Virgo Logbook

After the locking of the OMC it is possible to investigate the mirror drum modes in the Pr_B1_ACp channel at 20kHz. As described in the entry First investigation of the new main mirror frequency modes. the slightly increased thickness of the mirrors should push up the frequency of the drum mode (according to “Cypres": WE:5716Hz, NE:5702, WI:5694, NI:5732). In the first figure the dark fringe spectral amplitude is shown in the frequency range of interest. Four peaks are candidates to be the drum modes: 5671Hz, 5675, 5694, 5707. According to the entry OMC locked (that I erroneously considered wrong in the past, sorry) the 5671 and 5675 Hz should correspond to the input mirrors (probably NI:5671 and WI:5675); hence the other two frequencies should correspond to the end mirrors. The 5707 Hz corresponds probably to the NE; in fact observing just 2 locking periods (961349338 and 961355745) the frequency of the 5707 peak is the only one lowering and, at the same time, the NE tower is the only one where the temperature decreases. As shown in the second figure, the frequency reduction roughly corresponds to the temperature lowering. Hence, by exclusion, the 5694 Hz peak should correspond to the WE.
Finally, during the lock at GPStime=961349338 the two end mirror drum frequencies get excited; the Q measured for these modes are in the third and fourth figures (about 5M, a promising value).