M2 Assembly and Feed Optics

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M2 Assemblyand Feed Optics

• Ron Price
• August 25, 2003

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M2 Assembly Functional Requirements

• 60 cm diameter concave reflective optical
  surface.
• M2 surface figure quality 32 nm rms after active
  optics correction
• Six degree of freedom positioning of M2
• Fast tip-tilt motion of M2
• Operating Conditions
• Gravity Orientations - zenith angle of 0 to 80
• Thermal Conditions solar load and diurnal temp
• Wind Loading wind speeds up to 10 m/sec
• Interfaces
• Optical Support Structure (OSS) of the telescope
• Secondary Mirror Lifter
• Telescope Control System
• Utility Service

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M2 Assembly Critical Areas

• Several areas were identified early as exhibiting
  somewhat higher risk. Consequently, more time and
  effort has been directed into these areas to
  resolve the issues as much as possible.
• Surface figure change and thermal control of M2
  due to solar loading and diurnal temperature
  changes
• Print-thru of substrate structure due to solar
  loading
• Manufacturability of SiC substrate

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M2 AssemblyMajor Components
Hexapod Support
Tip-Tilt Mechanism
Area for thermal control
Flexure Mounts
M2
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M2 Blank

• Configuration
• Diameter 62 cm nominal
• Thickness
• Central area 75 mm
• Edges taper to 30 mm
• Lightweighted structure triangular pockets with
  5mm face sheet and rib thickness

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M2 Environment Drives Blank Requirements

• Thermal Environment
• Print-thru of the ribbed structured onto the
  optical surface due to solar load
• Surface figure change due to solar load, diurnal
  temperature change and thermal control
• Desire to have optical surface of M2 track
  ambient temperature
• Mechanical Environment
• Surface figure change due to changing gravity
  vector
• Surface figure change due to fast tip-tilt motion

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Comparison of M2 Candidate Materials
  ? E k c ? a   gm/cm³ 10¹º W/m/K J/kg/K - ppm/
K Ek/a k/?/c E/?   N/m² ULE 2.21 6.76 1.31 767
.170 .03 295.6 .79 30.6 Zerodur 2.53 9.06 1.65 81
2 .120 .05 124.5 .80 35.8 Beryllium
1.85 30.40 220.00 1820 .025 11.20 597.1 65.00 164.
3 SiC 3.10 46.60 200 700 .280 2.40 3883.3 89.00 15
0.3      Ek/a is a measure of resistance to
bending due to thermal transients k/?/c is
called thermal diffusivity E/? is referred to as
specific stiffness   ? density c specific
heat   E - Elastic modulus ? Poissons ratio
k - thermal conductivity a thermal expansion
coefficient         
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M2 Thermal Mechanical Analysis

• Thermal analysis of M2 is being performed to
• Quantify the level of rib structure print-thru on
  optical surface due to thermal gradients caused
  by solar loading
• Evaluate the baseline cooling methods for their
  ability to control M2 surface temperature under
  solar loading and track diurnal temperature
  changes
• Evaluate global surface figure changes
• Baseline cooling uses ambient temp air jets into
  pockets on back of M2

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M2 Analysis Finite Element Model
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M2 Thermal Mechanical Analysis - Preliminary
Results

• Print-thru less than 10 nm with SiC or Zerodur
• Tracking of ambient air temperature within 0.9
  to 1.3 deg C for SiC depending on cooling flow
  rates

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SiC M2 Thermal Profile-no air jet under flexure
M2 temperature profile for SiC substrate during
peak solar load - 0.14 C range 0.9 C above
ambient
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SiC M2 Surface Profile no air jet under flexure
Global figure change for SiC substrate during
peak solar load - 680 nm P-V
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SiC M2 Surface Profile uniform air jet cooling
Global figure change for SiC substrate during
peak solar load - 40 nm P-V
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M2 Substrate Material

• Although other materials have excellent
  performance in specific areas, silicon carbide
  appears to have the best overall performance in
  all of the required areas
• Further analysis will be performed, including
  dynamic performance under tip-tilt conditions, to
  confirm silicon carbide as the best substrate
  material
• Demonstrated capability by several vendors in
  silicon carbide at the 65 cm scale

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SiC Fabrication Methods

• CVD (Chemical Vapor Deposition) - Gaseous
  chemicals react on a heated surface (usually
  graphite mandrel) to form solid SiC.
• Reaction Bonded SiC - SiC slurry is poured into a
  mold, freeze dried, sintered to form a porous SiC
  structure, then subjected to a high temperature
  process that introduces silicon and results in
  high density.
• Direct Sintered SiC- Very small SiC particles are
  Cold Isostatically Pressed into shape, machined,
  then sintered at 2500 deg C.
• Hot Pressed SiC - Very small SiC particles are
  Hot Isostatically Pressed (HIgh temp and
  Pressure) into shape.
• C/SiC - Carbon felt made of short randomly
  oriented carbon fibers is machined to shape. This
  green body is heated in a vacuum and
  infiltrated with liquid silicon this results in
  a silicon carbide matrix.

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M2 Blank Status

• Currently evaluating vendors and processes for
  SiC substrates
• Boostec (CoorsTek USA) - Direct Sintered SiC
• ECM (GE Power System Composites USA) - C/Sic
• POCO Graphite - Reaction Bonded
• Xinetics - Reaction Bonded
• Obtaining material properties and ROM cost and
  schedule estimates

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M2 Polishing Specifications

• Surface Parameters
• Surface Shape Off-axis Ellipsoid
• Conic Constant K -0.53936
• Radius of Curvature -2,081.259
• Surface Roughness 20 A rms or better
• Preliminary specifications are being developed
  that meet the error budget allocation for surface
  figure yet allow large spatial frequency errors
  correctible by the active optics system

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M2 Support System Functional Requirements

• Functional Requirements
• Mirror Support - Support M2 weight and minimize
  surface figure changes over operational zenith
  angles
• Mirror Defining - Control the position and
  orientation of M2
• Provide fast tip-tilt motion
• Configuration
• Commercial hexapod to provide basic positioning
  motion
• Custom Tip-Tilt mechanism will mount on hexapod
  to provide fast image motion compensation
• M2 will attach to tip-tilt mechanism
  kinematically via three flexures

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M2 Support System Performance Requirements

• Performance Requirements
• Positioning
• Six degrees of freedom 1 micron accuracy
• Range of motion 5 to 10 mm
• Tip-Tilt
• Amplitude 10 arc seconds
• Rate 10 hz goal of 25 hz

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Safety Restraint System

• Requirement - the M2 Restraint System provides
  protection of the primary mirror in the event of
  shock and vibration due to seismic activity.
• Configuration safety restraints between rear of
  M2 structure and tip-tilt mechanism

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M2 Control System

• Functional Requirements
• Control M2 positioning system
• Control M2 thermal management system
• Control M2 fast tip-tilt system
• Interface to AOCS
• Interface to TCS

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Feed Optics
From M2
M4
M6
M3
M5
To coude
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Feed Optics

• M3 Flat Fold
• 12 cm
• Heat Load 27.2 watts
• M4 Concave Ellipsoid
• 34 cm
• Heat Load 24.5 watts
• M5 - Deformable Mirror Device
• 33 cm
• Heat Load 22.1 watts
• M6 Flat Fold
• 26 cm
• Heat Load 19.9 watts

M2 Thermal and Mechanical Analysis will be
extended to Feed Optics to determine optimum
substrate and configuration for each optic