Fabrication of the LSST monolithic primary-tertiary mirror

摘要

As previously reported (at the SPIE Astronomical Instrumentation conference of 2010 in San Diegosup1/sup), the LargeSynoptic Survey Telescope (LSST) utilizes a three-mirror design in which the primary (M1) and tertiary (M3) mirrorsare 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 ObservatoryMirror Lab. We will provide an update to the status of the mirrors and metrology systems, which have advanced fromconcepts to hardware in the past two years. In addition to the normal requirements for smooth surfaces of the appropriateprescriptions, 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 astructure function, related to seeing in excellent conditions. Both the pointing and centration of the two optical axes areimportant parameters, in addition to the axial spacing of the two vertices. This paper details the manufacturing processand metrology systems for each surface, including the alignment of the two surfaces. M1 is a hyperboloid and can utilizea standard Offner null corrector, whereas M3 is an oblate ellipsoid, so it has positive spherical aberration. The nullcorrector 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 looseabrasivegrinding.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.