I hadn't re-locked the DRMI after the work on the AS55 demod board. Tonight, I was able to recover the DRMI locking with the old settings.

The feature in the PRCL spectrum (uncalibrated, y-axis is cts/rtHz) at ~1.6kHz is mysterious, I wonder what that's about.

Wasted some time tonight futzing around with various settings because I couldn't catch a DRMI lock, thinking I may have to re-tune demod phases etc given that I've been mucking around the LSC rack a fair bit. But fortunately, the problem turned out to be that the correct feedforward filters were not enabled in the angular feedforward path (seems like these are not SDF monitored). Clue was that there was more angular motion of the POP spot on the CCD than I'm used to seeing, even in the PRMI carrier lock.

After fixing this, lock was acquired within seconds, and the locks are as robust as I remember them - I just broke one after ~20mins locked because I went into the lab. I've been putting off looking at this angular feedforward stuff and trying out some ideas rana suggested, seems like it can be really useful.

As part of the pre-lock work, I dither aligned arms, and then ran the PRCL/MICH dithers as well, following which I re-centered the ITM, PRM and BS Oplevspots onto their respective QPDs - they have not been centered for a couple of months now.

I'm now going to try and measure some other couplings like PSL RIN->MICH, Marconi phase noise->MICH etc.