Download Finite number of Guide Star

Adaptive Optics in the VLT and ELT era
Beyond Classical AO
François Wildi
Observatoire de Genève
Page 1
Issues for designer of AO systems
• Performance goals:
– Sky coverage fraction, observing wavelength, degree
of compensation needed for science program
• Parameters of the observatory:
– Turbulence characteristics (mean and variability),
telescope and instrument optical errors, availability of
laser guide stars
• AO parameters chosen in the design phase:
– Number of actuators, wavefront sensor type and
sample rate, servo bandwidth, laser characteristics
• AO parameters adjusted by user: integration time on
wavefront sensor, wavelength, guide star mag. & offset
Reminder #1: Dependence of Strehl on l and
number of DM degrees of freedom
 
5/3
S  exp  2  exp  0.28 d / r0  


r0 l   r0 0.5  m l / 0.5  m 
6 /5
5/3
2



d
 0.5  m 

S  exp 0.28 

 

l
 r0 0.5  m 


Deformable mirror fitting error only
• Assume bright
natural guide
star
• No meas’t error
or iso-planatism
or bandwidth
error
Reminder #1: Dependence of Strehl on l and
number of DM degrees of freedom (fitting)
• Assume bright
natural guide
star
Decreasing fitting error
Deformable mirror fitting error only
• No meas’t error
or iso-planatism
or bandwidth
error
Reminder #2: Strehl vs l and seeing (r0)
• Assume bright
natural guide
star
Decreasing fitting
error
Deformable mirror fitting error only
• No meas’t error
or iso-planatism
or bandwidth
error
Basics of wavefront sensing
• Measure phase by measuring intensity variations
• Difference between various wavefront sensor schemes
is the way in which phase differences are turned into
intensity differences
• General box diagram:
Guide
star
Turbulence
Telescope
Wavefront sensor
Optics
Detector
Reconstructor
Computer
Transforms aberrations into
intensity variations
Types of wavefront sensors
• “Direct” in pupil plane: split pupil up into subapertures
in some way, then use intensity in each subaperture to
deduce phase of wavefront. REAL TIME
– Slope sensing: Shack-Hartmann, pyramid sensing
– Curvature sensing
• “Indirect” in focal plane: wavefront properties are
deduced from whole-aperture intensity measurements
made at or near the focal plane. Iterative methods take a lot of time.
– Image sharpening, multi-dither
– Phase diversity
Shack-Hartmann wavefront sensor
concept - measure subaperture tilts
f
CCD
Pupil plane
Image plane
CCD
WFS implementation
• Compact
• Time-invariant
Reconstruction
• …
How to reconstruct wavefront from
measurements of local “tilt”
Effect of guide star magnitude
(measurement error)
Because of the photons statistics, some noise is
associated with the read-out of the Shack-Hartmann
spots intensities
 S2 H
 6.3 

 SNR 
2
2

 6.3  
2
S  exp   S  H   exp   
 

 SNR 
1
SNR 
N photons
Assumes no fitting error or other error terms
Effect of guide star magnitude
(measurement error)
Assumes no fitting error or other error terms
bright star
Decreasing
measurement error
dim star
Reminder #3: Strehl vs l and guide star
angular separation (anisoplanatism)
    5/3 
2
  exp     
S  exp   iso
   0  
r0
 0   l 6 /5
h
5/3
2
 
  0.5  m  

S  exp   

 

l
   0 (0.5  m) 

