AURA New Initiatives Office
The GSMT Point Design
Larry Stepp and Brooke Gregory
AURA New Initiatives Office
Advances of the Past Decade
AURA New Initiatives Office
What Lies Ahead?
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Astronomers already see the need for more powerful O/IR
telescopes, to:
– Extend the reach of current ground-based O/IR facilities
– Complement space-based telescopes (e.g. NGST)
– Complement next generation radio facilities (ALMA; SKA)
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What type of facility will provide the needed capabilities a
decade hence?
AURA New Initiatives Office
Decadal Review
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In May 2000, the astronomy decadal review
committee recommended, as its highest priority
ground-based initiative, the construction of a 30meter Giant Segmented Mirror Telescope (GSMT)
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In response, AURA formed a New Initiatives Office
(NIO) to support scientific and technical studies
leading to the creation of a GSMT
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Goal of ensuring broad astronomy community access to a
30m telescope contemporary with NGST.
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AURA New Initiatives Office
Approach to GSMT Design
Parallel efforts:
• Understand the scientific context for GSMT in NGST era
– Develop the key science requirements
• Address challenges common to all ELTs
– Site testing and selection
– Cost-effective mirror fabrication
– Characterization of wind loading
– Hierarchical control systems
– Adaptive optics
• Develop a Point Design
– Approach integrates initial science goals & instrument concepts
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What is a “Point Design”?
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A point design is a learning exercise that:
– Explores a single, plausible design
– Helps identify key technical issues
– Helps define factors important to the science requirements
– Provides an opportunity to develop necessary analytical
methods
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A point design is not:
– A trade study that evaluates all possible options
– A design that anyone is proposing to build
AURA New Initiatives Office
GSMT Point Design:
Scientific Motivations
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Provide a practical basis for wide-field, native seeinglimited instruments
– Origin of large-scale structure in the universe
Enable high-Strehl performance over ~ arc-minute fields
– Stellar populations; galactic kinematics; chemical abundances
Enable high sensitivity mid-IR spectroscopy
– Detection of stars & planetary systems in formation
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Key Point Design Features
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Fast aspheric primary
– Stigmatic image after two reflections
Radio telescope-type design
– Structural advantages
– Accommodates large instruments
Adaptive secondary
– Wind-buffeting compensation
– Atmospheric correction in IR, with low emissivity
– First stage in higher-order adaptive systems
Prime focus instrument
– Convenient plate scale for seeing-limited observations
– Enables wide-field science
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Optical Design
Optical design:
M1 diameter:
M1 focal ratio:
Classical Cassegrain
30 meters
f/1
M2 diameter: 2 meters
M2 focal ratio: f/18.75
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Radio Telescope Structural Design
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Lightweight steel truss structure
Fast primary focal ratio
Small secondary mirror
M2 supported on tripod structure
Elevation axis behind M1
– Span between elevation bearings is less than M1 diameter
– Allows direct load path
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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
Primary Mirror Segments
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Segment dimensions
– 1.15-m across flats -- 1.33-m corner to
corner
– 50 mm thickness
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Number of segments: 618
Maximum departure from sphere 110
microns
– Comparable to Keck
Axial support is 18-point whiffletree
– FEA Gravity deflection 15 nm RMS
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Initial Structural Analysis
Z
X
Y
Output Set: Mode 1, 2.156537 Hz, Deformed(0.0673): Total Translation
Horizon Pointing - Mode 1 = 2.16 Hz
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Structural Analysis
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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
Large size and low resonant frequency make wind
buffeting a key issue.
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Gemini 8-meter Telescope
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Sensor Locations
Ultrasonic anemometer
Ultrasonic anemometer
Pressure sensors
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Simultaneous Animations
(c00030oo)
Wind Pressure (N/m2)
Mirror Deformation (microns)
Wind Speed at 5 Locations (m/sec)
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Response of structure to wind
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Controllable Elements
Active Systems:
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M2 rigid body motion
– ~ 5-10 Hz
– Five axes
M1 segment rigid body position
– ~ 1 Hz
– Piston, tip & tilt
M1 segment figure control
– Based on look-up table ~ 0.1 Hz
– Astigmatism, focus, trefoil, coma
Active structural elements
– Active alignment
– Active damping
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Controllable Elements
Adaptive Systems:
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Adaptive mirror in prime focus corrector
Adaptive secondary mirror
– ~ 20-50 Hz
– ~ 1000-10,000 actuators
Multi-conjugate wide-field AO
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~ 3 DMs
Laser Guide Stars
High-order narrow-field conventional AO
– ~ 10,000 – 50,000 actuators
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Controls Approach:
Hierarchical Subsystems
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
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Active and Adaptive Optics will be integrated into
Telescope and Instrument concepts from the start.
