Lecture 16 The applications of tomography: MCAO, MOAO, GLAO

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Lecture 16The applications of tomographyMCAO,
MOAO, GLAO

• Claire Max
• AY 289C
• UC Santa Cruz
• March 6, 2008

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Next week (the last week of class!)

• Tuesday March 11th lecture
• AO for imaging the living human retina.
• Jason Porter, Univ. of Houston
• Thursday March 13th lecture
• Extreme AO for imaging planets around nearby
  stars
• Bruce Macintosh, Lawrence Livermore National Lab
• Final exam take-home, open-book
• Distributed at lecture Thursday March 13th
• Due in my office (or my computer) on or before
  Thurs March 20th at noon. This is a hard
  deadline.

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The final homework assignment

• Will be on web by end of this week
• Due Thurs March 13
• Will be intended to help you review the whole
  course and get ready for the exam
• What approaches have you found useful in
  reviewing classes like this? What makes the
  material stick ?

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Outline of lecture

• Review of AO tomography concepts
• AO applications of tomography
• Multi-conjugate adaptive optics (MCAO)
• Multi-object adaptive optics (MOAO)
• Ground-layer AO (GLAO)

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What is Tomography ?1. Cone effect
Credit Rigaut, MCAO for Dummies
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What is Tomography ?2. Wider field of view, no
cone effect
Tomography lets you reconstruct turbulence in the
entire cylinder of air above the telescope mirror
90 km
Credit Rigaut, MCAO for Dummies
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Ragazzonis Tomography Cartoon
Credit Ragazzoni, Nature 403, 2000
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Concept of a metapupil

• Can be made larger than real telescope pupil
• Increased field of view due to overlap of fields
  toward multiple guide stars

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How tomography works from Don Gavel
kZ
kX
Fourier slice theorem in tomography (Kak,
Computer Aided Tomography, 1988)

• Each wavefront sensor measures the integral of
  index variation along the ray lines
• The line integral along z determines the kz0
  Fourier spatial frequency component
• Projections at several angles sample the kx,ky,kz
  volume

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How tomography works from Don Gavel
kZ
kX

• The larger the telescopes primary mirror, the
  wider the range of angles accessible for
  measurement
• In Fourier space, this means that the bow-tie
  becomes wider
• More information about the full volume of
  turbulence above the telescope

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How tomography works some math

• where
• y vector of all WFS measurements
• x value of d?OPD) at each voxel in turbulent
  volume above telescope

x
y
A is a forward propagator (entries 0 or 1)

• Assume we measure y with our wavefront sensors
• Want to solve for x value of d?OPD)
• The equations are underdetermined there are
  more unknown voxel values than measured phases Þ
  blind modes. Need a few natural guide stars to
  determine these.

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Solve for the full turbulence above the telescope
using the back-propagator
x
y vector of all WFS measurements x value of
d?OPD) at each voxel in turbulent volume above
telescope
y
AT is a back propagator along rays back toward
the guidestars
x
Use iterative algorithms to converge on the
solution.
y
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LGS Related Problems Null modes

• Tilt Anisoplanatism Low order modes (e.g.
  focus) Tip-Tilt at altitude
• ? Dynamic Plate Scale changes

• Five Null Modes are not seen by LGS (Tilt
  indetermination problem)
• ? Need 3 well spread NGSs to control these modes

Credit Rigaut, MCAO for Dummies
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Outline of lecture

• Review of AO tomography concepts
• AO applications of tomography
• Multi-conjugate adaptive optics (MCAO)
• Multi-object adaptive optics (MOAO)
• Ground-layer AO (GLAO)

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What is multiconjugate AO?
Deformable mirror
Credit Rigaut, MCAO for Dummies
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What is multiconjugate AO?
Credit Rigaut, MCAO for Dummies
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The multi-conjugate AO concept
Telescope
Turb. Layers
WFS
2
1
DM2
DM1
Atmosphere
UP
Credit Rigaut, MCAO for Dummies
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Star Oriented MCAO

• Each WFS looks at one star
• Global Reconstruction
• n GS, n WFS, m DMs
• 1 Real Time Controller

• The correction applied at each DM is computed
  using all the input data.

Credit N. Devaney
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Layer Oriented MCAO

• Layer Oriented WFS architecture
• Local Reconstruction
• x GS, n WFS, n DMs
• n RTCs

• Wavefront is reconstructed at each altitude
  independently.
• Each WFS is optically coupled to all the others.
• GS light co-added for better SNR.

