Here are a couple plots of oplevs and OSEM signals during our last lock, focusing on the change related to turning on the OM2 TSAMS. I plotted test mass top mass OSEMs and oplevs (in case they told different stories), and the OSEMs for SRM, SR2, OM3 and OMC.
Conclusion: there doesn't appear to be an alignment change in the arms. We hypothesized that a change in the signal to WFS 45 could change the DHARD and MICH alignment loops. That doesn't appear to be the case. However, we do see a pretty clear change in the OM3 and OMC alignment. There might be some OMC ASC alignment offsets we could try in order to mimic the changes we are seeing from the OM2 change.
From squeezer ASC trends as OM2 heats up, we also see an alignment shift, most strongly related to AS_A_RF45_Pitch (corresponds to SQZ-ASC_ANG_P_OUTPUT). Not implausible that the output alignment is what's changing; this would also be consistent with having to reset SQZ ASC offsets 70890 with a hot OM2.
At 76W, Camilla and I tried walking OMC ASC QPD offsets and didn't find anything better (69307). Before at 60W, Koji had success increasing the optical gain by walking the OMC ASC QPD offsets, but that was in mid-March before great TCS tuning, 67994. I thought this helped to improve and stabilize the range then. He also light heated OM2 after the new OMC ASC offsets 68048 and saw a similar ~2% reduction in optical gain, consistent with now 70859.
So, I also think it makes sense to try walking the OMC alignment, and generally the IFO's output alignment.
