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Loose Abrasive Grinding of the M1/M3
The LSST M1/M3 optic is now in the final phase of loose abrasive grinding as optical processing continues at the Steward Observatory Mirror Laboratory (SOML). Damage to the mirror sustained during the fixed abrasive grinding has been repaired, completely restoring the optical and mechanical properties of the mirror.
These photos show the 1.2-m diameter stress lap attached to the Large Optical Generator (left) and (right) processing the M1 surface, where the first few hours of loose abrasive grinding removed the fixed grinding tool marks.
Image credit: B. Gressler/NOAO/LSST (left) & J. Kingsley/SOML/UA/LSST (right)
The wet slurry of 40-micron grit particles is currently in use as the computer controlled lap is actively shaped during the coordinated movement to remove material and converge towards the final mirror surface shape. This phase also provides a “shining” of the surface that enables a thorough inspection of the face sheet internal glass quality that has revealed no unexpected scratches, bubbles, or inclusions which would affect continued processing. Also during this phase, additional measurements are made to ensure the wedge (or tilt), the vertex position, and concentricity of the optical surface are all within specification relative to the substrate and the M3 surface.
The SOML processing plan calls for approximately 200 hours of grinding with this grit size before transitioning to a 20-micron particle size, and then to a 12-micron size. Throughout these iterative grinding steps, the surface is measured to guide the removal process and maintain a convergence on the final specifications and prepare it for the final polishing phase. Each grinding phase produces a progressively smoother surface with a shape closer to the final expected form. Processing of the M1 surface through these grit sizes is expected to take a little over 3 months. Once the M1 surface is ready for polishing, the lap will be re-tooled to enable a similar sequence of grit size processing on the M3 surface.
Article written by B. Gressler
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:
Adler Planetarium; Brookhaven National Laboratory (BNL); California Institute of Technology; Carnegie Mellon University; Chile; Cornell University; Drexel University; Fermi National Accelerator Laboratory; George Mason 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 (KIPAC) – Stanford University; Las Cumbres Observatory Global Telescope Network, Inc.; Lawrence Livermore National Laboratory (LLNL); Los Alamos National Laboratory (LANL); National Optical Astronomy Observatory; Princeton University; Purdue University; Research Corporation for Science Advancement; Rutgers University; SLAC National Accelerator Laboratory; Space Telescope Science Institute; Texas A & M University; The Pennsylvania State University; The University of Arizona; University of California at Davis; University of California at Irvine; University of Illinois at Urbana-Champaign; University of Michigan; University of Pennsylvania; University of Pittsburgh; University of Washington; Vanderbilt University
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