Credit N. Devaney
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MCAO Predicted Performance
1
1
0

• Surface plots of Strehl ratio over a 1.5 arc min
  FoV.
• 13x13 actuator system, K band, Cerro Pachon
  turbulence profile.

Credit Rigaut, Le Louarn
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MCAO Performance PredictionsNGS, Mauna Kea
Atmospheric Profile
2 DMs / 5 NGS
1 DM / 1 NGS
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320 stars / K band / 0.7 seeing
Stars magnified for clarity
Credit Rigaut, MCAO for Dummies
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MCAO Simulations, 3 laser guide stars
Strehl at 2.2 ?m 3 NGS, FoV 1 arc min
Strehl at 2.2 ?m 3 NGS, FoV 1.5 arc min
Average Strehl drops, variation over FoV
increases as FoV is increased
Credit N. Devaney
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Results from ESOs Multiconjugate AO Demonstrator
(MAD)
Single Conjugate
Multi Conjugate
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Gemini South MCAO
Credit Eric James Brent Ellerbroek, Gemini
Observatory
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Outline of lecture

• Review of AO tomography concepts
• AO applications of tomography
• Multi-conjugate adaptive optics (MCAO)
• Multi-object adaptive optics (MOAO)
• Ground-layer AO (GLAO)

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Distinctions between multi-conjugate and
multi-object AO

• Only one DM per object, conjugate to ground
• Guide star light doesnt bounce off small MEMS
  DMs in multi-object spectrograph

• DMs conjugate to different altitudes in the
  atmosphere
• Guide star light is corrected by DMs before its
  wavefront is measured

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Science with MOAO multiple deployable spatially
resolved spectrographs

• A MEMS DM underneath each high-redshift galaxy,
  feeding a narrow-field spatially resolved
  spectrograph (IFU)
• No need to do AO correction on the blank spaces
  between the galaxies

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Why does MOAO work if there is only one
deformable mirror in the science path?

• Tomography lets you measure the turbulence
  throughout the volume above the telescope

90 km
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Why does MOAO work if there is only one
deformable mirror in the science path?

• Tomography lets you measure the turbulence
  throughout the volume above the telescope
• In the direction to each galaxy, you can then
  project out the turbulence you need to cancel out
  for that galaxy

90 km
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Outline of lecture

• Review of AO tomography concepts
• AO applications of tomography
• Multi-conjugate adaptive optics (MCAO)
• Multi-object adaptive optics (MOAO)
• Ground-layer AO (GLAO)

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Ground layer AO do tomography, but only use 1 DM
(conjugate to ground)
GLAO
GLAO uses 1 ground-conjugated DM, corrects
near-ground turbulence
Credit J-M Conan
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Correcting just the ground layer gives a very
large isoplanatic angle

• Strehl 0.38 at ? ?0
• ?0 is isoplanatic angle
• ?0 is weighted by high-altitude turbulence (z5/3)
• If turbulence is only at low altitude, overlap is
  very high.
• If you only correct the low altitude turbulence,
  the isoplanatic angle will be large (but the
  correction will be only modest)

Common Path
Telescope
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Ground Layer AO (GLAO) typically decreases
natural seeing by a factor of 1.5 to 2

• Example GLAO calculation for Giant Magellan
  Telescope (M. Johns)
• Adaptive secondary conjugation at 160 m above
  primary mirror.
• Performance goals
• ? 0.8 mm
• Field of view 10
• Factor of 1.5-2 reduction in image size.

Modeled using Cerro Pachon turbulence profile.
(M-L Hart 2003)
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Many observatories have ambitious GLAO projects
planned

• Near term on medium sized telescopes SOAR
  (4.25m), William Herschel Telescope (4.2m), MMT
  (6.5m)
• Medium term on VLT (8m), LBT (2x8m)
• Longer term on Giant Magellan Telescope etc.
• Is it worth the large investment just to
  decrease seeing disk by factor of 1.5 to 2 ?
• Depends on whether existing or planned large
  spectrographs can take advantage of smaller image
• Potential improved SNR for background-limited
  point sources

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time
Credit A. Tokovinin
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Credit A. Tokovinin
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Summary

• Tomography a way to measure the full volume of
  turbulence above the telescope
• Once you have measured the turbulence and know
  its height distribution, there are several ways
  to do the wavefront correction to get wider field
  of view
• Multi-conjugate AO multiple DMs, each optically
  conjugate to a different layer in the atmosphere.
• Multi-object AO correct many individual objects,
  each over a small field.
• Ground-layer AO correct just the turbulence
  close to the ground. Gives very large field of
  view but only modest correction. Should work in
  both the visible and the IR.