Agenda
GSMT Controls Workshop, 11 September, 2001
9:00 am
9:30 am
10:00 am
10:30 am
11:00 am
11:30 am
12:00 pm
1:00 pm
5:00 pm
GSMT Overview – Brooke Gregory, Larry Stepp
Pointing Control for a Giant Segmented Mirror Telescope
– Patrick Wallace
Implications of Wind Testing Results on the GSMT Control
Systems -- David Smith
To be defined – Mark Whorton
MSFC's Heritage in Segmented Mirror Control Technology
– John Rakoczy
Current concepts and status of GSMT control system
– George Angeli
Lunch
Informal discussions
Adjourn
GSMT Overview
B. Gregory, L. Stepp
11 September 2001
AURA NIO: Mission
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In response to the AASC call for a Giant Segmented
Mirror Telescope (GSMT) AURA formed a New
Initiatives Office (NIO)
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collaborative effort between NOAO and Gemini to explore
design concepts for a GSMT
NIO mission
“ to ensure broad astronomy community access to a 30m
telescope contemporary in time with ALMA and NGST, by
playing a key role in scientific and technical studies leading to
the creation of a GSMT.”
AURA New Initiatives Office
Management Board
Matt Mountain
Jeremy Mould
William Smith
Engineering Oversight
Jim Oschmann
Project Scientist
Steve Strom
Program Manager
Larry Stepp
Contracted Studies
studies
Contracted
TBD
Systems Scientist
Brooke Gregory
Part time support
NOAO & Gemini
Administrative assistant
Jennifer Purcell
Optics
Opto-mechanics
Larry
Stepp
Myung
Cho
Controls
Controls
George
Angeli
(NOAO)
George
Angeli
Adaptive Optics
Adaptive
Optics
Brent Ellerbroek (Gemini)
Ellerbroek/Rigaut
(Gemini)
Mechanical Designer
Rick Robles
Structures
Paul Gillett
Site Testing
Alistair Walker (NOAO)
Instruments
Sam Barden (NOAO)
Objectives: Next 2 years
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Develop point design for GSMT & instruments
Develop key technical solutions
– Adaptive optics
– Active compensation of wind buffeting
– Mirror segment fabrication
Investigate design-to-cost considerations
Carry out conceptual design activities that support and
complement other ELT efforts
Develop a partnership to build GSMT
AURA New Initiatives Office
Approach to GSMT Design
Parallel efforts:
• Understand the scientific context for GSMT
• Develop the key science requirements
• Address challenges common to all ELTs
• Wind-loading
• Adaptive optics
• Site
• Develop a Point Design
• Based on initial science goals
• Key part is conceptual design of instruments
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
Point Design: Scientific Motivations
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Enable high-Strehl performance over several arc-minute
fields
– Stellar populations; galactic kinematics; chemistry
Provide a practical basis for wide-field, native seeinglimited instruments
– Origin of large-scale structure
Enable high sensitivity mid-IR spectroscopy
– Detection of forming planetary systems
Telescope design should be driven by needs of instruments
Point Design: Basic Design Concepts
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Explore a radio telescope approach
– Possible structural advantages
– Possible advantages in accommodating large instruments
Use aspheric optics – good image after two reflections
Incorporate an adaptive M2
– Compensate for wind-buffeting
– Reduce thermal background
– Deliver enhanced-seeing images
Explore prime focus option
– Attractive enabler for wide-field science
– Cost-saving in instrument design
Radio Telescope Structural Design
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Fast primary focal ratio
Lightweight steel truss structure
Small secondary mirror
Secondary supported on tripod structure
Elevation axis behind primary mirror
Span between elevation bearings is less than
diameter of primary mirror – allows direct load path
Optical Design
Optical design: Classical Cassegrain
Primary diameter:
30 meters
Primary focal ratio:
f/1
Secondary diameter: 2 meters
Secondary focal ratio: f/18.75
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
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.
