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April 2010  •  Volume 3 Number 1

M1/M3 Mirror Update

LSST mirror blank on the Large Optical Generator

A mid-March visit to the Steward Observatory Mirror Lab shows the exciting work underway on the LSST M1/M3 mirror with the Large Optical Generator (LOG): diamond generating the front surface curvature of the combined mirror surfaces.

Grinding wheel removing glass from the LSST tertiary mirror (Photo courtesy D. Ketelsen, SOML)

Recall the primary (M1) and tertiary (M3) mirrors are built into a single piece of glass, like two concentric rings, with the outer 8.4-m primary mirror enclosing the 5.0-m tertiary mirror. It will be necessary to remove nearly 5 tons of glass to achieve the approximate shape before polishing can begin. In late February, work began on the M1 surface; attention now has shifted inward to the M3 surface, removing 2 cubic centimeters of glass per second while generating the steeply curved shape to within 5mm of its final thickness. Working from the outer edge to the center, it’s a bit like the needle on a record player spiraling inward, removing a ridge of material in a circular pattern as you work your way around.

Reflections of B. Gressler and S. Jacoby can be seen in the front mirror surface; photo by D. Sweeney.

The rough generating work shown here is done with a rotating wheel of 20 diamond “pellets”, seen as shiny “teeth” on the underside of the black grinding wheel. A subsequent pass will use a wheel with finer pellets, followed by another pass with resin-bond pellets, each pass smoothing the surface a bit more. Coolant is applied through the blue tubes as the grinding proceeds. The flash image above freezes the wheel rotation to show the diamond pellets and coolant droplets. The safety roller to the right provides real-time feedback to the computer of the glass position.

All this grinding takes place behind the plastic baleen curtain shown at left, while the mirror substrate slowly rotates about once per minute. The generating of the M1/M3 figure should be finished by early May; then on to the grinding and polishing!

Suzanne Jacoby, Dean Ketelsen, and Bill Gressler contributed to this article.


LSST is a public-private partnership. Funding for design and development activity comes from the National Science Foundation, private donations, grants to universities, and in-kind support at Department of Energy laboratories and other LSSTC Institutional Members:

Brookhaven National Laboratory; California Institute of Technology; Carnegie Mellon University; Chile; Cornell University; Drexel University; Google Inc.; Harvard-Smithsonian Center for Astrophysics; Institut de Physique Nucléaire et de Physique des Particules (IN2P3); Johns Hopkins University; Kavli Institute for Particle Astrophysics and Cosmology at Stanford University; Las Cumbres Observatory Global Telescope Network, Inc.; Lawrence Livermore National Laboratory; Los Alamos National Laboratory; National Optical Astronomy Observatory; Princeton University; Purdue University; Research Corporation for Science Advancement; Rutgers University; SLAC National Accelerator Laboratory; Space Telescope Science Institute; The Pennsylvania State University; The University of Arizona; University of California, Davis; University of California, Irvine; University of Illinois at Urbana-Champaign; University of Michigan; University of Pennsylvania; University of Pittsburgh; University of Washington; Vanderbilt University

LSST E-News Team:

  • Suzanne Jacoby (Editor-in-Chief)
  • Anna Spitz (Writer at Large)
  • Mark Newhouse (Design & Production: Web)
  • Emily Acosta (Design & Production: PDF/Print)
  • Sidney Wolff (Editorial Consultant)
  • Additional contributors as noted

LSST E-News is a free email publication of the Large Synoptic Survey Telescope Project. It is for informational purposes only, and the information is subject to change without notice.

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