Ethan, Sheila, Evan, Jeff, Corey

At 2019-12-18 16:07 UTC (1260720463), the PCAL X optical follower servo loop turned off with no discernable cause (orange button centre screen on PCALX screen). There was no changes made to the SDF for this to occur. The SDF information is located under EX_ECAT_PLC3 with channel names including PCALX_OPTICALFOLLOWERSERVO. This is distinctly different from the OFS saturating, which would not turn off the loop. H1:CAL-PCALX_OPTICALFOLLOWERSERVOENABLE encodes the state of the OFS loop, and has been plotted in OFSENABLE_turnoff.png. At 400s I turned the loop on briefly before turning it off for fear of losing lock (large excursion of broadband PCALX noise into DARM due to lack of ramp time). The OFS loop was off for ~50 minutes, fortunately we were not observing during this time.

Attached are two plots (OFS_off_worst_case.pdf and OFS_off_typical.pdf) which, as their filenames suggest, show the worst case of the elevated noise from PCAL X when the OFS control loop was off, and the typical elevated PCAL X noise observed during this period of time.

We should look into this further.