Giant Segmented Mirror Telescope
L. M. Stepp, G. Z. Angeli, S. C. Barden, M. K. Cho, B. L.
Ellerbroek , P. E. Gillett, B. Gregory, S. E. Strom
OSA Conference on Optical Fabrication and Testing
May 3, 2002
AURA New Initiatives Office
Extremely Large Telescopes
• Astronomers are already planning telescopes larger
•
than the 6-10-meter current generation
Larger ground-based telescope will be needed to:
– Understand the origin and formation of
• Large scale structure in the Universe
• Galaxies
• Stars
• Planetary systems
– Complement other planned observing facilities
• NGST
• ALMA
• SKA
AURA New Initiatives Office
The USA Decadal Review
•
•
In May 2000, the US astronomy decadal review committee
recommended the construction of a 30-meter Giant Segmented
Mirror Telescope (GSMT) as its highest ground-based initiative
In response, AURA formed a New Initiatives Office (NIO) to support
scientific and technical studies leading to creation of GSMT
–
NIO is a joint venture of the National Optical Astronomy Observatory
(NOAO) and the Gemini Observatory
–
Goal is to ensure broad astronomy community access to a 30m
telescope contemporary with NGST and ALMA.
AURA New Initiatives Office
AURA New Initiatives Office
Approach to GSMT Design
Three Parallel efforts:
• Understand the scientific context for GSMT in NGST / ALMA era
– Develop the key science requirements
• Develop a Point Design
– Based on initial science goals & instrument concepts
• Address challenges common to all ELTs
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–
–
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Site testing and selection
Cost-effective segment fabrication
Characterization of wind loading
Hierarchical control systems
– Adaptive optics
– Cost control techniques
AURA New Initiatives Office
Science Goals Driving the Point Design
Telescope design should provide:
• High-Strehl performance over ~ arc-minute fields
– Stellar populations; galactic kinematics; chemical abundances
• High sensitivity mid-IR spectroscopy and high dynamic
range imaging
– Forming and mature planetary systems
• Wide-field, native seeing-limited multi-object spectroscopy
– Origin of large-scale structure in the universe
AURA New Initiatives Office
NIO Point Design Philosophy
The design of a next-generation telescope is a systems challenge
– Requires an integrated approach that takes advantage of the dynamic
compensation available from AO systems
The point design should:
– Be responsive to the science goals
– Help identify key technical issues
– Help define factors important to the science requirements
– Provide an opportunity to develop needed analytical methods
The point design does not need to be:
– Completely detailed
– 100% consistent
AURA New Initiatives Office
Point Design Optical System
Optical Design:
– 30-m aperture
– F/18.75
– Classical Cassegrain
Primary Mirror:
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–
–
–
–
Aspherical
Segmented
Fast focal ratio -- F/1
Hexagonal segments
Segment size -- 1.33 m across corners
Secondary Mirror:
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–
–
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Small -- 2-m diameter
Convex
Aperture stop
Adaptive
AURA New Initiatives Office
Optical Performance
Cassegrain Focus: Narrow Field
Linear diameter
of 2-arcmin
field is 0.33 m
Spot diagrams at center of field and at radius of one arc minute.
The circles indicate the Airy disk diameter for  = 2.5 microns.
AURA New Initiatives Office
Optical Performance
Cassegrain Focus: Wide Field
Linear diameter
of 12-arcmin
field is 1.96 m
Spot diagrams at center of field and at radius of 6 arc minutes.
The circle diameter is 0.5 arcsec.
AURA New Initiatives Office
Telescope Emissivity
Source
Telescope emissivity (%)
Telescope emissivity (%)
Aluminum coatings
Silver coatings
M1 coating
2.0
1.3
M2 coating
2.0
1.3
M2 obscuration
0.4
0.4
Segment joints
0.9
0.9
M2 support tripod
2.7
2.7
Total
8.0
6.6
AURA New Initiatives Office
Structural Design Concept
Based on Radio Telescope
• Lightweight steel truss structure
• M2 supported on tripod
• Elevation axis behind M1
AURA New Initiatives Office
Initial Point Design Structure
Concept developed by Joe Antebi
of Simpson Gumpertz & Heger
• Based on radio telescope
• Space frame truss
• Single counterweight
• Cross bracing of M2 support
AURA New Initiatives Office
Initial Point Design Structure
Plan View of Structure
Pattern of segments
Typical 'raft', 7 mirrors per raft
1.152 m mirror
across flats
Gemini
Special raft - 6 places, 4
mirrors per raft
Circle, 30m dia.
