Agenda GSMT Controls Workshop, 11 September, 2001

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AgendaGSMT Controls Workshop, 11 September, 2001

• 900 am      GSMT Overview Brooke Gregory,
  Larry Stepp
• 930 am      Pointing Control for a Giant
  Segmented Mirror Telescope                
  Patrick Wallace
• 1000 am      Implications of Wind Testing
  Results on the GSMT Control
  Systems -- David Smith
• 1030 am      To be defined Mark Whorton
• 1100 am     MSFC's Heritage in Segmented Mirror
  Control Technology                 John
  Rakoczy
• 1130 am      Current concepts and status of GSMT
  control system                 George Angeli
• 1200 pm       Lunch
• 100 pm       Informal discussions
• 500 pm Adjourn

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GSMT Overview

• B. Gregory, L. Stepp 11 September 2001

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AURA NIO Mission

• In response to the AASC call for a Giant
  Segmented Mirror Telescope (GSMT) AURA formed a
  New Initiatives Office (NIO)
• 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.

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AURA New Initiatives Office

Contracted Studies
Adaptive Optics Ellerbroek/Rigaut (Gemini)
Controls George Angeli
Opto-mechanics Myung Cho
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Objectives Next 2 years

• 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

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AURA New Initiatives OfficeApproach 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

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What is a Point Design?

• 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
• A point design is not
• A trade study that evaluates all possible options
• A design that anyone is proposing to build

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Point Design Scientific Motivations

• Enable high-Strehl performance over several
  arc-minute fields
• Stellar populations galactic kinematics
  chemistry
• Provide a practical basis for wide-field, native
  seeing-limited 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

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Point Design Basic Design Concepts

• 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

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Radio Telescope Structural Design

• 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

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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
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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

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Point Design Structure
Plan View of Structure Pattern of segments

Gemini
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Lower Elevation Structure
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Primary Mirror Segments

• Factors favoring large segment size
• Reduces number of position sensors actuators
• Simplifies alignment procedures
• Reduces overall complexity
• Reduces number of unique segment types
• Factors favoring a small segment size
• Reduces complexity of segment support (i.e.
  whiffletree)
• Reduces shipping costs (big jump at 2.4 meters)
• Reduces size cost of equipment for polishing,
  ion figuring, coating handling
• Reduces asphericity of individual segments
• Asphericity goes as square of segment diameter
• Reduces sensitivity to segment position rotation

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Primary Mirror Segments

• Size chosen for point design
• 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)

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Segment Support

• Point design axial support is 18-point
  whiffletree
• FEA Gravity deflection 15 nm RMS

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Wind Loading

• 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

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Initital Structural Analysis
Horizon Pointing - Mode 1 2.16 Hz

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Structural Analysis

• 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

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Instruments

• 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

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Multi-Object Multi-Fiber Optical Spectrograph
(MOMFOS)

• 20 arc-minute field
• 60-meter fiber cable
• 700 0.7 fibers
• 3 spectrographs, 230 fibers each
• VPH gratings
• Articulated collimator for different resolution
  regimes
• Resolution Example ranges with single
  grating
• R 1,000 350nm 650nm
• R 5,000 470nm 530nm
• R 20,000 491nm 508nm
• Detects 13 - 23 of photons hitting the 30m
  primary

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Mid-Infrared High Dispersion AO Spectrograph
(MIHDAS)

• Adaptive Secondary AO feed
• On-Axis, Narrow Field/Point Source
• R120,000
• 3 spectrographs
• 2-5 mm (small beamed, x-dispersed), 0.2
  arc-second slit length
• 10-14 mm (x-dispersed), 1 arc-second slit
• 16-20 mm (x-dispersed), 1 arc-second slit
• 10-14 mm spectrograph likely to utilize same
  collimator as 16-20 mm instrument. Different
  Gratings and Camera.
• 2-5 mm spectrograph may require additional AO
  mirrors.

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Near Infra-Red Deployable Integral Field
Spectrograph (NIRDIF)

• MCAO fed
• 1.5 to 2.0 arc-minute FOV
• 1 2.5 mm wavelength coverage
• Deployable IFU units
• 1.5 arc-second FOV per IFU probe
• 31 slices per IFU probe (0.048 per slice)
• 26 deployable units

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MCAO Near-IR Imager

• f/38 input with 11 reimaging optics
• 1.5 to 2 arc-minute field of view
• Monolithic imager -
• 5.5 mm/arc-second plate scale!
• 685 mm sized detector array for 2 arc-min field!
• 28K by 28K detector!
• 7 by 7 mosaic of 4K arrays
• 0.004 arc-second per pixel sampling
• Alternative approach is to have deployable
  capability
• for imaging over a subset of the total field.

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Instrument Locations on Telescope

• Fixed Gravity Cass
• Direct-fed Nasmyth
• Fiber-fed Nasmyth
• Prime Focus
• Co-moving Cass

View showing Fixed Gravity Cass instrument
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MOMFOS with Prime Focus Corrector

Conceptual design fits in a 3m dia by 5m long
cylinder
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Instrument Locations on Telescope
View showing Co-moving Cass instrument
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MCAO/AO foci and instruments
Oschmann et al (2001)
MCAO opticsmoves with telescope
elevation axis
4m
MCAO Imager at vertical Nasmyth
Narrow field AO or narrow field seeing limited
port
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MCAO System Current Layout

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Instrument Locations on Telescope
Fiber-fed MOMFOS
MCAO-fed NIRDIF or MCAO Imager
Cass-fed MIHDAS
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Mayall, Gemini and GSMT Enclosuresat 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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Key Point-Design Features

• 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

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Key Point-Design Features

• 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

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Key Point-Design Features

• 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

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Key Point-Design Features

• 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
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Key Point-Design Features

• 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

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Key Enabling Techniques Active and Adaptive
Optics

• Active Systems
• 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

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Key Enabling Techniques Active and Adaptive
Optics

• Adaptive Systems
• Adaptive secondary mirror
• 20-50 Hz
• 1000-10,000 actuators
• Adaptive mirror in prime focus corrector
• Multi-conjugate wide-field AO
• 3 DMs
• Laser Guide Stars
• High-order narrow-field conventional AO
• 10,000 50,000 actuators
• Active and Adaptive Optics will be integrated
  into GSMT Telescope and Instrument concepts from
  the start