M1 Assembly

1
M1 Assembly

• Ron Price
• August 25, 2003

2
M1 Assembly Functional Requirements

• 4 meter diameter clear aperture
• M1 surface figure quality 32 nm rms
• Operating conditions
• Gravity Orientations - zenith angle of 0 to 80
• Thermal Conditions solar load and diurnal temp
• Wind Loading wind speeds up to 5 m/sec
• Interfaces
• Optical Support Structure (OSS) of the telescope
• M1 Lifter
• Lifting Cart
• Telescope Control System
• Utility Service

3
M1 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.
• Polishing of the 4 meter off-axis asphere
• Performance of M1 under wind loading
• Thermal control of M1 due to solar loading and
  diurnal temperature changes

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M1 AssemblyMajor Components (cont)
Thermal Control Air Jet System
Aperture Stop
M1
M1 Lateral Supports
M1Axial Supports
Thermal Control Heat Exchangers
M1 Cell
5
M1 Blank

• Configuration
• Diameter 4.24 meters
• Thickness Constant 100 millimeters (almost)
• Edge perpendicular to optical surface
• Material
• Selection of material is driven by large
  temperature gradients from front to back due to
  solar loading and thermal control
• Ultra-low expansion (050 x 10-9 /C) fused
  silica or glass-ceramic is needed to maintain
  optical figure under these conditions
  
• Material Choices
• Corning ULE (3144 kg)
• Schott Zerodur (3598 kg)

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M1 Blank Physical Configuration
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M1 Blank Thickness

• M1 blank thickness is a trade-off based on
  several competing factors

Blank Thickness 100 mm

• Thicker
• Less support print-thru
• Increased weight
• Increased thermal inertia
• Increased resistance to wind buffeting
• Lower handling stress
• Higher resonant frequency

• Thinner
• More support print-thru
• Decreased weight
• Decreased thermal inertia
• Decreased resistance to wind buffeting
• Higher handling stress
• Lower resonant frequency

8
M1 Blank Fabrication Methods

• Corning ULE
• Production of boules
• Edging to hexagonal shape
• Fusing hexes into monolithic flat blank
• Grind plano-plano
• Slumping blank over convex refractory mold to
  rough shape
• Generating to near net shape
• Delivery time 18 months

• Schott Zerodur
• Pouring of glassy material into mold
• Annealing
• Rough shaping
• Ceramizing of blank into glass-ceramic state
• Generating to near net shape
• Delivery time approximately 30 months

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M1 Procurement Schedule

• General Discussions/Visits to Blank
  Fabricators 2 months
• Prepare/Issue RFP for M1 Blank 1 month
• Contractor Response Time 2 months
• Source Selection Process 1 month
• Contract Negotiations/Approval/Award 2 months
• M1 Blank Fabrication 20 months
• M1 Blank Generation 6 months
• Acid Etching of rear and sides of M1 blank 1
  month
• Transportation of M1 blank to polisher 1 month
• Grinding/Polishing/Testing 30 months
• Transportation of finished M1 to site 1 month
• Integration of M1 into M1 cell 2 months
• Coating of M1 1 month
• Total time required 70 months
• (6 years)

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M1 Blank Status

• Completed M1 has the longest lead time of any
  single component - 5 to 6 years
• If M1 blank procurement is delayed until
  construction phase, M1 becomes critical path for
  telescope construction
• Ongoing discussions with Schott and Corning
• ROMs for cost and schedule provided
• Effort being made to obtain funding for early
  procurement of M1 blank

11
M1 Polishing Specifications

• Preliminary specifications have been developed
  that meet the error budget allocation of 32 nm
  rms surface figure
• Surface Shape Off-axis Paraboloid
• Conic Constant K -1.000
• Radius of Curvature 16,000 50 mm (f/2)
• Surface Roughness 20 A rms or better

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Required Optical Tests

• Full Aperture Interferometry
• ?632.8 nm
• Null corrector lens
• Pixel size lt 18 mm
• Sub-Aperture Interferometry
• Pixel size lt 2-4 mm
• Conic constant and paraxial radius of curvature
  shall be verified using a completely independent
  test method that does not utilize a null
  corrector lens
• Surface roughness
• M1 to be supported and tested on actual system
  support hardware or equivalent

13
M1 Polishing - Risk Reduction

• Off-axis highly aspheric surface of ATST M1 was
  identified early as a potential risk area
• ATST contracted with four firms in August 2002 to
  produce Polishing Feasibility Studies
• Brashear LP - Pittsburgh, PA
• Rayleigh Optical - Baltimore, MD
• SAGEM/Reosc - Paris, France
• U of A/Steward Obs. Mirror Lab - Tucson, AZ
• Goodrich Inc. also provided equivalent study
  information at a briefing January, 2003