AURA New Initiatives Office
Initial Structural Analysis
• Total weight of elevation structure – 700 tonnes
• Total moving weight – 1400 tonnes
• Gravity deflections ~ 5-25 mm
– Primarily rigid-body tilt of elevation structure
•
Lowest resonant frequencies ~ 2 Hz
Z
X
Y
Output Set: Mode 1, 2.156537 Hz, Deformed(0.0673): Total Translation
AURA New Initiatives Office
Current Structural Concept
AURA New Initiatives Office
Instrument Locations
Prime Focus
AURA New Initiatives Office
Instrument Locations
Co-moving Cassegrain Focus
AURA New Initiatives Office
Instrument Locations
Fixed-gravity Cassegrain Focus
AURA New Initiatives Office
Instrument Locations
MCAO-fed Nasmyth Focus
AURA New Initiatives Office
Opto-mechanical Features
•
Segments grouped into rafts
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•
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7 segments per raft
9B
9C
9G
9A
16 types of rafts
9F
9D
9E
6G
6F
6E
7F
3A
3F
3E
4B
4G
4A
2D
15E
12E
5A
5F
2A
15D
15F
12D
5C
5G
2C
2F
12A
5B
4E
2G
15A
8E
4D
2B
15C
15G
12C
12F
8D
8F
4F
12B
8C
15B
14E
12G
8A
14D
14F
11E
8G
4C
14C
14A
11D
8B
7E
3D
11C
11F
7D
14B
14G
11A
7C
7A
3C
11B
11G
7B
13D
13E
10D
10E
7G
3B
3G
13F
10A
Ø30M
13C
13A
10C
10F
6D
13B
13G
10B
6C
6A
16D
16A
10G
6B
91 rafts total
16C
16B
5D
5E
2E
1
AURA New Initiatives Office
Summary of Segment Properties
• Segment dimensions
– 1.15-m across flats -- 1.33-m corner to corner
– 50 mm thickness
• Segment weight: 157 kg if Zerodur; 133 kg if ULE
• Number of segments: 618
• Maximum departure from sphere 110 microns
– Comparable to Keck
AURA New Initiatives Office
Segment Supports
• Axial support: 18-point whiffletree
– FEA Gravity deflection 15 nm RMS
• Lateral support: 3 bipods -- line of action at mid-plane
– FEA Gravity deflection 2.2 nm RMS
AURA New Initiatives Office
Stray light baffles (if required)
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•
•
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M1 baffle 13.5 m long
M2 baffle 3 m diameter
Central obscuration 3 m diameter
Fully baffle 5 arcmin diameter field
AURA New Initiatives Office
Adaptive Optics Systems
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Adaptive mirror in prime focus corrector
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Adaptive secondary mirror
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Multi-conjugate wide-field AO
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High-order narrow-field conventional AO
AURA New Initiatives Office
Direct Cassegrain AO
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Conventional AO
– Single guide star
•
Uses adaptive M2
– 2400 actuators
– 20-40 Hz
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F/18.75 image
– At Cassegrain Focus
•
Serves as first stage in
higher AO systems
AURA New Initiatives Office
MCAO
3 DMs
ADC
Off-axis Parabola
Fold Mirrors
Beam Splitter
Off-axis Parabola
WFS beams
Tip-tilt Mirror
Fold Mirrors
Optical Design by Richard Buchroeder
F/38 Focus
AURA New Initiatives Office
MCAO
• System parameters
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–
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3 DMs at conjugate ranges of 0, 5, and 10 km
5 sodium laser guide stars at center & corners of 1' square
3 natural guide stars
Diameter of DMs 0.5 m
Final focal ratio: f/38
FOV: 2 arcmin
AURA New Initiatives Office
High-performance NGS AO
Optical Design by Richard Buchroeder
AURA New Initiatives Office
Prime Focus AO System
• Corrects M1 warping and ground-level turbulence
– Achieves moderate improvement over 20-arcmin FOV
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Multiple NGS wavefront sensors
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Adaptive mirror conjugate to M1
– ~ 1000 actuators
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Tip-tilt mirror
AURA New Initiatives Office
Performance of Point Design AO Systems
AURA New Initiatives Office
Initial Instrument Concepts for GSMT
• Design concepts driven by science objectives
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–
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Multi-Object, Multi-Fiber, Optical Spectrograph MOMFOS
– Science: 3-D map of the early universe
Near IR Deployable Integral Field Spectrograph NIRDIF
– Science: deconstructing young galaxies and pre-galactic fragments
Mid-IR, High Dispersion, AO Spectrograph MIHDAS
– Science: origins of planetary systems
Near IR, AO Echelle Spectrograph NIrES
– Science: origins of planetary systems
MCAO-fed near-IR imager
– Science: stellar populations
Diffraction-Limited Near-IR Coronagraph
– Science: characterization of mature planets
AURA New Initiatives Office
Summary of Instrument Concepts
