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Can you please explain how the seismic noise coupling is estimated for the noise budget?
Since the cavities are now on the same stack it seems tricky. I guess that some stack tilts produce differential vertical accelerations on the cavities. How much of the noise below 100 Hz is from scattered light?
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The estimated seismic noise in the plot uses the coupling calculated by COMSOL model, see PSL:1060 and PSLL1065. It is the coupling between acceleration to the cavity displacement noise. It is just an upper bound of the seismic noise, no common mode rejection is used in the calculation. (I have to check if the seismic noise data in the noise budget is from floated or unfloated table). So far, only displacement noise from vertical seismic motion is calculated in the noise budget.
I'm certain that the noise bump below 100 Hz is mostly scattered light induced by any vibration on the chamber. The reasons are:
- The behavior of the noise. It slouches back and forth when I slightly tap on the vacuum chamber.
- We have seen this before and after the table was floated. See the measurement from unfloated table, psl:878, and floated tablePSL:966. The hugh bump from DC to 100Hz disappears and the mehcanical peaks around 20-50 Hz show up after being buried by the big bump. The causes of these mechanical peaks are not known. I never found out where they came from. I thought they might be some modes of the seismic stacks, but the previous measurements do not have the peaks around 20-50Hz. See PSL:1314.
So I don't think we really hit the actual seismic-driven displacement noise yet ,and what we see is mostly from seismic-driven scattered light which I don't know how to calculate. I ordered a new set of table leg to replace the current ones that leak. They should be here next month.
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