Mechanical improvement of the interim LMT/GTM primary surface actuators

The design of the Large Millimeter Telescope/Gran Telescopio Milimetrico (LMT/GTM) is such that it relies on an active surface system for the primary reflector in order to meet its surface accuracy specifications. Specifically, each of the primary surface segments is supported by four linear actuators, one for each corner. In the current state of the telescope, the interim active surface system must provide control for 84 segments long enough to allow completion of the rest of the reflecting surface and installation of the final actuators. During the LMT/GTM first light program in 2011, the interim active surface system of 336 actuators suffered from a wide range of problems that prevented useful operation. As a result, the LMT engineering team began an extensive testing program in 2012 to determine the sources of the problem and to evaluate possible mitigation strategies. As a result of these tests, the project implemented a substantial improvement program to the actuators in the first half of 2013, including a set of mechanical modifications and a replacement of the control system electronics. In this paper, we present the original mechanical design of the actuator, the design issues, and the modifications that were implemented. Details are provided about how the actuators have been improved from the perspective of repeatability, accuracy, and robustness. Finally, additional comments and recommendations are made for applying the lessons learned to the final actuator system.