The Giant Magellan Telescope (GMT) is an optical-infrared 25 Meter ELT to be located in Chile. It is being designed
and constructed by a group of U.S. and international universities and research institutions1.
Structural performance of large telescopes can be enhanced significantly with the added stiffness that results from
distributing loads to many points in the structure. In defining the two rotating assemblies in an altitude-over-azimuth
mount more than a kinematic set of constraints can lead to hydrostatic bearing oil film failure due to unintended forces
that result from runner bearing irregularities. High Frequency Over Constraint (HFOC) increases stiffness without risk of
oil film failure. It was used successfully on the Magellan 6.5 Meter Telescopes.
GMT will employ this and two additional methods to enhance stiffness at frequencies from DC wind up through the
telescope primary mode frequencies of ~11 Hz. This will be achieved without excessive hydrostatic bearing pad forces.
Detailed discussion of GMT's hydrostatic constraints, azimuth track and optics support structure (OSS) runner bearing
illustrations, and performance criteria are provided for the design.