Figure 4 : Concept of interferometric 8-meter telescope studied for the IGT, having a spherical open-frame mount. A mobile version also studied has 6 robotic legs replacing the A-frame support shown in this fixed version.. Arc-shaped elements of the spherical truss are carried by a carriage C (right) driven along the supporting ring Ri . The arc A is in frictional contact with roller R, which acts only on the horizontal component of the sphere's local velocity. Carriages can be disengaged form the sphere and re-engaged at a different location to maintain a correct distribution of loads. A fully geared, rather than frictional, variant of the drive is also considered. The protruding mast on top of the M2 ensemble carries a small mirror at the center of curvature of the primary mirror for the laser alignment system. Optional Nasmyth platforms, not represented, and a Cassegrain cage can receive auxiliary instruments, for use during non-interferometric nights, when poor and fast "seeing" prevails. For interferometric uses, a coudé output exits horizontally from the center of the sphere.