A question is if the optical gain for differential signals created inside the CITF is different from the DARM optical gain as a function of SR alignment. The 56MHz optical gain which involves being recycled by SR is not affected by the SR misaligment, because the ~1.5urad SR misalignment is small compared to the ~12m length of the CITF, and the CITF is marginally stable. Is it the same for differential perturbations created by the BS?
Figure 1 shows a lock acqusition, with SR aligned at the beginning (pole at 400Hz) and then 10 minutes later misaligned (pole at 200Hz)
Figure 2 shows three times, 21:10 UTC with SR still aligned (purple), 21:20 UTC with SR misaligned (red), 21:30 UTC with SR aligned (blue)
Figure 3 zooms on the BS drum mode (1872Hz VIR-0476A-16 https://tds.virgo-gw.eu/?content=3&r=12696). It is decreasing over the 10 minutes of SR misalignment, but then in the following 10 minutes is decreases again by the same fraction in h(t). Hence this decrease is not a change in optical gain, but just the drum mode damping. Meanwhile on B1 the first step is clearly larger, as the optical gain at high frequency changes due to the SR misalignment. So it seems the optical gain evolves the same way as the DARM gain. And there is no strange effect that the SR recycling work differently for perturbations created in the CITF and perturbations created in the arms.