Long day, short alog

We continued to try to lock PRMI today. After consulting with Sheila, we moved IMs1-3 back to the alignment they had during the arm peek last week. Ryan S has noted the osem values for each alignment, so we can switch back and forth between these two configurations. Then, Louis and I reverted IM4, PR2, PRM, SRM, SR2, BS to their alignments during a DRMI lock last week. I also moved SR3 to its current alignment (TCS team had moved it back to old O4 alignment for HWS work).

This alignment was very bad for flashes, so I iterated between initial alignment work on PRX and MICH, and running a PRMI lock. We found that it was very difficult to lock PRMI, both because the alignment was poor and the beamsplitter was moving a lot. Once we turned back on BS oplev damping, PRMI locking was more frequent due to less BS motion, allowing us to better touch up the alignment. However, there was then a large 6 Hz oscillation. After checking many things, it turns out the oplev damping is causing this oscillation, both PRCL and MICH are stable. So, we need oplev damping to lock but we can't keep it on or we will unlock.

After talking with Sheila again, I moved to try commissioning the BS M3 feedback as a way to help this problem. Louis and I tried measuring the beamsplitter plant with the offloaded feedback to M2 and M1 and then M3 and M1. We found that we needed about a gain of -300 on M3 to replicate the M2 plant. However, we could not lock MICH dark with this feedback.

We tried many different things after this which I won't detail, but Keita and I performed a different type of measurement that confirmed the work Louis and I did earlier was correct. When we locked again, we saw that the issue is that the DAC rails. Looking at the master outs on M3, the saturation occurs below 10 Hz. I see that we should replicate the LLO feedback that is distributed between M2 and M3 with a crossover of 7 Hz. See 82200. I will pick this up tomorrow.

Editing to add spectrum of BS M3 master out with length feedback applied and the appropriate gains. I put a horizontal line roughly at 134e6 DAC counts, which is where we expect the saturation to occur on our 28 bit DACs. A crossover of 7 Hz as LLO has would work well. Louis is copying their filters over now.