Instrument
Wavelength
Image
Resolution
Spectral
Resolution
FOV
Multiplex
MOMFOS
0.4 - 1 m
1”
2000 - 20,000
20 arcmin
700
NIRDIF
1 - 2.5 m
0.1” x 1”
5000 - 10,000
2 arcmin
26
MIHDAS
16 - 20 m
0.2” (DL)
100,000
1 arcsec
1
NIrES
1 - 5 m
0.03” (DL)
100,000
0.1 arcsec
1
MCAO
Imager
1 - 2.5 m
0.03” (DL)
Imager
1.5 - 2
arcmin
1
MEIFU
0.4 - 1 m
0.1” x 0.18”
500 - 1500
5 arcmin
5,000,000
1 - 5 m
0.03” (DL)
Imager
2 arcsec
1
Coronagraph
AURA New Initiatives Office
ELT Control Systems Face Tough Challenges
• Enemies of image quality gain strength as the telescope aperture
grows:
– Gravity
• Predictable, telescope orientation varies slowly
– Temperature gradients
• Slowly varying
– Atmospheric turbulence
• Dynamic, can be modeled statistically
– Wind buffeting
• Dynamic, hard to predict
• GSMT’s large size and low resonant frequency make wind buffeting
a key issue
• For a given Strehl ratio, required RMS wavefront is same as for
smaller telescope
AURA New Initiatives Office
Multiple Controls Systems
Systems Overlap in Parameter Space
LGS MCAO
~100
spatial & temporal avg
~50
AO (M2)
spatial & temporal avg
~20
spatial avg
aO (M1)
temporal avg
~10
spatial avg
Secondary
rigid body
2
spatial & temporal avg
Main Axes
0.001
0.01
0.1
1
10
100
AURA New Initiatives Office
Control Philosophy
• Goal is to decouple control loops by separating them in
– Space
– Spatial frequency
– Temporal frequency
• Allows decentralization of control laws
• Decoupling simplifies control system
– Design
– Implementation
– Troubleshooting
AURA New Initiatives Office
Site Evaluation Studies
•
Survey of candidate sites by
remote sensing (satellite data)
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Wind flow and atmospheric
turbulence modeling
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Characterization of turbulent
layers
•
Sodium layer measurements
•
On-site Measurements
AURA New Initiatives Office
Technical Challenges for an ELT
• Active and adaptive compensation for wind buffeting
• Adaptive correction of atmospheric turbulence
• Segment co-alignment and phasing
• Tip-tilt control of secondary mirror
• Large (10-20 m3) cryogenic (~ 10 K) instruments
• Cost-effective segment fabrication
• Fabrication of adaptive secondary mirror
AURA New Initiatives Office
Segment Fabrication Challenges
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•
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Aspheric departures > 200 microns P-V
Mechanical dimensions accurate to ~ 0.1 mm
Bevel size <1 mm
Surface figure accuracy ~ 20 nm RMS
Production rate of ~ 200 segments / year
Large number of different:
– segment shapes
– orientations
– asphericities
AURA New Initiatives Office
Optical Testing Challenges
• Aspheric departures > 200 microns P-V
• With respect to the optical test equipment:
– Segment position must be known to ~ 0.3 mm
– Segment clocking must be known to ~ 0.1 mrad
•
•
•
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Figure measurement accuracy ~ 5 nm RMS
Radius of curvature repeatability ~ 0.5 mm in 60 m
Production rate of ~ 200 segments / year
Large number of different:
– segment shapes
– orientations
– asphericities
AURA New Initiatives Office
We view segment fabrication as primarily
a mass-production challenge
• Cost
• Schedule
• Quality control
NIO is collaborating with other ELT design groups, and
consulting with commercial polishing firms, to
develop cost-effective segment fabrication methods
AURA New Initiatives Office
Secondary Mirror Fabrication Challenges
• 2-meter deformable facesheet ~ 3 mm thick
• Bevel size <1 mm
• Surface figure accuracy ~ 20 nm RMS with active
•
•
correction
Figure must be good to the outer edge
Conformal backing structure of thermally-stable material
– Must accommodate AO actuators
– Must be stiff enough to allow fast tip-tilt & focus
AURA New Initiatives Office
Secondary Mirror
Optical Testing Challenges
• Convex aspheric surface
• Figure measurement accuracy ~ 5 nm RMS
• Facesheet extremely flexible
– In-process testing should match acceptance test
• Metrology mount with ~ 2400 actuators
AURA New Initiatives Office
Information on AURA NIO activities is
available at:
www.aura-nio.noao.edu
AURA New Initiatives Office
NOAO is operated by the Association of Universities for Research in
Astronomy (AURA), Inc. under cooperative agreement with the National
Science Foundation.
Gemini is an international partnership managed by the Association of
Universities for Research in Astronomy under a cooperative agreement
with the National Science Foundation. Partner countries include the
United States, United Kingdom, Canada, Chile, Australia, Argentina, and
Brazil.
AURA New Initiatives Office