On Tuesday we moved the spot in the SRC to one of the spots (-P, second column) that TJ listed in 77694 . This was partially motivated by some modeling that Alenna has done which suggests that our original (now possibly burned) spot in the OFI was below the center, so we might benefit from trying to move to a spot above the burned spot to be closer to center in the OFI. Jenne measured the spot position on SR2 and SRM in 788119, minimizing the coupling of angle dithers to DARM, rather than SRCL, which indicated that we actually moved lower on SRM. We'd like to check if we get the same result when measuring the A2L coefficient in using SRCL rather than DARM.
The second attached screenshot shows how our kappa C (normalized optical gain) has changed as we've moved in the SRC, with the inital recovery after the OFI damage shown at ~38 days, where the optical gain dropped to 96%, recovery to 98% with the large SR3 yaw move 35 days ago. Around -29 days ago, Jennie Wright updated the OMC QPD offsets to bring kappa c back to near 1, as she noted in 78143 this change in offsets wasn't due to the damage of the Faraday, we had run a similar QPD spot scan before the OFI incident which indicated that we would have gained optical gain by moving to these same offsets before the OFI was damaged. This implies that if we hadn't lost any optical gain due to the Faraday damage, we should now have 1.02 for kappa c with the optimized QPD offsets. In the last few days when SR3 was at the -P position from 77694, we had slightly more optical gain (1.01 kappa C, meaning 2% better throughput.).
In this -P position, the range was not as good as at the position we've been using since late April (+Y in 77694). The first attachment shows a low frequency noise comparison. Interestingly below 15Hz the noise was better in the -P alignment, but worse from 20-45Hz. Some of this extra noise is from LSC FF which seems to need to be retuned at this alignment. I did make an attempt yesterday to rescale the MICH FF, but this wasn't very sucsesfull as it seems that we would need to refit the filter to get good feedforward subtraction. The LSC coherence though wouldn't explain all of this excess noise, especially not the 25Hz peak.
Today, we decided to try the other pitch spot, to see if that would put us closer to the center of the OFI. TJ moved us to the spot called +P in 77694 . As soon as the ASC started to come on in full lock, it was clear that this is not a good location, with hughe power fluctuations in the PRC, POP 18 and POP90. We were sitting in RF lock with this from 5/30 20:44-21:20 UTC. During this time we injected a 31Hz line on SRM pitch, and adjusted P2L gain to minimize it's appearance in both SRCL and Rf DARM. This wasn't a very precise measurement, but for both SRCL and DARM the line was smallest at -1.5 P2L gain (compared to -1 or -2), indicating that this position is closer to centered on SRM than the other two positions.
We've now gone back to the position that we had been using since late April, as that has the best range. We may want to continue looking for better spots next week, to see if we can fully recover optical gain to 1.02.
