Error reduction and modeling for hexapod positioners of secondary mirrors for large ground-based telescopes

The positioning requirements for secondary mirrors and instruments for large ground-based telescopes are becoming increasingly challenging. Modern telescope designs, such as LSST and TMT, are specifying repeatability and/or absolute accuracy limits below 10 mu m and 10 mu rad for the hexapod positioning systems generally used for these applications. Hexapod error sources, including lead screw pitch variations, windup, backlash, friction, thermal expansion, compliance, sensing, and joint node location uncertainties, are examined along with methods for reducing or eliminating these errors by mechanical means or through calibration. Alternative sensing approaches are discussed and their relative benefits are evaluated. Finally, a model-based design approach is presented for conducting initial design trade studies, assessing technical risk, predicting achievable performance, establishing subsystem and component requirements, and tracking positioning error budgets through the entire development process. A parametric actuator model and its initial results are described, and testing approaches are outlined to identify key model parameters and verify subsystem and component performance.