M1 Assembly

Ron Price

August 25, 2003

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

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

M1 AssemblyMajor Components (cont)

Aperture Stop

M1

M1 Cell

Thermal ControlAir Jet System

M1 LateralSupports

M1AxialSupportsThermal Control

Heat Exchangers

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 (0±50 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)

M1 Blank Physical Configuration

Ø12100 PARENT PARABOLOID

4000

(4100)

100

ALL DIMENSIONS IN MM.

Ø4237

SURFACE A

SURFACE B

ATST PRIMARY MIRROR

PARENT PARABOLOID

GEOMETRICAL AXIS OF ATST PRIMARY MIRROR BLANK

AXIS A,GEOMETRICAL AXIS OF

PARENT PARABOLOID

A A

SECTION A-A

M1 Blank Thickness

• Thinner:

More support print-thru

Decreased weight

Decreased thermal inertia

Decreased resistance to wind buffeting

Higher handling stress

Lower resonant frequency

• Thicker:

Less support print-thru

Increased weight

Increased thermal inertia

Increased resistance to wind buffeting

Lower handling stress

Higher resonant frequency

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

Blank

Thickness

100 mm

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

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)

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• ROM’s for cost and schedule provided• Effort being made to obtain funding for early

procurement of M1 blank

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

Required Optical Tests

• Full Aperture Interferometry– λ=632.8 nm– Null corrector lens– Pixel size < 18 mm

• Sub-Aperture Interferometry– Pixel size < 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

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

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

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

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

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

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

M1 Support System

Lateral Supports(6 equally spacedaround mirror)

Axial Supports(120 in 5concentricrings)

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

M1 Orientation vs Zenith Angle

ELEVATION AXIS

ZENITH ANGLE 80°

ZENITH ANGLE 0°

ZENITH ANGLE 76°

ATST PRIMARY MIRROR MOTION - ALT/AZ CONFIGURATION

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

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

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

Axial Support System Performance

M1 in zenith pointing position

Support print-thru will be polished out

Low

High

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

Lateral Support System Performance

M1 in horizon pointing position

Low

High

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)

Lateral Support Local Effects (cont)

Deformation measured from the edge

0

50

100

150

200

250

300

0 28 59 89 119

distance from the edge (mm)

Z-d

efo

rmat

ion

(n

m)

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)

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– < 0.05 hz - Active optics system can compensate for

quasi-static wind loads– > 0.05 hz – Beyond range of active optics

compensation; must be reacted by stiffness of M1 and support system or attenuated by enclosure

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

Application of Data to ATST

Mirror Deformation vs Wind Speed

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.0 2.0 4.0 6.0 8.0 10.0

Wind Speed at M1 (m/sec)

RM

S s

urfa

ce (

mic

rons

)

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: D=8m, t=0.2m

Deformation=0.65µ rms

ATST: D=4m, t=0.1m

Deformation=0.325µ rms

Predicted ATST performance

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

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

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

M1 Cell (cont)

Mounting interface to OSS

Internal Rib Structure

Actuators located within pockets

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

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.

M1 Cleaning Concept

•Horizon pointing position

•CO2 snow applied as mirror cover opens

•Particulates collected at lower edge of mirror

M1 Washing Concept

Edge seal around M1

Collection trough

•Telescope moved into position and equipment installed

•M1 washed and rinsed

•Liquid effluent collected at lower edge of mirror

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 1°C 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.

Industry Participation

• RFP’s 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