Presentation on theme: "A Stress Analysis of Sputter Coating on the LSST Primary-Tertiary Mirror Substrate temperature and strain during sputter deposition of aluminum on cast."— Presentation transcript:
A Stress Analysis of Sputter Coating on the LSST Primary-Tertiary Mirror Substrate temperature and strain during sputter deposition of aluminum on cast borosilicate glass in a Gemini Observatory coating chamber by Jacques Sebag et al. by Blake Coughenour A Summary of Results from
Scientific Concepts of LSST 6 Band: 300nm-1.1μm Wide Field: f/1.25 viewing 9.62 deg 2 (50x moon area) Camera: 3200 MP Data: 30 TB of data nightly
Reflective Optical Coatings MetalsAve Refl Vis / IRHigh AbsorptionComments Aluminum92 % / 98 %700-950nm Sensitive deposition necessary to prevent "Blueing.” Thin layer of protective Al 2 O 3 forms on surface. Gold94 % / 98 %200-550nm Adhesion issues with glass. Very soft surface. Use Chrome as binderlayer. Silver95 % / 98 %UV Issues with tarnishing. Very soft surface.
Common Coating Processes Aluminum Evaporation Well Tested, Cheap, Simplest Process Recoated every 12-18 months Loss of throughput for Visible Observations "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 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
Evaporation vs. Sputtering Thin Film EvaporationThin Film Sputtering Pros Fast deposition; Heats Whole Substrate Surface; Good history. Better adhesion – lasts longer; Target can supply large amounts of material Cons Filament limits amount of material; Non-uniform adhesion and thickness Slow deposition rate; Ballistic atoms heat up substrate surface locally Use Borosilicate Monolithic MirrorsULE Monolithic Mirrors and Zerodur Figures: http://ns.kopt.co.jp/English/ca_jou-gi/joutyaku.html
Strain Measurements 10 kW40 kWTensile LimitSurface Limit Back Surface14psi32psi100 psi300 psi Front Surface24psi54psi100 psi300 psi Samples had strain gauges mounted on the back surface to measure strain directly (Thermal + Mechanical)
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
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