On the left is shown a photograph of the girder metrology gondola at the curvature center of the diluted primary mirror. We drew in yellow and red the cables with low contrast on the picture. The convex metrology mirror is attached in such a way that it mainly turns around his own curvature center Ω when the balloon oscillates in the wind. The bottom tripod (in red) is stabilized. On the right, a close view of the girder gondola: two corner cubes attached at the bottom of the girder are used by the servo loop system (Sect. 5.1.1). The green filter at the center of the mirror creates a shadow on the metrology table on the ground. It improves the contrast of the white fringe return image. Behind the green filter, a circular sight is used to align vertically the gondola. Six holes have been drilled in the metrology mirror using a diamond tool. The bottom tripod cables (red) converge through three holes toward the point where the balloon is attached. The three attachment points of the focal gondola are oriented toward the middle of the girder (C point) in such a way that it will turn around the curvature center of the diluted primary mirror. The laser distance meter’s light (Fig. 7) is reflected on the white sheet on the left of the metrology mirror.