Schema of the OHP interferometer prototype. In order to show the whole experiment, this is not a scale drawing. The black lines represent the cables. The blue circles symbolize the three primary mirrors at the ground level around the metrology table (Fig. 10). The convex metrology mirror is attached at the bottom of a girder gondola. The helium balloon keeps the cables under tension by pulling to the lower part of this girder gondola. Point C which corresponds to the center of the spherical diluted primary mirror, is at the middle of the girder. Ω at the top of the girder is the curvature center of the convex metrology mirror. It is passively positioned by three cables attached to the ground. This mechanical design allows the metrology mirror to rotate around its own center of curvature Ω, if the balloon oscillates under the wind. The white cylinders (MW1, MW2, MW3) are the three motorized winches stabilizing the metrology mirror. The devices to measure the displacements of the metrology gondola are also shown: red lasers in front of the 20   cm lens and corner cubes attached on the metrology gondola (Sect. 5.1.1). For the sake of clarity, the focal gondola under the metrology gondola is not represented.