Optical testing of the LSST combined primary/tertiary mirror

Michael T. Tuell, Hubert M. Martin, James H. Burge, William J. Gressler, Chunyu Zhao

Research output: Chapter in Book/Report/Conference proceedingConference contribution

21 Scopus citations


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. 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. 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 describes the basic metrology systems for each surface, with particular attention to the alignment of the two surfaces. These surfaces are aspheric enough to require null correctors for each wavefront. Both M1 and M3 are concave surfaces with both non-zero conic constants and higher-order terms (6th order for M1 and both 6th and 8th orders for M3). M1 is hyperboloidal and can utilize a standard Offner null corrector. M3 is an oblate ellipsoid, so has positive spherical aberration. We have chosen to place a phase-etched computer-generated hologram (CGH) between the mirror surface and the center-of-curvature (CoC), whereas the M1 null lens is beyond the CoC. One relatively new metrology tool is the laser tracker, which is relied upon to measure the alignment and spacings. A separate laser tracker system will be used to measure both surfaces during loose abrasive grinding and initial polishing.

Original languageEnglish (US)
Title of host publicationModern Technologies in Space- and Ground-Based Telescopes and Instrumentation
StatePublished - 2010
EventModern Technologies in Space- and Ground-Based Telescopes and Instrumentation - San Diego, CA, United States
Duration: Jun 27 2010Jul 2 2010

Publication series

NameProceedings of SPIE - The International Society for Optical Engineering
ISSN (Print)0277-786X


OtherModern Technologies in Space- and Ground-Based Telescopes and Instrumentation
Country/TerritoryUnited States
CitySan Diego, CA


  • Alignment
  • Aspheric surface
  • Computer-generated hologram
  • Interferometry
  • Laser tracker
  • Metrology
  • Null corrector
  • Survey telescope

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics
  • Computer Science Applications
  • Applied Mathematics
  • Electrical and Electronic Engineering


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