A New Methodology for the Assessment of Relative Pointing Accuracy and Active Alignment Requirements of the Large Synoptic Survey Telescope

Abstract

The Large Synoptic Survey Telescope (LSST) is a large (8.4 meter) wide-field (3.5 degree) survey telescope, which will be located on the summit of Cerro Pachón in Chile. The Telescope Mount Assembly (TMA) acquires and tracks fields on the sky by providing motions about the azimuth and elevation axes and, therefore, it provides pointing, tracking and slewing system performance requirements needed for the LSST to fulfill its survey mission. In this scenario, this paper presents a new measurement methodology for the dimensional assessment of pointing performance requirements. Based on laser tracker technology, a new measurement methodology is developed for the measurement of the optical axis for any motion within the pointing range of the LSST. In addition, laser tracker is also employed for the measurement of the alignment between secondary (M2) and primary/tertiary (M1M3) mirrors for different orientations around the elevation angle as a part of the active alignment system of the telescope. Monte-carlo based simulation results show that the methodology is fit for purpose even if floor movement occurs due to temperature variation of the environment. 1. Introduction TMA is one of the most important contributors to LSST. The TMA includes all the moving mass excluding the payloads which includes three optical systems: 1) M1M3 mirror cell assembly, 2) M2 mirror cell assembly 3) camera, their positioning systems (M2 hexapod and camera hexapod and rotator) and the M2 light baffle.