Large Synoptic Survey Telescope Mechanical Structure and Design

Abstract

The Large-aperture Synoptic Survey Telescope, LSST, will have an effective aperture of ~6.5 m and a 3 degree field of view. Its 3-mirror optical system with 8.4 m primary, 3.5 m secondary, 4.2 meter tertiary mirrors and a trapped focus offer unique telescope design challenges. The operation of this telescope will require quick slewing, accurate tracking and alignment maintained actively for 0.25 arcsec images in the presence of wind and gravity perturbations. We describe our current design for which finite element models show a lowest frequency resonance above 7 Hertz. Further refinement promises an even stiffer structure. The design has been optimized for low mass (230 tons), minimal inertia (2.4×10⁶ kg-m² in elevation, 3.2×10⁶ kg-m² in azimuth) for fast response. It takes advantage of several concepts proven in the Large Binocular Telescope mount, which has shown high performance at low cost. These include elevation motion on C rings placed under the primary mirror, a primary mirror cell built as an integral part of the structure, and the elevation axis placed behind and off to the side of the primary vertex, to achieve balance with minimum mass.