jenne.driggers@LIGO.ORG - posted 13:08, Wednesday 19 July 2023 - last comment - 11:49, Thursday 20 July 2023(71510)
Range improves with AS36Q offset removed when OM2 is hot
Last night and this morning, we've turned the OM2 heater back on. Our range didn't improve, however. We still had in place the AS36Q yaw offset from last Thursday (alog 71309). At the start of our Wed commissioning period, I turned that offset off. This improved our range back to ~140 Mpc according to CAL-DELTAL on the wall, but our range is still not back to where it was the last time we had OM2 hot. We may need to think about more alignment changes / checks.
Naoki just reminded me that we probably need to revert the MICH LSC feedforward to the hot OM2 setting. We'll do that once the PEM team is between tests.
I've just removed the AS36Q offset from ISC_LOCK's ADS_to_camera state and reloaded. I've accepted the SDF with the offset of 0. I've also accepted the TRAMP of 10 sec, which is what it should come back to when the safe.snap is reverted. So, hopefully operators won't see any SDF diffs that they need to address.
The MICH FF change between cold and hot OM2 is written in alog71285. We switched the MICH FF filter from FM5 to FM3 while NLN. We also implemented it in the ISC LOCK guardian and reloaded it following alog71285.
Unfortunately, I'm not so sure that we like this version of MICH FF. In the attachment, the green dashed trace is what we have right now (LSC MICH FF is FM3 and FM10, the hot OM2 settings), and the red trace is what we had before Naoki reverted to the hot OM2 settings (FM5 and FM10, nominally the cold OM2 settings). The hot OM2 FF has increased our coherence with LSC MICH today.
It's possible that OM2 isn't all the way thermalized yet and that matters, or more likely, we have some other unknown alignment change that has caused this to be less effective than it was in the past.
LSC-MICHFF diff accepted
Here are two quick plots comparing today's hot OM2 configuration versus late June when we had excellent 70 Hz noise. The traces and data are the same, just the x-axis is different.
The seafoam is June 28th at 08:00:00 UTC, hot OM2, no deliberate ASC offsets. Brown is today's hot OM2 (19 July 2023 18:15:27 UTC) before I removed the AS36Q offset, and purple is after removing that offset (19 July 2023 19:41:09 UTC). So, purple should be the same as seafoam since in principle the configuration is all the same, but clearly it's not.
One suggestion is that perhaps there are some squeezer adjustments that could affect the 70 Hz region; I believe that Sheila and Naoki were discussing some potential tests.
We should double check, but I don't think the residual LSC MICH coherence explains the whole difference.
Its' certainly interesting to note that the brown trace (hot OM2, but with the AS36Q MICH ASC offset that we had found while OM2 was cold) seems to have very little jitter coupling, and also a relatively small amount of laser noise at high frequency. It does however have lots of excess noise below 60 Hz, which is consistent with what Marissa had been seeing earlier today.
Indeed most of the sensitivity around 70 Hz was recoveredy by MICH FF: See alog 71550.