Today, Viktor and I started installing the two-mirror cavity. - Firstly, we cleaned the environment and the dust counter showed good cleanliness - After opening the cavity we tried to determine the source of the strange spot with a laser detection card and found that the beam was very close to the front edge of the longitudinal D-shaped mirror. In addition there was nothing else strange. - The setup of the two-mirror cavity is shown in Figure 1. We have to use the menhir laser of 216MHz. The mirrors used are shown in Figure 2. - We have installed the M2 and will continue the installation tomorrow.