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A Stress Analysis of Sputter Coating on the LSST Primary-Tertiary Mirror

by Blake Coughenour. A Stress Analysis of Sputter Coating on the LSST Primary-Tertiary Mirror. A Summary of Results from. Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber by Jacques Sebag et al.

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A Stress Analysis of Sputter Coating on the LSST Primary-Tertiary Mirror

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  1. by Blake Coughenour A Stress Analysis of Sputter Coatingon the LSST Primary-Tertiary Mirror A Summary of Results from Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber by Jacques Sebag et al.

  2. Scientific Concepts of LSST • 6 Band: 300nm-1.1μm • Wide Field: f/1.25 viewing 9.62 deg2 (50x moon area) • Camera: 3200 MP • Data: 30 TB of data nightly

  3. Reflective Optical Coatings

  4. Common Coating Processes • Aluminum Evaporation • Well Tested, Cheap, Simplest Process • Recoated every 12-18 months • Loss of throughput for Visible Observations • Silver Sputtering • Allows for Multi-Layered Coating with Thickness Control • Recoated every 12 months • High throughput for Visible-IR observations • Low Absorption yields Low Thermal Background noise "The main advantage of silver is that it reduces the total thermal emission of the telescope. This in turn increases the sensitivity of the mid-infrared instruments on the telescope and allows us to see warm objects like stellar and planetary nurseries significantly better” - Scott Fisher , Gemini Scientist

  5. Evaporation vs. Sputtering Figures: http://ns.kopt.co.jp/English/ca_jou-gi/joutyaku.html

  6. Sputtering Procedure

  7. Monolithic Borosilicate Mirrors

  8. Temperature Stress Recommended by SOML: 5°C Maximum

  9. Test System Tested 28mm Borosilicate Samples with Thermocouples and Strain Gauges in Gemini Observatory Coating Chamber

  10. Temperature Measurements

  11. Temperature Extrapolation

  12. Strain Measurements Samples had strain gauges mounted on the back surface to measure strain directly (Thermal + Mechanical)

  13. Summary • A Cast Borosilicate Mirror can be safely coated with Sputtering Deposition Techniques • Max Temperature Variation below 5°C • Resulting Stresses below 100psi Tensile Stress Acknowledgements: Jacques Sebag, John Andrew, Douglas Neill, Michael Warner, Blain Olbert, and Roger Angel

  14. Questions?

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