The Finesse of the arm cavities has been extracted using two different methods, and the results have been found to be compatible between them within the error bars.

Method 1 - fit of the Airy peak ringing fit 

The arm cavities free swinging data (#38458) have been analyzed in order to extract the finesse of each cavity. The free swinging have been obtained by giving a small kick to the mirror and leaving the dampers open.

The ringing fit has been performed as explained in #33795 and #34541. Only the fits with a residual lower than 0.5 have been selected. The time series of the analyzed data for the North arm is attached (the one for the West is similar, so it has not been reported) in figure 1, while figure 2 shows one single peak and its fit superimposed. The histograms of the Finesses are shown in figure 3 and 4. It must be noticed that if the cavity speed is too high (greater than 3 um/s) the fit doesn't work properly. This measurement can be repeated by kicking the mirror "more gently", in such a way to have less ringing.

Method 2 -  cal lines demodulated in hrec

The Finesse has been computed as reported in #38326. This time, the low frequency line has been moved 10 Hz ahead, and the accuracy of the value as extracted by the demodulation of this line is not very good. On the other hand, the vaue extracted by the high frequency line (358Hz) is much cleaner and has a smaller sigma. See figures 5 and 6 for the North and West arm respectively.

To summarize:

Arm cavity finesse