14
M1 Polishing Feasibility Study Results

• All studies noted the substantial aspheric
  departure from a best fit sphere of the ATST M1,
  but none noted it as a high risk area
• Variety of existing polishing methods exist to
  handle the high slope differences
• Small laps to maintain contact over high slope
  areas
• Deformable or stressed laps to conform to
  surface
• Computer controlled polishing
• Testing and independent verification of optical
  surface figure and characteristics is probably
  most challenging area development of a suitable
  null corrector lens was noted as a significant
  task by all studies
• No show-stoppers
• Reasonable cost and schedules proposed

15
M1 Aperture Stop

• Functional Requirements
• Absorb and remove solar load surrounding M1
• Define clear aperture of M1
• Mounted above optical surface of M1 perpendicular
  to the geometrical axis of M1

Aperture Stop
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M1 Support System

• Functional Requirements
• Mirror Support - Support M1 weight and maintain
  nominal surface figure over operational zenith
  angles and thermal environments
• Mirror Defining - Control the position and
  orientation of M1
• Active Optics - Vary the axial forces on M1 to
  control its surface figure during operation

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M1 Active Optics Requirements

• Functional Requirements
• Maintain M1 surface figure over 0 to 80 zenith
  angle
• Compensate for M1 figure errors due to polishing
• Compensate for M1 figure errors caused by thermal
  gradients in primary mirror
• Compensate for variations in M1 coating thickness
• Compensate for M2 figure errors due to polishing
• Compensate for changes in M2 figure as a function
  of zenith angle and thermal gradients
• Compensate for changes in shape of M1 cell
• Performance Requirements and an active force
  budget will be developed allocating force levels
  to each of the above areas

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M1 Support Points

• Axial Supports
• Configuration - 120 support points arranged in
  five concentric rings on back of M1
• Lateral Supports
• Configuration - 6 support points equally spaced
  around periphery of M1

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M1 Support System
Axial Supports (120 in 5 concentric rings)
Lateral Supports (6 equally spaced around mirror)
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M1 Support System Actuators

• Axial Support Actuators Design Options
• Passive/Active System
• Passive hydraulic 3 zone system with superimposed
  forces for active optics control
• Completely Active System
• Electro-mechanical actuators
• Lateral Supports
• 6 passive links between edge of M1 and M1 cell
• All active optics correction will be applied
  through axial support actuators

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M1 Orientation vs Zenith Angle
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Support System Optimization

• Finite element model of M1 was developed to
  analyze the effect of each axial support actuator
• Axial support ring locations and forces were
  optimized to minimize deflection of optical
  surface
• Performance of lateral support system at horizon
  pointing was analyzed correction forces were
  applied by active axial support actuators

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M1 Finite Element Model

• Finite Element model
• One half mirror model
• 1260 thin shell elements
• 1248 nodal points
• Analysis performed by Dr. Myung Cho of NOAO New
  Initiatives Office/GSMT Project

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Axial Support System Performance

• Axial support print-through
• P-V 90 nm surface
• RMS 18 nm surface
• Optimized axial support forces
• support forces between 180 N and 320 N
• Optimized support radial locations
• Ring 1 0.332 m
• Ring 2 0.774 m
• Ring 3 1.193 m
• Ring 4 1.584 m
• Ring 5 1.967 m

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Axial Support System Performance
Low
M1 in zenith pointing position Support
print-thru will be polished out
High
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Lateral Support System Performance

• Lateral support system
• Six (6) supports equally spaced around the edge
• Lateral support forces
• nominal lateral support force 6000 N
• Surface P-V 36 microns
• Active optics corrections (aO)
• P-V 63 nm surface
• RMS 6 nm surface
• maximum active force required 186 N

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Lateral Support System Performance
Low
M1 in horizon pointing position
High
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Lateral Support Local Effects

• Lateral supports
• 6000 N nominal forces (at 6 locations)
• Cause localized deformations due to Poisson
  effect
• Max. local deformation of 270 nm at the lateral
  supports (red and blue spots)

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Lateral Support Local Effects (cont)
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Stress in M1 Substrate due to Lateral Support Pads

• Lateral support force
• 6000 N nominal force
• Lateral support pad
• 50 x 180 x 5mm stainless steel
• Von Mises stress
• 1.5 Mpa (200 psi)

