Performance testing of the LMT/GTM primary surface actuators

The design of the Large Millimeter Telescope (LMT/GTM) relies on an active surface system for the primary reflector in order to meet its surface accuracy specifications. Each of the 180 segments of the 50m diameter primary surface is supported by four linear actuators, one for each corner. Currently, while awaiting completion of the reflector panels needed to populate the outer portion of the surface, the precision reflecting surface is limited to the inner three rings of segments, which provide a 32.5m diameter primary with 84 segments. The interim active surface system must provide control for these segments long enough to allow completion of the rest of the reflecting surface and procurement 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. Following extensive diagnostic testing in 2012, the project implemented a substantial actuator improvement program in the first half of 2013, including mechanical modifications and replacement of the control system electronics. Later, each actuator was characterized on an external test bench, providing demonstration of actuator operation and readiness, as well as calibration information to further improve absolute accuracy. In this paper, we present results of the accuracy and repeatability of the refurbished actuators, as well as results from the accelerated lifetime testing on site. The uniformity of the calibration coefficients is also presented and discussed. The results demonstrate that the refurbished actuator units are sufficient for the needs of the interim active surface system.