There are still many mysteries remaining - e.g. the MICH-->PRCL contribution still can't be nulled. But for now, I have the settings that keep the PRMI locked fairly robustly with REFL55I/Q or REFL165I/Q (I quadrature for PRCL, Q for MICH in both cases), see Attachment #1 and Attachment #2 respectively. For the 1f locking, the REFL55 digital demod phase was fine-tuned to minimize the frequency noise (generated by driving MC2) coupling to the Michelson readout (as the Michelson is supposed to be immune) - the coupling was measured to be ~60dB larger at the PRCL error point than MICH. There was still nearly unity coherence between my MC2 drive and the MICH error point demodulated at the drive frequency, but I was not able to null it any better than this. With the PRMI (ETMs misaligned) locked on the 1f signals, I measured Attachment #1 and used it to determine the demod phase that would best enable REFL165_I to be a PRCL sensor. Rana thinks that if there is some subtle effect in the marginally stable PRC, we would not see it unless we do a mode scan (time consuming to set up and execute). So I'm just going to push on with the PRFPMI locking - let's see if the clean arm mode forces a clean TEM00 mode to be resonant in the PRC, and if that can sort out the lack of orthogonality between MICH/PRCL in the 1f sensors (after all, we only care about the 3f signals in as much as they allow us to lock the interferometer). I'll try the PRMI with arms on ALS tomorrow eve.

I have no idea what to make of how the single frequency lines I am driving in MICH and PRCL show up in REFL11 and REFL33 - the signals are apparently completely degenerate (in optical quadrature). How this is possible even though the PRMI remains stably locked, POP22/POP110/AS110 report stable sideband buildup is not clear to me.