Following 24177. It seems pitch jitter of the input beam would contribute to DARM now.
To check the assumption that the angular jitter of the light beam incident on the main interferometer would become noise in DARM somehow, I projected
K1:IMC-IMMT1_TRANS_QPDA1_DC_PIT_OUT_DQ onto K1:CAL-CS_PROC_DARM_DISPLACEMENT_DQ. Firstly I tried to use pastavi noise budget mode but failed somehow (TF xml file was not reloaded...), so I just did it by hand.
To estimate the transfer function from the beam jitter to DARM, I took advantage of the data when MCO to DARM transfer function was measured on 2023 Feb 14 JST. More specifically, ...
Measurements/NoiseBudget/TFs/2023/0214/MCO_P_256.xml, where someone (Tamaki-kun?) shook MCO in pitch to measure transfer function from it to DARM. Fortunately IMMT1T QPDs were also recorded simultaneously in the same xml file, it is possible to re-construct the transfer function from IMMT1T pitch to DARM while shaking MCO in pitch, which would correspond to the situation that a certain level of beam jitter is being injected.
As far as my rough check, somehow the coherence of this transfer function seemed acceptable (Fig. 1), and Fig. 2 is the transfer fucntion; points are measured data, while the curve is a very rough fit: -4*f^2.
Now I obtain the fit transfer function. Then I used DARM data and IMMT1T pitch data recorded this morning, and made a projection; see Fig. 3. Most of the peaks seemed well explained. Due to maybe ADC noise or dark noise of the IMMT1T QPD, other than such peak structures are just flat. If a whitening filter can be applied to these QPDs, more interesting results might be obtained. For reference, the corresponding screen shot of diaggui is shown in Fig. 4.
So is the principle sculptor the input beam jitter? But where?
Anyway, hopefully do the same measurement again by careful persons to confirm it. There might be my mistake that the coherence of the transfer function measured would not be due to the injected beam jitter, but due to the originally existing peak structures both in DARM and IMMT1T P. In fact, to see such possiblity, I overplot measured/fit transfer function on Fig. 3 with arbitrary scaled.