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.