About 1.5 hours after the last DAS step I have tried to lock in LN3, with a MICH offset to make the sideband balanced, and with the gain of the SR TY loop to reduce the DCP divided by a factor 2 to have a slowe motion.

Figure 1 shows that the reduced SR TY gain did reduce the speed of the SR misalignment, but B1s still increased and crossed the 42mW threshold, it only reduced the speed of the increase. This would disprove the theory that the issue is a gain of a loop being too low, the issue is somewhere else.

Figure 2. Comparing to last night, when the power on B1s was slightly smaller during the peak, the peak occurs at the same relative SR misalignment of slightly less than 1urad. 

Figure 3 shows the B1s image during the peak, Figure 4 a few minutes after (with SR more misaligned) and Figure 5 a few minutes before. The SR misalignment amplifies some spurious beam on the right side of the main one. That spurious beam is odd, and maybe teaching us something important about the interferometer.

We could remove B1s from the trigger of the fast shutter once we are in LN1. As in that case the B1 photodiodes are a better sensor of unlocks, and so is DARM, and the trigger on B1s is no longer critical as it is during the lock acqusition of the OMC itself.