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Reports until 10:42, Thursday 09 November 2023
H1 CAL (DetChar)
louis.dartez@LIGO.ORG - posted 10:42, Thursday 09 November 2023 - last comment - 10:44, Friday 10 November 2023(74113)
Line at 17.7 Hz interferes with kappaTST line, mysteriously got louder on 10/13
JoeB, M. Wade, A. Viets, L. Dartez

Maddie pointed out an issue with the computation of the kappa_TST starting since 10/13. After looking into it a bit further, Aaron found a noisy peak at 17.7 Hz just above the kappa_TST line (which is very close at 17.6 Hz). It turns out that the peak has been there for a while, but it got louder on 10/13 and has been interfering with the kappa_TST calculation since then. 

For a quick reference take a look at the top left plot on the calibration summary pages for October 12 and October 13. Taking a look at the DARM spectra for those days, JoeB, noticed a near 17.7Hz on 10/12 at about 23:30 UTC (darm_spect_oct12_2023.png). Interestingly, he noted that it looks like the 17.7Hz line, which was present before the glitch, got louder after the glitch (darm_spect_oct13_2023.png). 

I've attached an ndscope screenshot of the moment that the glitch happens (darm_17p7hz_glitch_kappaTST_uncertainty.png). Indeed, there is a glitch at around 23:30 on 10/12 and it is seen in by the KappaTST line and the line's uncertainty. Interestingly, after the glitch the TST line uncertainty stayed high by about 0.6% compared to its value before the glitch occurred. This 0.6% increase pushed the mean KappaTST line uncertainty above 1%, which is also the threshold applied by the GDS pipeline to determine when to begin gating that metric (see comment LHO:72944 for more info on the threshold itself).

It's not clear to us what caused the glitch or why the uncertainty stayed higher afterwards. I noticed that the glitch at 23:30 was preceded by a smaller glitch by a few hours. Oddly, the mean KappaTST uncertainty also increased (and stayed that way) then too. There are three distinct "steps" in the kappaTST uncertainty shown in the ndscope I attached. 

I'll note that I initially looked for changes to the 17.7Hz line before and after the TCS changes on 10/13 (LHO:73445) but did not find any evidence that the two are related.

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Until we identify what is causing the 17.7Hz line and fix it, we'll need to do something to help the kappaTST estimation. I'd like to see if I can slightly increase the kappaTST line height in DARM to compensate for the presence of this noisy peak and improve the coherence of the L3 sus line TF to DARM.

Images attached to this report
Comments related to this report
camilla.compton@LIGO.ORG - 11:54, Thursday 09 November 2023 (74117)

The TCS Ring Heater changes were reverted 16 October 74116.

On Oct 12th we retuned MICH and SRCL LSC Feedforwards and moved the actuation from ETMX to ETMY PUM 73420.

gabriele.vajente@LIGO.ORG - 12:47, Thursday 09 November 2023 (74119)

The MICH FF always has a pole / zero pair at about 17.7 Hz. In the latest filter, the peak is a few dB higher than in previous iterations

camilla.compton@LIGO.ORG - 15:12, Thursday 09 November 2023 (74121)

The change in H1:CAL-CS_TDEP_SUS_LINE3_UNCERTAINTY is when the MICH and SRCL feedforward filters were changed. See attached.

Images attached to this comment
louis.dartez@LIGO.ORG - 10:44, Friday 10 November 2023 (74137)
this is due to a 17.7Hz resonance in the new MICH FF. See LHO:74136.
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