Performance prediction of the LSST secondary mirror

Abstract

The Large Synoptic Survey Telescope (LSST) is an 8.4 meter telescope with a field of view of 10 square degrees. This telescope will be capable of mapping the entire visible sky every few nights via sequential 15-second exposures, opening new windows on the universe from dark energy to time variable objects. The LSST optics calls for an annular 3.5 m diameter Secondary Mirror (M2), which is a large meniscus convex asphere (ellipse). The M2 converts the beam reflected from the f/1.2 primary mirror into a beam for the f/0.83 Tertiary Mirror (M3). The M2 has a mass of approximately 1.5 metric tons and the mirror support system will need to maintain the mirror figure at different gravity orientations. The optical performance evaluations were made based on the optimized support systems consisting of 72 axial supports, mounted at the mirror back surface, and 6 tangent link lateral supports mounted around the outer edge. The predicted print-though errors of the M2 supports are 8nm RMS surface for axial gravity and 10nm RMS surface for lateral gravity. The natural frequencies were calculated for the M2 dynamic performance. In addition, thermo-elastic analyses of M2 for thermal gradient cases were conducted. The LSST M2 support system has an active optics capability to maintain optical figure and its performance to correct low-order aberrations has been demonstrated. The optical image qualities and structure functions for the axial and lateral gravity print-through cases, and thermal gradient effects were calculated.

LSST secondary mirror assembly baseline design

“…The actuators are arranged in a three concentric ring configuration, [3] fig 11. The configuration of the 72 axial actuators was chosen because: it requires the minimum number of actuators to meet the optical performance specification; minimizes the variation in actuator forces; and it allows for the utilization of more structurally efficient straight ribs in the mirror cell main structure. …”

Performance Analysis for Mirrors of 30 cm Cryogenic Space Infrared Telescope

“…Since the performance of a mirror depends on the deformation of a mirror surface, even if a mirror with high surface accuracy is fabricated, if a support of the mirror goes wrong, its quality cannot be guaranteed. Therefore, when designing support structures or polishing mirrors, it is crucial that the optical performance analysis of the mirror (Yoder 1993, Heo & Kwak 2008, Cho et al 2009 2010) is preceded. This paper focuses on calculating optical deformation map and root mean square (RMS) wavefront error (WFE) from the raw displacement of a mirror surface of a 30 cm cryogenic space infrared telescope in external loads.…”