Evan, Stefan, Sheila, Matt, Lisa The official goal of tonight was to make measurements for the ultimate noise budget plot that Evan is about to produce (work still in progress). The unofficial goal was to improve the sensitivity between 50 - 100 Hz, and investigate the noise that is obsessing Stefan these days . More details will follow tomorrow, but after Tuesday maintenance, three earthquakes and some in principle innocuous but effectively disruptive measurements, we finally managed to relock robustly and make a couple of (alleged) improvements (1/1.5 in Valera's scale of awesomeness): - despite some recent MICH feed-forward tuning, MICH still had some residual coherence (< 0.3) with DARM between 50 and 100 Hz. We saw room for improving the current cut-off, so Matt added another filter to the LSC-MICH filter bank (FM9, EBS50) which only loses a few degrees of phase at 10 Hz, but provides about 10 dB of attenuation between 50 and 100 Hz. The coherence is now reduced < 0.1 above 50 Hz; - starting by staring at some recent Bruco reports which showed some suspicious (even if small) coherence with the ASC AS DC signals below 100 Hz, we ended up (re)discovering that the current WFS centering loops ASC_AS_A / AS_AS_B --> OM1 / OM2 have a simple cut-off at 100 Hz (CLP100 in H1ASC-DC4/DC3 P/Y). The existing LP8 filter was not stable, so now there is a new cut-off at 20 Hz (ELF20). Both these filters have been tested a couple of times during the entire locking sequence, so we leave them on. We leave the interferometer locked undisturbed ~ 2:15 AM (July 29 9:15 UTC). P.S.: For the morning crew, as Sheila reported , the locking sequence doesn't currently work automatically to low noise because the ETMX ESDs are not turning off.
Another task is to automate some CARM gain redistribution in the guardian.
Right now in full lock the CMB common gain is −22 dB, the fast gain is +7 dB, and the LSC-MCL gain is 240 ct/ct. We can reduce the input-referred noise of the SNB+CMB by a factor of 2 by changing these values to −15 dB, 0 dB, and 107 ct/ct.
[In fact, from a noise perspective it would probably be even better to do a little more redistribution by removing some gain from the IMC ao path (currently it's −2 dB) and adding it to the CMB common gain.]
Since the IFO was still locked when I came in, at around 8:06AM local time I started manually reallocating the CM and MC board gain as Evan suggested:
[CM common, CM fast, LSC MCL, MC AO] =
originally [-22, 7, 240, -2]
then to [-15, 0, 107, -2]
then to [-12, 0, 107, -5]
Finished at around 8:11 AM.
[update] After running in this state for ~30min, the IFO lost lock during the morning meeting. I manually reverted the CM board and MC board gains back as I wasn't sure which part of these is NOT touched by the guardian when locking.