AURA New Initiatives Office
Instruments
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NIO team currently developing design concepts for 4
instruments:
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Multi-Object, Multi-Fiber, Optical Spectrograph – MOMFOS
Near IR Deployable Integral Field Spectrograph – NIRDIF
MCAO-fed near-IR imager
Mid-IR, High Dispersion, AO Spectrograph – MIHDAS
Paper by Sam Barden et al immediately after this one.
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Instrument Locations on Telescope
Prime Focus
Fiber-fed Nasmyth
Co-moving Cass
Direct-fed Nasmyth
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Instrument Locations on Telescope
Prime Focus
Fiber-fed Nasmyth
Co-moving Cass
Direct-fed Nasmyth
Fixed Gravity Cass
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MCAO System
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MCAO System
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MCAO System
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Paper on:
Adaptive optics requirements, concepts and performance
estimates for Extremely Large telescopes
by Brent Ellerbroek and Francois Rigaut at 1:10.
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Mayall, Gemini and GSMT Enclosures
at same scale
Mayall
Gemini
GSMT
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McKale Center – Univ of Arizona
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GSMT – at same scale
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Some Possible GSMT Enclosure Designs
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Summary: Key Point-Design
Features
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F/1 primary mirror
– Advantages:
• Reduces size of enclosure
• Reduces flexure of optical support
structure
• Reduces counterweights required
– Disadvantages:
• Increased sensitivity to
misalignment
• Increased asphericity of segments
AURA New Initiatives Office
Summary: Key Point-Design
Features
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Paraboloidal primary
– Advantages:
• Good image quality over 10-15 arcmin field
with only two reflections
• Lower emissivity for mid-IR
• Compatible with laser guide stars
– Disadvantages:
• Higher segment fabrication cost
• Increased sensitivity to segment alignment
AURA New Initiatives Office
Summary: Key Point-Design
Features
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Radio telescope structure
– Advantages:
• Direct load path to elevation bearings
• Can have short back focal distance
• Allows small secondary mirror – can
be adaptive
• Allows MCAO system ahead of
Nasmyth focus
• Allows many gravity-invariant
instrument locations
– Disadvantage:
• Requires counterweight
• Sweeps out larger volume in
enclosure
AURA New Initiatives Office
Summary: Key Point-Design
Features
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2m diameter adaptive secondary mirror
– Advantages:
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Correction of low-order M1 modes
Enhanced native seeing
Good performance in mid-IR
First stage in high-order AO system
– Disadvantages:
• Increased difficulty (i.e., cost)
AURA New Initiatives Office
Summary: Key Point-Design Features
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Prime focus location for MOMFOS
– Advantages:
• Fast focal ratio leads to reasonablysized instrument
• Adaptive prime focus corrector allows
enhanced seeing performance
– Disadvantages:
• Issues of interchange with M2
• Requires fibers instead of slits
AURA New Initiatives Office
Plans for Next 15 Months
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Involve community in defining GSMT scientific context
Continue structural analysis
Construct hierarchical system control model
Simulate system performance in presence of
disturbances
Extend AO development efforts
Continue site testing
Develop cost-reduction strategies
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Segment fabrication
Telescope structure
Adaptive optics
Instrument technologies
Enclosures
AURA New Initiatives Office
Acknowledgements
Many people have contributed to this work, including:
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George Angeli
Joe Antebi and Frank Kan of
SG&H
Sam Barden
Dick Buchroeder
Myung Cho
Brent Ellerbroek
Paul Gillett
Brooke Gregory
Charles Harmer
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Ming Liang
Matt Mountain
Joan Najita
Jim Oschmann
Jennifer Purcell
Francois Rigaut
Rick Robles
Mike Sheehan
David Smith of MERLAB
Steve Strom
Plus many NOAO & Gemini scientists working on the GSMT science case
AURA New Initiatives Office
Information on AURA NIO activities is
available at:
www.aura-nio.noao.edu
AURA New Initiatives Office