This morning the ITF could not be locked (the WE arm cavity can could be barely locked but very noise. This was due to a failure, @ 3:58UTC, of the BS suspension, see Figure 1. Valerio has been contacted and it will detail the issue.

Strange unlock at 8:11UTC due to SRCL corr becoming immediatly large, probably due to a reset of SRCL SET FLT (not understood), figure 2.

The OS has been checked and ideed the 0 value of OS signal corresponds to ~ 0Hz of real optical spring.

Then I've checked the effect of the SRCL set on LSC error signal. Lowering the SRCL set we can slightly improve (really a poor improvement) LSC MICH, no effect at low frequency, but the effect on DARM is dramatic, see Figure 3.

The best choice is then to keep the OS @ 0Hz.

we unlocked for DARM failure at OS @ 15Hz.

I tried also to explore the MICH set to lower the coupling of the RIN. see Figures 4 and 5, a more accurate test has been performed after.

The new corrector for SRCL has been tested (purple new , blue old) the gain is in the 30Hz-60Hz bandwidth and improves the accuracy of a factor 2. Not in the automation. Figure 6.

Then I optimized the fmoderr (of -0.1Hz) and I start again the MICH set study.

The minimum of the RIN at high frequency does not correspond to the balancing og  B1p 56MHz sidebands but on the balancing of the 6MHz. Moreover the low frequency coupling minimum does not correspond to the same position of the high frequency one. see Figure 7.

Minimum of PSTAB 1 & 2 @ MICH 0.6 (B1p 6MHz balanced)

Minumum of SSFS line @ MICH 1.2 (B1p 56MHz balanced)

Minumum of PSTAB 0 @ MICH 2

I tried also to repeat the test of yesterday to lower the modulation index of 56MHz but no effect on the coupling was visibile.

At ~14:45 I've removed the 81MHz to check the effect on the sensitivity. No visible effect was observed.

I repeated the scan of MICH, only to check again the wp. no changes (see figure 8)

I leave 0.65 of MICH, next shift can decide how to proceed.