Ryan S, Jenne D, Sheila

The ALS_COMM and DIFF nodes will check on the green arm transmission when finding IR to ensure that the arms are aligned well enough to move on. This threshold has been set to 0.75 but I lowered it to 0.65 since we've ran into this threshold as few times in the last week and have been unable to improve the arm transmission.

Last week this happened on Thursday (alog74655) and I just manualed around the state to continue locking. It worked for that lock to get us back up, and then it wasn't a problem with future locks until today. Today, the arm transmission was low for both arms so we went to confirm that there was no PR3 movement, and then we restored optics back to the start of the previous lock. This also didn't help. Looking at the arm power over the last few weeks (attachment 1), after the PR3 and table work from last Tuesday (alog74618) it has been lower than before then but as low as we were seeing today. The current thinking is that the PR3 move last week brought our powers lower, and while the table work improved the COMM beatnote, it did nothing for the arm powers. Once the IFO gets to a better full lock alignment, this alignment tends to give us better ALS power next lock acquisition, so lowering this threshold should help us get to that point and hopefully help sequential locks.

The long term solution would be to go back on table to realign the rest of the COMM path to camera, then renormalize the arm powers so that 1 is actually our max power, then bring this threshold back to the 0.75 that it was*. Going on that table always carries rick of making things worse, so we definitely don't want to be doing this right before the holidays. We will reconvene in 2024 on this issue if it's still an issue.

*There's a comment in the guardian code that it was 0.85 from Aug 12, 2020.