Advances in Edge Sensors for the Thirty Meter Telescope Primary Mirror

The out-of-plane degrees of freedom (piston, tip, and tilt) of each of the 492 segments in the Thirty Meter Telescope primary mirror will be actively controlled using three actuators per segment and two edge sensors along each inter-segment gap. We address two important topics for this system: edge sensor design, and the correction of fabrication and installation errors. The primary mirror segments are passively constrained in the three lateral degrees of freedom. We evaluate the segment lateral motions due to the changing gravity vector and temperature, using site temperature and wind data, thermal modeling, and finite-element analysis. Sensor fabrication and installation errors combined with these lateral motions will induce errors in the sensor readings. We evaluate these errors for a capacitive sensor design as a function of dihedral angle sensitivity. We also describe operational scenarios for using the Alignment and Phasing System to correct the sensor readings for errors associated with fabrication and installation.