Yesterday people found that PR3 and BS seemed to move when PRC is carrier locked.
For PR3, there's no feedback and we see a clear correlation between the carrier power (sorry the first plot doesn't have the carrier power, only the AS power) and both OL and witness OSEMs. All in all PR3 oplev seems to slowly go negative in PIT after the carrier lock and goes back positive after lock loss, though this doesn't completely describe the behavior shown on the first plot. PR3 DC OPLEV servo should be able to counter this if needed.
For BS it's somewhat more difficult to see partly because of the length feedback, but there's definitely a correlation between the carrier buildup and the BS oplev. Unlike PR3, every time there's a carrier lock OPLEV PIT goes positive and slowly comes back. BS oplev doesn't seem to be well correlated to the mean length feedback, so this is not the effect of the DC length push tilting the BS.
Note that the plots posted above show that BS and PR3 moved about 1 to 1.5urad over 10 minutes.
Our PSL power is 10W, recycling gain at the moment is about 10.
These numbers sound as if this is a factor of 10-ish smaller than what the LLO people observed, but I'd appreciate any input.
These are the LLO numbers we were using for comparison: BS drift (without BS baffle): 5 urad @ 60 W on BS (LLO entry 9920); PR3 drift (without PR3 baffle): 12 urad / min (most likely still with 60 W on BS, entry 8960).