1-5Hz have always been there, but the amplitude has grown by a factor of ~3-6 in the past two weeks.
In the attached plot, the ampltiude of the 3Hz peak on Mar 6 was about 0.004 / rt[Hz], and today it is 0.02 / rt[Hz].
We've added rolloffs to the ASC loops, turned on the 'resg1' filter in the ETMY_M0_DAMP_L filter bank (this adds a RG at 1Hz and an overall gain of +6dB), and looked accusingly at the OMC TRANS camera image. No change so far. Next idea is to try a boost in the DARM loop gain at low frequency.
The cut offs we added are elliptical low passes at about a factor of 10 above the ugf of several of our ASC loops (INP1, PRC1, PRC2, SRC1, SRC2 pitch and yaw). These are engaged in the gaurdian now. We do not see anylarge changes in the noise, neither the low frequency noise that dominates the OMC DC PDs nor in the bucket. With all of these loops and CHARD (cETM) off we saw that the noise bellow 10 Hz was a little better. We attempted to add a ELP30 to MICH pitch, which broke the lock.
Also, for SUS SDF, we have added notch filters for the Bounce and roll mode to the ITM L2 stages, as well as compensation for the new ESD low passes in ETMY L3 ESDOUTF
The OMC transmitted power fluctuations at frequencies below 20 Hz is completely coherent with the power fluctuations before the OMC (see for example ASAIR_A_LF or ASAIR_B_LF), as shown by the attached plots taken from the brute force coherence report.
I believe this is telling us that the OMC flicker is due to power fluctuations inside the IFO, and not generated by the OMC control. Maybe improving the longitudinal and angular stability will help.