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M1 Wind Loading

• Uniform Wind Loading not a problem for the M1
  support system because it is a very small
  fraction of the mirror weight at low velocities
• Non-Uniform Wind Loading
• lt 0.05 hz - Active optics system can compensate
  for quasi-static wind loads
• gt 0.05 hz Beyond range of active optics
  compensation must be reacted by stiffness of M1
  and support system or attenuated by enclosure

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Gemini South Studies

• Extensive wind related measurements were made
  during commissioning of Gemini South
• These measurements provided
• Wind velocity and wind pressure at M1
• Structure functions for the time-varying pressure
  patterns on M1 as a function of wind angle of
  attack, zenith angle and vent positions
• Based on this data, M1 surface deformation can be
  estimated as a function of pressure variation and
  wind speed

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Application of Data to ATST
Assuming a 3-zone hydraulic mirror support, ATST
M1 deformation under wind loading may be
determined by the scaling law D4 / t³. For 10 m/s
average wind Gemini D8m, t0.2m Deformation0.
65µ rms ATST D4m, t0.1m Deformation0.325µ rms
Predicted ATST performance
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Predicted Baseline Performance

• Gemini allowed a max wind-induced M1 surface
  deformation of 60 nm rms which limited the
  average wind velocity over the mirror to 3 m/s
• Assuming 60 nm limit and scaling, this would
  allow a maximum average wind velocity at the ATST
  M1 of about 5 m/s
• Assumes a 3 zone hydraulic whiffle tree support
  system with 120 axial supports as the ATST
  baseline M1 support

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Options for Improving Wind Buffeting Performance

• Modifications to 3-zone hydraulic whiffle-tree
  support
• Add damping to improve M1 stiffness
• Add six-zone mode capability for higher wind
  conditions
• 120 discrete actuators
• Baseline design for the SOAR telescope
• Could increase M1 stiffness by as much as a
  factor of 4, allowing a max average wind velocity
  of 10 m/s for 60 nm rms surface deformation
• M1 cell deformations directly affect M1 surface
  figure

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M1 Cell

• Functional Requirements
• Stiff
• Serves as a base for support system components,
  thermal control hardware and cleaning/washing
  hardware
• Interfaces to telescope Optical Support Structure
• Configuration
• Welded steel structure
• Honeycomb pattern to provide maximum stiffness
  for support actuators

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M1 Cell (cont)
Mounting interface to OSS
Actuators located within pockets
Internal Rib Structure
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M1 Safety Restraint System

• Requirement - the M1 Restraint System provides
  protection of the primary mirror in the event of
  shock and vibration due to seismic activity.
• Configuration safety clips around periphery of
  M1

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M1 Cleaning and Washing

• Requirements
• Daily cleaning of M1 with CO2 snow
• Periodic in-situ washing of M1
• Cleaning
• CO2 dispersal device will be attached to the M1
  cover for cleaning at the beginning of each day
• Telescope near horizon pointing
• Washing
• Telescope near horizon pointing
• Sealing system around periphery of M1
• Liquid effluent collected at lower edge of M1
• Resource
• Gary Poczulp, NOAO Coating Supervisor, is serving
  as a consultant to ATST on these issues.

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M1 Cleaning Concept

• Horizon pointing position
• CO2 snow applied as mirror cover opens
• Particulates collected at lower edge of mirror

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M1 Washing Concept
Edge seal around M1

• Telescope moved into position and equipment
  installed
• M1 washed and rinsed
• Liquid effluent collected at lower edge of mirror

Collection trough
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M1 Control System

• General Functional Requirements
• Control application of active forces to M1.
• Control M1 thermal management system
• Provide relevant and timely status information.
• Interface to the TCS, GIS, and OCS.
• Protect personnel and equipment.
• Provide an engineering console and a simulation
  mode.
• General Performance Requirements
• Accept input mirror figure information at up to
  10 Hz.
• Blend and average mirror figure information at up
  to 0.1 Hz.
• Control temperature of front side of M1 and
  aperture stop to within 1C of ambient.
• Store and apply a 24 hour thermal profile
  estimation.
• Provide status information at up to 10 Hz.
• Respond to interlock conditions within 1 second.

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Industry Participation

• RFPs were issued in July for Design Evaluation
  and Cost Studies of M1 and M2 Assemblies
• Contracts issued to three firms
• EOS Technologies Tucson, AZ
• Goodrich Corporation Danbury, CT
• SAGEM/Reosc St Pierre du Perray, France
• Kick-off to these contracts at this CoDR
• Studies will be completed by November 15 2003 for
  incorporation into ATST Construction Proposal