Continuing my efforts to check and update the LSC coupling (see 86423 and 86370) I ran yet another bruco, which showed that now the dominant low frequency coherence is coming from SRCL.
In review, I have:
- updated PRCL feedforward, which has greatly reduced the remaining coherence
- updated MICH feedforward, again reducing coherence
- and I did a minimal update to SRCL, which has not really done much (see linked SRCL coherence above)
I think we can do better. The main challenge is fitting the <100 Hz coupling while fitting the >100 Hz coupling, or at least not significantly worsening it. I think the fit would be easier if we made use of the parallel SRCLFF banks to fit a low and high frequency feedforward. This would require fitting the low frequency coupling, engaging it, and then remeasuring the high frequency coupling to fit separately.
I have what I think is a better fit to the low frequency coupling, currently saved in FM5 of the SRCLFF1 bank. To test this new filter:
- take SRCL1FF gain to zero
- turn off FM6 (current) and turn on FM5
- ramp SRCL1FF gain back to 1
- run SRCL injection template and compare with the reference (best practice would be getting a same day injection first on FM6 to compare with)
- use SRCL_excitation_20240824.xml saved in /opt/rtcds/userapps/release/lsc/h1/scripts/feedforward/
- if the filter performs better, change the guardian code in LOWNOISE_LENGTH_CONTROL to use FM5 and update SDF
Time permitting, if the new filter works, taking a better measurementing of the coupling from 100 Hz and up while on the new filter would be a useful next step, which may require adjusting the excitation shape or increasing the excitation gain.
To motivate the use of commissioning time on this, I did a very simple coherence based subtraction of the SRCL noise to see what it could get us in range improvement. I used the same time as the bruco I linked above, so this time includes the PRCL and MICH improvements. I copied some of Oli's range compare code to help me generate this nice plot comparing 1 hour of strain with coherence subtraction of the SRCL noise. Even if we can't improve the feedforward above 100 Hz, there is possible a 2 Mpc improvement from below 100 Hz.
