Control system for an alternative actuator for the primary surface of the Large Millimeter Telescope (LMT/GTM)

The Large Millimeter Telescope/Gran Telescopio Milimetrico (LMT/GTM) is a bi-national project between INAOE in Mexico and UMASS in the USA. It is an open air radio telescope designed for astronomical observations in wavelengths ranging from 0.85 mm to 4 mm. Its 50-meter diameter primary reflector is so massive that its shape deviates from the theoretical parabola due to gravitational effect as it moves in elevation, which ultimately affects gain, one of the most important features of the telescope. To correct this elevation-dependent deformation, the primary surface is divided into 180 segments that are automatically positioned by means of four electro mechanical actuators. Unfortunately, the lifetime expectancy of the interim actuators installed in rings 1 to 3 during 2013, are below specs and the cost of substituting them with the new actuators, now under development for rings 4 and 5, may not be affordable. In this paper an alternative actuator control system that re uses most of the current electronics coupled to a completely redesigned mechanism is presented. The results of the performance tests under load show that the system is capable of achieving positioning with RMS error of 4 micron and that the accuracy is dominated by the LVDT characterization error.