Virgo Logbook

Since the last lock the dihedron peaks (78 Hz, 82 Hz) were really too high in the sensitivity. It was already observed an abnormal level of the two peaks since a few days, and there was some idea about a possible correlation to the finesse asymmetry.
The further worsening, occurred on Aug 28th after an unlock (fig 1), was not due to a variation of finesse asymmetry, but to a mistuning of modulation frequency. In fact the macro ResetFmodErr, performed at the relock, had to recover a big mistuning, but it seems that it did partially the work (fig 2). This morning the macro has been performed again, during the calibration stuff, and a further step was needed (more than 2 Hz), and still not enough (fig 3). Finally the tuning converged because the macro was performed several time during the maintenance period (fig 4). By the way, a calibration of ResetFmodErr is needed.
A this point our peaks were back the level of morning 28th; better than before, but stiil too high (fig 5).
Two possibility:
1) the 'finesse asymmetry' theory is correct;
2) there is an offset in fModErr (which was not there ten day ago).

We decided to adjust fMod at step 12, trying to minimize the amplitude of the peak in the dark fringe (we put the ITF in commissioning mode).
Moving it by 6 Hz, the minimum was crossed and the similar level on the opposite side was reached (fig 6). It was decided to set a new value of fMod, 3 Hz far from the 'standard tuning'. The peaks were reduced near the noise floor (fig 7), and we could say, with very low confidence, that some other improvement has been obtained at low frequency (fig 8).

The two hypotesis are stii open: we cannot exclude that actually we performed a mistuning of fMod in order to compensate some coupling of the peaks created by the big finesse asymmetry. To have an answer, I think we have to change again the etalon working point.