Vicky, Camilla
IFO was Out of Observing 15:40 to 15:56UTC as SQZ FC Unlocked. After 5 failed automatic SQZ_FC TRANSION_IR_LOCKING attempts, Vicky and I kept the FC GRD in DOWN, closed the Input 1 loop (SQZ Overview > FC Servo > H1:SQZ-FC_SERVO_IN1EN) to manually lock FC on green and aligned FC2 using the GR camera and maximizing H1:SQZ-FC_TRANS_C_LF_OUTPUT. Accepted FC2 alignment sdfs and went back to observing. Updated "Issues with SQZ FC Guardian" wiki
Think the issue could have been due to alignment drifts over the long 21 hour lock and as the M3 length was cleared, this couples in with pitch and yaw alignments, plot attached. Enough that the FC2 alignment needed to be slightly tweaked to relock. From plot, can see that M2 Pit (grey trace, bottom left) changed ~40urad on FC unlock?! This is a lot of drift over 20 hours, zoomed out plot here.
We have previously decoupled L2A for FC2's M1 stage, see LHO:67957, March 2023, most L2A decoupling was needed in pitch. But we have not decoupled L2A for FC2's lower M3 stage (in principle we should not have to do this as most DC gain is on M1). More relevant is probably that since March 2023, the beam spots on FC1/2 mirrors have since drifted away from center so the previous L2A decoupling may be less valid now. May be worth checking and re-centering FC cavity axis on the optic centers now, where our previous L2A decoupling should work better, and may overall be more stable.
I think that if L2A decoupling coefficients are very different now, that could impact FC alignment and relocking after long lock stretches. That's basically what happened before, which led us to do the decoupling FC2 M1 L2A in the first place (i.e., after long locks, the FC alignment drifts a lot due to the length-to-pitch coupling, such that when FC unlocks and we stop pushing on FC2 in length, then FC2 pitch changes suddenly and we lose FC alignment). Something to keep an eye on as Camilla's fixing of FC2 pitch alignment seemed to recover the filter cavity lock transition from green to red, which feels similar to before.