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 % nm Sensitive deposition necessary to prevent "Blueing.” Thin layer of protective Al 2 O 3 forms on surface. Gold94 % / 98 % nm 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 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:

Sputtering Procedure

Monolithic Borosilicate Mirrors Thermal Characteristics Thermal Diffusivity (D)1.11 x10 -7 m 2 /s CTE (  ) 2.8 ppm/K Specific Heat (c) 710 J/kg  C Strength Characteristics Young’s Modulus58.6 Gpa (8.5 Msi) Typical Breaking Strength13.8 Mpa (2000 psi) Max Tensile Stress0.7 Mpa (100 psi) Max Polished Surface Stress2.1 Mpa (300 psi)

Temperature Stress Recommended by SOML: 5°C Maximum

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

Temperature Measurements

Temperature Extrapolation

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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