Performance verification testing for HET wide-field upgrade tracker in the laboratory

Abstract

To enable the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX), the McDonald Observatory (MDO) and the Center for Electro-mechanics (CEM) at the University of Texas at Austin are developing a new HET tracker in support of the Wide-Field Upgrade (WFU) and the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). The precision tracker is required to maintain the position of a 3,100 kg payload within ten microns of its desired position relative to the telescope's primary mirror. The hardware system to accomplish this has ten precision controlled actuators. Prior to installation on the telescope, full performance verification is required of the completed tracker in CEM's lab, without a primary mirror or the telescope's final instrument package. This requires the development of a laboratory test stand capable of supporting the completed tracker over its full range of motion, as well as means of measurement and methodology that can verify the accuracy of the tracker motion over full travel (4m diameter circle, 400 mm deep, with 9 degrees of tip and tilt) at a cost and schedule in keeping with the HET WFU requirements. Several techniques have been evaluated to complete this series of tests including: photogrammetry, laser tracker, autocollimator, and a distance measuring interferometer, with the laser tracker ultimately being identified as the most viable method. The design of the proposed system and its implementation in the lab is presented along with the test processes, predicted accuracy, and the basis for using the chosen method^*.