This afternoon we spent about 3 hours struggling to lock ALS.

At 20:45 UTC we lost lock while transitioning to Earthquake mode (though the ground motion wasn't really that high)

While reacquiring we kept losing lock while trying to lock ALS. We saw a few different problems:

• We were frequenctly losing the IMC. Sheila went to the PSL racks and measured the IMC UGF while it was locked at 2W. It was ~39 kHz, comparable to the last measurement of 40kHz.
• For a while we could not get through ALS_COMM's Handoff_Part_2, where we transition the IMC control from IMC_MCL to LSC_MCL and ALS_COMM. As we increased the analog gain of the LSC refl servo we saw glitches in the Y arm and sometimes in both arms.
  • We temporarily reduced the fianl analog gain from 17 dB to 10 dB.
  • First attachment shows some glitches seen in the transmission of both arms during/after the slider adjustment, followed by the y-arm losing lock, followed by the x arm losing lock
  • Second attachment shows another time when the glitches were more obvious in the y arm, but both arms lost lock at the same time
  • We tried locking COMM before DIFF (normally we lock DIFF first following the saga where ALS-DIFF PLL caused glitches in COMM documented here). And at this point we survived, but not because we changed the locking order - we lost lock again afterwards and reacquired just fine with the normal old guardian settings

In conclusion: for some reason we don't know, we couldn't lock ALS for a couple of hours this afternoon. Then for some reason we don't know, we could lock it again.