Performance testing of the LMT/GTM M2 positioner

Prior to the early science campaign of Spring 2013, the engineering team at the Large Millimeter Telescope/ Gran Telescopio Milimetrico (LMT/GTM) conducted a series of performance tests on the hexapod used for positioning the secondary reflector (M2 mirror). The tests were of particular interest to the project due to the high mass of the existing aluminum M2 mirror. The testing was conducted in a lower foundation room at the LMT site on a fixture that allowed the positioner and mirror to be oriented at both zenith and horizon orientations. In each of these positions, the repeatability of the system zero position was tested, along with both single degree-of-freedom (DOF) and combined DOF motions. Additionally, the tests investigated the stability of the system at constant command position to changes in the orientation of the unit with respect to gravity. Throughout these tests, a laser tracker was used for measurement of the position of targets on both the fixed base of the hexapod and on the outer rim of the M2 mirror. In this way, motions of the tracker head or of the support fixture could be eliminated from the analysis. In this paper, we present results of the accuracy and repeatability of the system, as well as comments on the effects of the laser tracker measurement geometry with respect to the system at the zenith and horizon orientations.