Sheila, Ronaldas

Sheila suggested to investigate bullseye sensor noise contribution to DARM. It is basically a coherence based projection of jitter from the bullseye sensor upstream of the PMC, which was contributing to the DARM noise from 30Hz-1 kHz in this lock stretch (4th of July).

Bullseye sensor readings seem to be coherent with DARM signal on YAW and PIT d.o.f., see 'bullseye_yaw_pit_coherence.jpg'.

This sensor measures jitter that we believe is driven by the turbulent flow of water over the crystals in the high power oscillator. The acoustic or PZT injections in the PSL which can reproduce the jitter peaks seen by sensors downstream of the PMC cannot explain the level of noise coupling that the coherence suggests for jitter sensors before the PMC. We are relying on the observed coherence between this sensor and DARM to estimate the noise contribution.  

Nonetheless we tried the following:
1) choose maximum coherence d.o.f. for bullseye sensor (either YAW or PIT)
2) if coherence with DARM is bigger than 0.01 for a frequency bin, include it as a noise
3) then the bullseye noise is simply sqrt(coherence) * DARM

Result can be seen in 'bullseye_sensor_coherence_noise.jpg'. This noise contributes quite a lot for f>100Hz and improves noise budget, see 'compact_nb.jpg' and 'full_nb.jpg' [cyan dots in the plot]. For a comparison, previous noise budget plot can be found here (previous noise budget).

While the result seems encouraging, we need to investigate it for other locks. This coherence projection was done for 4th of July, 2017 (GPS time: 1183190418, duration: 600s), before the earthquake. Note that we expect this noise source to be reduced for commissioning/O3 because the high power oscillator has been replaced with the 70W amplifier.