Having seen the first flashes on the B7 photodiode, as described in entry 33783, we have made an analysis of the observed ringing effect to obtain the mirror speed and the cavity Finesse.

We have taken data from the channel 'SNEB_B7_PDviewport' starting from GPS: 1146932838 with a duration of 100 seconds. We have double checked that the RFC was locked during this period.

First of all we have selected all the peaks corresponding to a main resonance (we have discarded the HOM even if the ringing effect is visible on them as well). Then we have made a fit (including the ringing effect) using the analytical expression described in the PhD thesis of Rakhmanov (pp. 32-33), as it was made in the past by Gabriele Vajente (entry 26487). An example of the fit is shown in Figure 1, and it gives us the storage time of the cavity, as well as the speed at which the cavity resonance is crossed, and using this storage time we calculated the Finesse of the cavity as F = ts *pi*FSR.

We have repeated this process for all the peaks found on the data set taken. Figure 2 shows the calculated finesse values distribution. From this analysis the mean value has been found to be 445 /- 22(std), consistent with the expected Finesse (455) . Notice that only the fits with a residual lower than 50 has been taken in account, so we have analyzed 131 peaks.

Figure 3 shows the distribution of the resonance-crossing speed. Notice that the spread of the values is quite wide because the mirrors are free-swinging and the speed is not necessarily constant. Notice as well that the expected critical velocity for the cavity is 0.4um/s, which is consistent with the lower limit, which is around this value.