The mainframe is defined as the part of the structure that supports the optical sub-assemblies, including the guiders, slit mask, field lens, collimator, dispersers, and cameras. The mainframe is attached to the telescope by the Nasmyth Instrument Rotator (NIR) bearing flange, and at a secondary support point near the instrument center of gravity. The secondary support point provides a single degree of freedom constraint in the direction opposite to gravity. The mainframe may be of space frame or monocoque construction, or a hybrid of the two.