The GMT adaptive secondary mirror (ASM) is based on a "segmented" concept following the primary segment layout:
seven 1.05m diameter circular, independent adaptive mirrors are fed by the primaries and focus to the main telescope
focal stations. The adaptive unit's design is based on the consolidated thin mirror, contactless technology already
employed in several units (MMT, LBT, Magellan, VLT and one of the proposed E-ELT M4 designs), but nevertheless
the mirror's topology reveals several design challenges. In particular, the off-axis units are strongly aspheric and
therefore they require aspheric shaping of both thin mirror surfaces and of the thick reference body. The strong tilt of the
off-axis units forced us to consider a peculiar fine positioning hexapod design, maximizing its stiffness and also
implementing a special design of the last three rings of actuators to remain within the prescribed obstruction. From the
control point of view, the actuator density of the adaptive mirrors is remarkably lower than in all previous units: 672
actuators with 36mm spacing compared to 30mm typical separation adopted so far. This choice is validated by static and
dynamic performance computation though a sophisticated numerical simulator based on a full state space model
incorporating mechanics, control and fluid dynamics. The control system fulfills the dimensional constraints of the unit.
The design has completed the feasibility phase, including the cost estimate. The choice of making the GMT adaptive
secondary mirrors similar to the already existing ones strongly reduces the implementation risks and allows shortening
the remaining design path.