Telescope mount assembly pointing accuracy assessment for the Large Synoptic Survey Telescope: A large-scale metrology challenge

摘要

The Telescope Mount Assembly (TMA) is one of the main subsystems of the Large Synoptic Survey Telescope (LSST), a large (8.4 m) wide-field (3.5 degree) survey telescope, which will be located on the summit of Cerro Pachon in Chile. The TMA provides motions about the azimuth and elevation axes to comply with the survey mission, so it is of great interest to make sure that the TMA is pointing towards the intended location on the sky as accurately as possible. In this scenario, IK4-TEKNIKER has developed a custom engineered measurement procedure to assess the pointing accuracy error of the TMA, based on laser tracker technology and several fiducial points fixed to the floor. There are several metrology challenges to tackle, such as the large-scale measurement scenario, the visibility to the optical axis of the telescope or the working range of the TMA. A complete simulation model has been built based upon the Monte-Carlo technique within Spatial Analyzer (SA) software to anticipate the measurement uncertainty with the suggested methodology and simulation results show that the presented methodology is fit for purpose. These results show uncertainties better than 2 arcsec for every pointing position within the pointing range of the TMA, which means that the presented methodology is compatible with the pointing accuracy requirements, limited to 50 arcsec at a 95% confidence level (k = 2). Finally, the successful implementation of the suggested new verification method is done in situ. A fully automatic measurement procedure is performed reducing the test data acquisition down to 75 minutes and preliminary TMA pointing accuracy results show that the presented measurement procedure has performed according to the simulated performance.