For most of the week we have had extra broadband noise in DARM, in the 10-50 Hz region, coherent with our ASC signals. The obvious difference between last week and this week is on Sunday I moved three of the alignment dither lines to low frequency. Today Sheila and I moved those lines (PIT3 YAW3 and YAW5) back to their 20 Hz frequencies. This reduced, but did not eliminate, the extra noise in DARM and the coherence with the hard loops.
In the attached spectra the top left is DARM, brown is early this evening with the dithers all in the 6-10 Hz band and red is after returning PIT3, YAW3 and YAW5 back to the 18-20Hz band. Grey is a reference from Jan16, when the dither loops were in the same configuration as the red trace, but the noise was lower. The other plots show coherence with the ASC loops (Hard pitch and yaw on the bottom left and right, soft on top right [we are not using the soft yaw loops at the moment]). Brown and green references are from today when all the dithers were in the 6-10Hz band; red and blue are from after the three dithers were moved back to the 18-20 Hz band, and the loops allowed to re-converge. The hard loop coherence with darm was reduced, most noticably in the 35-60Hz region for DHARD P, and 25-40Hz refion for DHARD Y. The soft loop coherence got worse with the new dithers.
We're not sure why the A2L decoupling is worse with the lower frequency dithers. There is certainly cross-coupling between the loops.
I've reverted to the 18-20Hz dither lines for PIT3 YAW3 and YAW5 in the guardian, but weekend commissioners may have to proceed through ENGAGE_SOFT_LOOPS (and CHARD_BLEND and LOWNOISE_ASC) with caution. I've left the gains pretty low so it might take too long for things to converge. I tried testing out lock requisition tonight but we've had an earthquake that has made things a bit tricky.
Another warning to tomorrow's commissioners tonight in ENGAGE_SOFT_LOOKS CSOFT P, the dithers, INP1 and SCR1 all rang up at once and unclear if the cause was the earthquake, or something else. We later had a lockloss in CHARD_BLEND that looked like chard pitch was responsible...
Another thing to note during this evening's lock: We had period of continuous glitching of EX ESD, maybe a bad glitch caused other glitches? EX L3 saturated for a few minutes with many glitches from 03:15:57 - 03:24:12 UTC (Jan 26). Second attachment is a screenshot of the time seri
Keita suggested I look at the op lev trends while we were shifting the dither line frequencies, to see how much the test mass pointing changed over the course of the lock. First attachment shows dither frequencies, and outputs of the dither loops, test mass op-levs, and PRM witness sensor. When turning PIT3 and YAW3 on at the new dither frequency, ITMY pointing changes by ~1 urad, and PRM changes by a few counts. When YAW5 convergest with the new dither frequency, ITMY and ETMY have both moved by roughly 1urad.
I was initially concerned by this shift in pointing, but I had a look at the op levs over the course of a long lock earlier this month and it seems that we see much larger excursions over the course of a normal lock (second attachment).
