Factory characterization testing of a large precision hexapod for the LMT/GTM

摘要

As part of the telescope completion plan, the Large Millimeter Telescope (LMT/GTM) project replaced the hexapod positioner for the secondary (M2) mirror. The new hexapod was provided by Symetrie of Nimes, France. The particular challenges for the LMT/GTM hexapod are that it is both large and precise. After completion of the fabrication and internal contractor verification of the system, the project conducted a series of characterization tests, both at the fabrication facility and at the telescope site. During the factory tests, the project team tested the hexapod in both vertical and horizontal positions, verifying the motion range, accuracy, repeatability, and velocity, all at the maximum operational payload. Additionally, the team performed verification checks on the stiffness of the hexapod. The results were excellent, with calibrated errors of less than 5 microns in the translation degrees of freedom and less than 1 arcsec in the rotations, which was at the limit of the metrology of the tests.Following the successful factory test, the system was transported to the telescope site and the tests were repeated. While the calibration step was not performed during the site tests, the raw results were comparable to the factory values, clearing the way for installation on the telescope. The new hexapod was installed, along with a new M2 mirror, in the Fall of 2017 in advance of the LMT/GTM's first observing season as a 50 m telescope. This paper presents the test program, the metrology approach, the characterization tests, the calibration method, and the final factory acceptance test results.