MultiObject Adaptive Optics MOAO

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Multi-Object Adaptive Optics (MOAO)

• Brent Ellerbroek
• TMT AOWG/IWG Discussions
• March 19, 2004

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Presentation Outline

• Strawman MOAO Concept
• (Very) preliminary simulation results
• Selected instrument design issues

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MOAO Strawman Concepts
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Why Are We Doing This?

• MCAO has two fundamental error sources
• Atmospheric tomography with a finite number of
  guidestars
• Atmospheric fitting with a finite number of DMs
• MOAO eliminates the atmospheric fitting error
• Surpasses MCAO performance over the same FOV
• Matches MCAO performance over a broader FOV
• MOAO reduces the number of warm optical surfaces
• Significantly reduces emissivity in K, L, and M
  bands
• Applications for TMT include
• High spatial resolution multi-object IFU
  spectroscopy in J, H, and K over a 5 arc minute
  field
• High spatial resolution spectroscopy in L, M with
  high sky coverage

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Preliminary Simulation Results

• Idealized performance with D30m
• CP turbulence profile
• 5 FOV with 7 NGS
• Order 60 by 60 AO compensation
• No noise, servo lag, or aperture edge effects
• No implementation errors (e.g., DM/WFS
  registration)
• Somewhat less idealized simulations with D8m
• 5 LGS 4 tip/tilt NGS
• Order 16 by 16 AO compensation
• Include WFS noise and aperture edge effects

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Performance vs. Guide Star Asterism with D30m
7 guidestars, 0.5 radius
7 guidestars, 2.0 radius
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Performance vs. Guidestar Asterism with D8m
MOAO
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Instrument Design Considerations

• Open-loop and/or off-null operation of WFSs and
  DMs
• Button characteristics
• How large, how many, what do they contain?
• Number of focal planes
• Whats included in the cryostat?
• Filter wheels and ADCs

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Coping with Nonlinear AO Components

• Real-world AO components have limited linear
  dynamic ranges
• Each WFS might include its own local DM
• Each DM might include its own local WFS (designed
  for large linear dynamic range)
• MOAO system could located be behind an adaptive
  secondary (or another DM) for low order, large
  amplitude correction)

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Button Taxonomy
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Two Optical Design Concepts for IFU Buttons
OAP Relay
Single sphere
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Number of Focal Planes

• Should sources at different wavelengths be
  separated spatially, or via dichroics?
• Could conceivably use up to 4 focal planes
• Visible/NIR split
• Further split the visible into NGS and LGS
  channels
• Further split the NIR into science and NGS
  channels
• Provides more space for all those buttons
• Adds extra (large) optical components and
  decreases throughput

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Sample Focal Plane Configurations
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Other Options

• Extent of cryostat
• Output fibers only (6-7 warm surfaces)
• One small cryostat per IFU button (3-4 warm
  surfaces, much larger buttons)
• One large cryostat for IFU buttons (3-4 warm
  surfaces, large window)
• ADCs and filters
• Large elements for the full field of view?
• Separate elements for each button?