Transcript presentation

A Stress Analysis of Sputter Coating
on the LSST Primary-Tertiary Mirror
by Blake Coughenour
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.
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
Reflective Optical Coatings
Metals
Ave Refl Vis / IR
High Absorption
Comments
Aluminum
92 % / 98 %
700-950nm
Sensitive deposition necessary to prevent "Blueing.”
Thin layer of protective Al2O3 forms on surface.
Gold
94 % / 98 %
200-550nm
Adhesion issues with glass. Very soft surface.
Use Chrome as binderlayer.
Silver
95 % / 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
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
Evaporation vs. Sputtering
Thin Film Evaporation
Thin 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 Mirrors
ULE Monolithic Mirrors and Zerodur
Figures: http://ns.kopt.co.jp/English/ca_jou-gi/joutyaku.html
Sputtering Procedure
Monolithic Borosilicate Mirrors
Thermal Characteristics
Strength Characteristics
Young’s Modulus
58.6 Gpa (8.5 Msi)
Typical Breaking Strength
13.8 Mpa (2000 psi)
Max Tensile Stress
0.7 Mpa (100 psi)
Max Polished Surface Stress
2.1 Mpa (300 psi)
Thermal Diffusivity (D)
1.11 x10-7 m2/s
CTE ()
2.8 ppm/K
Specific Heat (c)
710 J/kg C
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
Samples had strain gauges mounted on the back surface to
measure strain directly (Thermal + Mechanical)
10 kW
40 kW
Tensile Limit
Surface Limit
Back Surface
14psi
32psi
100 psi
300 psi
Front Surface
24psi
54psi
100 psi
300 psi
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
Questions?