Lower Elevation Structure
Primary Mirror Segments
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Size chosen for point design:
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1.15-m across flats -- 1.33-m corner to corner
50 mm thickness
Number of segments: 618
Maximum asphericity ~ 110 microns (equal to Keck)
Segment Support
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Point design axial support is 18-point whiffletree
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FEA Gravity deflection 15 nm RMS
Wind Loading
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Primary challenge may be wind buffeting
– More critical than for existing telescopes
• Structural resonances closer to peak wind power
• Wind may limit performance more than local seeing
Solutions include:
– Site selection for low wind speed
– Optimizing enclosure design
– Dynamic compensation
• Adaptive Optics
• Active structural damping
Initital Structural Analysis
Z
X
Y
Output Set: Mode 1, 2.156537 Hz, Deformed(0.0673): Total Translation
Horizon Pointing - Mode 1 = 2.16 Hz
Structural Analysis
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Total weight of elevation structure – 700 tonnes
Total moving weight – 1400 tonnes
Gravity deflections ~ 5-25 mm
Wind buffeting response ~ 10-100 microns
Deflections are primarily rigid-body motions
Lowest resonant frequencies ~ 2 Hz
Instruments
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NIO team developing design concepts
– 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
Build on extant concepts where possible
Define major design challenges
Identify needed technologies
Instrument
Locations on
Telescope
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Fixed Gravity Cass
Direct-fed Nasmyth
Fiber-fed Nasmyth
Prime Focus
Co-moving Cass
View showing Fixed Gravity Cass
instrument
MOMFOS with
Prime Focus
Corrector
Conceptual design
fits in a 3m dia by
5m long cylinder
Instrument
Locations on
Telescope
View showing Co-moving Cass
instrument
MCAO/AO foci and instruments
Oschmann et al (2001)
MCAO optics
moves with telescope
elevation axis
Narrow field AO or
narrow field seeing
limited port
MCAO Imager
at vertical
Nasmyth
4m
MCAO System: Current Layout
Instrument Locations on Telescope
Fiber-fed
MOMFOS
MCAO-fed
NIRDIF
or
MCAO Imager
Cass-fed
MIHDAS
Mayall, Gemini and GSMT Enclosures
at same scale
Mayall
Gemini
GSMT
McKale Center – Univ of Arizona
GSMT – at same scale
Key Point-Design Features
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Radio telescope structure
– Advantages:
• Direct load path to elevation bearings
• Cass focus can be just behind M1
• 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
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
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
Key Point-Design Features
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2m diameter adaptive
secondary mirror
– Advantages:
• 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)
Goal: 8000 actuators
30cm spacing on M1
Key Point-Design Features
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Prime focus location for MOMFOS
– Advantages:
• Fast focal ratio leads to instrument of
reasonable size
• Adaptive prime focus corrector allows
enhanced seeing performance
– Disadvantages:
• Issues of interchange with M2
Key Enabling Techniques:
Active and Adaptive Optics
Active Systems:
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M1 segment rigid body position
– ~ 1 Hz
– Piston, tip & tilt
M1 segment figure control –
– Based on look-up table ~ 0.1 Hz
– Low order -- Astigmatism, focus, trefoil, coma
M2 rigid body motion
– ~ 5-10 Hz
– Five axes – active focus & alignment, image stabilization
Active structural elements (?)
– Active alignment
– Active damping
Key Enabling Techniques:
Active and Adaptive Optics
Adaptive Systems:
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Adaptive secondary mirror
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~ 20-50 Hz
~ 1000-10,000 actuators
Adaptive mirror in prime focus corrector
Multi-conjugate wide-field AO
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~ 3 DMs
Laser Guide Stars
High-order narrow-field conventional AO
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~ 10,000 – 50,000 actuators
Active and Adaptive Optics will be integrated into GSMT Telescope
and Instrument concepts from the start