13 September 2012 Fabrication of the LSST monolithic primary-tertiary mirror
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Abstract
As previously reported (at the SPIE Astronomical Instrumentation conference of 2010 in San Diego1), the Large Synoptic Survey Telescope (LSST) utilizes a three-mirror design in which the primary (M1) and tertiary (M3) mirrors are two concentric aspheric surfaces on one monolithic substrate. The substrate material is Ohara E6 borosilicate glass, in a honeycomb sandwich configuration, currently in production at The University of Arizona’s Steward Observatory Mirror Lab. We will provide an update to the status of the mirrors and metrology systems, which have advanced from concepts to hardware in the past two years. In addition to the normal requirements for smooth surfaces of the appropriate prescriptions, the alignment of the two surfaces must be accurately measured and controlled in the production lab, reducing the degrees of freedom needed to be controlled in the telescope. The surface specification is described as a structure function, related to seeing in excellent conditions. Both the pointing and centration of the two optical axes are important parameters, in addition to the axial spacing of the two vertices. This paper details the manufacturing process and metrology systems for each surface, including the alignment of the two surfaces. M1 is a hyperboloid and can utilize a standard Offner null corrector, whereas M3 is an oblate ellipsoid, so it has positive spherical aberration. The null corrector is a phase-etched computer-generated hologram (CGH) between the mirror surface and the center-of-curvature. Laser trackers are relied upon to measure the alignment and spacing as well as rough-surface metrology during looseabrasive grinding.
© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Michael T. Tuell, Michael T. Tuell, Hubert M. Martin, Hubert M. Martin, James H. Burge, James H. Burge, Dean A. Ketelsen, Dean A. Ketelsen, Kevin Law, Kevin Law, William J. Gressler, William J. Gressler, Chunyu Zhao, Chunyu Zhao, } "Fabrication of the LSST monolithic primary-tertiary mirror", Proc. SPIE 8450, Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II, 84504Q (13 September 2012); doi: 10.1117/12.926403; https://doi.org/10.1117/12.926403
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