Transcript Claire Max's slides on MCAO, MOAO, and GLAO

Lecture 16
The applications of tomography:
MCAO, MOAO, GLAO
Claire Max
AY 289C
UC Santa Cruz
March 6, 2008
Page 1
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.
Page 2
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” ?
Page 3
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)
Page 4
What is Tomography ?
90 km
1. Cone effect
Credit: Rigaut, MCAO for Dummies
“Missing” Data
Page 5
What is Tomography ?
90 km
2. Wider field of view, no cone effect
Credit: Rigaut, MCAO for Dummies
Tomography lets
you reconstruct
turbulence in the
entire cylinder of
air above the
telescope mirror
Page 6
Ragazzoni’s Tomography Cartoon
Credit: Ragazzoni, Nature 403, 2000
Page 7
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
Page 8
How tomography works: from Don Gavel
kZ
kx 
k z


1
z
kX

 x    n x   z, z dz
k x   N k x ,k x 
0
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 kz=0 Fourier spatial frequency component
• Projections at several angles sample the kx,ky,kz volume
9
How tomography works: from Don Gavel
kZ
kx 
k z


1
z
kX

 x    n x   z, z dz
k x   N k x ,k x 
0
• The larger the telescope’s 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
10
How tomography works: some math
y  Ax
• 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  blind modes. Need a few natural guide
stars to determine these.
Page 11
Solve for the full turbulence above the
telescope using the back-propagator
xA y
T
y = vector of all WFS measurements
x
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
Page 12
LGS Related Problems: Null modes
• Tilt Anisoplanatism :
Low order modes (e.g.
focus) > Tip-Tilt at
altitude
 Dynamic Plate
Scale changes
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
• 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
Page 13
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)
Page 14
What is multiconjugate AO?
Turbulence Layers
Deformable mirror
Credit: Rigaut, MCAO for Dummies
Page 15
What is multiconjugate AO?
Deformable mirrors
Credit: Rigaut, MCAO for Dummies
Turbulence Layers
Page 16
The multi-conjugate AO concept
Turb. Layers
#2
#1
Telescope
WFS
DM1
DM2
Atmosphere
UP
Credit: Rigaut, MCAO for Dummies
Page 17
“Star Oriented” MCAO
Guide Stars
• Each WFS looks at one star
• Global Reconstruction
High Altitude Layer
• n GS, n WFS, m DMs
• 1 Real Time Controller
Ground Layer
• The correction applied at
each DM is computed using
all the input data.
Telescope
DM2
DM1
WFC
WFSs
Credit: N. Devaney
Page 18
Layer Oriented MCAO
• Layer Oriented WFS architecture
Guide Stars
• Local Reconstruction
• x GS, n WFS, n DMs
High Alt. Layer
• n RTCs
Ground Layer
• Wavefront is reconstructed at
each altitude independently.
• Each WFS is optically coupled to
all the others.
• GS light co-added for better SNR.
Telescope
DM2
DM1
WFC1
WFC2
WFS1
WFS2
Credit: N. Devaney
Page 19
MCAO Performance Predictions
NGS, Mauna Kea Atmospheric Profile
No AO
Classical AO
1 DM / 1 NGS
320 stars / K band / 0.7’’ seeing
Credit: Rigaut, MCAO for Dummies
165’’
MCAO
2 DMs / 5 NGS
Stars magnified for clarity
Page 21
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
Page 22
Results from ESO’s Multiconjugate AO
Demonstrator (MAD)
Single Conjugate
Multi Conjugate
Page 23
Gemini South MCAO
Science
Path
NGSWFS
WFS
Path
LGS
Path
DM0
DM9
SCIBS
TTM
WFS
WFSBS
OAP1
ADC
OAP2
DM4.5
ADC
Zoom Focus
OAP3
Lens
LGS WFS
Credit: Eric James & Brent Ellerbroek, Gemini Observatory
Page 24
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)
Page 25
Distinctions between multi-conjugate
and multi-object AO
?
1-2 arc min
• DMs conjugate to different
altitudes in the atmosphere
• Only one DM per object,
conjugate to ground
• Guide star light is corrected by
DMs before its wavefront is
measured
• Guide star light doesn’t
bounce off small MEMS DMs in
multi-object spectrograph
Page 26
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
Page 27
Why does MOAO work if there is only one
deformable mirror in the science path?
90 km
• Tomography lets you
measure the turbulence
throughout the volume
above the telescope
Page 28
Why does MOAO work if there is only one
deformable mirror in the science path?
90 km
• 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
Page 29
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)
Page 30
Ground layer AO: do tomography, but
only use 1 DM (conjugate to ground)
MCAO
GLAO
single DM
conjugated to
ground layer
GLAO uses 1 ground-conjugated DM, corrects near-ground turbulence
Credit: J-M Conan
Page 31
Correcting just the ground layer gives a
very large isoplanatic angle
• Strehl = 0.38 at  = 0
0 is isoplanatic angle
3 / 5



2
8/3
2
5/3
 0  2.914 k (sec  )  dz CN (z) z 


0
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 Page 32
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 m
– Field of view: >10’
– Factor of 1.5-2
reduction in image
size.
Modeled using Cerro Pachon
turbulence profile. (M-L Hart 2003)
Page 33
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
Page 34
time
Credit: A.
Page 35
Credit: A.
Tokovinin
Page 36
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.
Page 37