Reminder #3: Strehl vs l and guide star
angular separation (anisoplanatism)
Sky coverage accounting for
guide star densities
LGS coverage ~80 %
Tip/tilt sensor magnitude limit
Hartmann sensor magnitude limit
Galactic
latitude
NGS coverage 0.1 %
Isoplanatic angle 0
Isokinetic angle k
(Temporary) conclusion:
• With 0.1% sky coverage, classical AO is of limited use
for general astronomy.
• This is perticularly true for extra-galactic astronomy,
where the science object is diffuse, often faint and
cannot be used for wavefront sensing.
The way out:
• To circumvent the sky coverage problem, several ways
have been devised and are actively pursued:
1. Multi-Conjugate Adaptive Optics (MCAO)
2. Multi Object Adaptive Optics (MOAO)
3. Ground Layer Adaptive Optics (GLAO)
4. Laser Tomography Adaptive Optics (LTAO)
MULTI CONJUGATE ADAPTIVE
OPTICS
MCAO definition
• To increase the isoplanatic patch, the idea is to design
an adaptive optical system with several deformable
mirrors (DM), each correcting for one of the turbulent
layer
Each DM is located at an image of the corresponding
layer in the optical system. (By definition, the layer and
the DM are called conjugated by the optical system.
What is multiconjugate? Case without
Turbulence Layers
Deformable mirror
What is multiconjugate? Case with it
Deformable mirrors
Turbulence Layers
Multiconjugate AO Set-up
Turb. Layers
#2
Atmosphere
UP
#1
Telescope
WFS
DM#2 DM#1
Proper use of the system
requires several wavefront
sensors to perform
Tomography
Altitude Layer
(phase
aberration = +)
Ground Layer = Pupil
(phase aberration = O)
Tomography = Stereoscopy
WFS#1
WFS#2
How this works
• Altitude of aberration proportional to shear at WFS ->
retrieve altitude from 2D wfs info. Tomography = stereoscopy
• In MCAO case: Restricted problem -> limited number of
DM. Treated as a whole. Similarly to AO were one does not
explicitely reconstruct the phase, in MCAO the 3D phase
distribution is not reconstructed, and then projected to the
DMs. The system computes directly the DM commands that
will minimize the error as measured by the WFS.
• More stable.
MAD, ESO’s Multi-conjugate Adaptive
optics Demonstrator (1st ligth may ‘07)
MCAO proposal (Initial TMT)
• 2-3 conjugate DMs
• 5-7 Laser Guide Stars
• 3 Tip-Tilt Stars
The reality…: GEMINI MCAO Module
LGS source
Science ADC simulator
NGS source
simulator
DMs
shutters
TTM
Beamsplitter
NGS
WFS
NGS
ADC
Diagnostic WFS
LGS WFS
LGS zoom corrector
Effectiveness of MCAO: no correction
Numerical simulations:
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view ~ 1.2’
• H band
Effectiveness of MCAO: classical AO
Numerical simulations:
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view ~ 1.2’
• H band
Effectiveness of MCAO: MCAO proper
Numerical simulations:
• 5 Natural guide stars
• 5 Wavefront sensors
• 2 mirrors
• 8 turbulence layers
• MK turbulence profile
• Field of view ~ 1.2’
• H band
MCAO Performance Summary
Early NGS results, MK Profile
No AO
Classical AO
1 DM / 1 NGS
320 stars / K band / 0.7’’ seeing
165’’
MCAO
2 DMs / 5 NGS
Stars magnified for clarity
Example of MCAO Performance
•
•
•
•
•
•
•
13x13 actuators system
K Band
5 LGSs in X of 1 arcmin on a
side
Cerro Pachon turbulence
profile
200 PDE/sub/ms for H.Order
WFS
Four R=18 TT GS 30” off axis
(MCAO)
One R=18 TT GS on axis(AO)
MCAO Performance
1
Classical LGS AO
MCAO
Strehl
1
0
Surface plots of Strehl ratio over a 1.5 arc min FoV.
13x13 actuator system, K band, CP turbulence.
Average Strehl (triangles)
• Robustness
• Sensitivity to noise1 is fairly better than with AO
Prop noise AO / Prop noise MCAO  sqrt( NGS )
• Predictive algorithms possible ?
4
.5
+
+
+
2
+
+
+
0
Profile number
Strehl St. dev across FoV % (+)
Other nice features of MCAO
Strehl in Fov
AverageFitting/AO
Fitting
Generalized
Generalized Fitting
(Finite number of DMs)
Geometry of the problem
Geometry of the problem
Highest spatial
frequencies projected
5/3
d
out ofdact
the command
Simulations

0
4
Model
1.75
13
13
.h
/ dact
Altitude [km]
c(h) = (h)-(h).h
< c(h) 2 >
vs .h
0.23(dact /r0)5/3
Error [rd2]  (.h)5/3
Generalized Anisoplanatism
(Finite number of Guide Star)
Additional error terms are necessary to
represent laser guide star MCAO.
Tomography error arises from the
finite number and placement of guide
stars on the sky. Generalized
anisoplanatism error results from the
correction of the continuous
atmosphere at only a finite number of
conjugate layer
altitudes.
Generalized Fitting
(Finite number of DMs)
Error [rd2]  (.h)5/3
Design Criteria e.g. Error balanced  hmax(,dact)
DM Spacing = 2 x hmax
dact
FoV [arcmin]
hmax [m]
NDM/GS
0.5
1
3000
3
0.2
1
1200
5-6
0.2
10
120
50
Generalized Anisoplanatism
(Finite number of Guide Star)
• Turbulence altitude estimation error
• OK toward GS, but error in between GS: Strehl “dips”
100”
FoVDM
= 70”
• Maximum FoV depends upon
pitch.
• Example for 7x7 system
Generalized Anisoplanatism goes down
with increasing apertures
2D info only
3D info
3D info
2D info only
Aperture
On MCAO for ELTs
• Generalized Fitting  FoV 5/3 for a fixed DM
configuration, regardless of D
NGSs or LGSs ?
• NGSs -> FoV of 15-20 arcmin to get S.C > 50% with 4
stars
• Gen.Fitting error blows up the error budget, unless
many DMs are used
• Many DMs mean many GS -> 20 arcmin not enough
-> NGS do not work for ELTs. Need LGS.
MCAO Pros and Cons
PROS:
• Enlarged Field of View
– PSF variability problem drastically reduced
• Cone-effect solved
• Gain in SNR (less sensitive to noise, predictive
algorithms)
• Marginally enlarged Sky Coverage (LGS systems)
CONS
• Complexity: Multiple Guide stars and DMs
• Other limitations: Generalized Fitting, anisoplanatism,
42
aliasing
MULTI OBJECTS ADAPTIVE
OPTICS
• In certain case, the user does not want to (or need to)
have a fully corrected image. He/she might be satisfied
with having only specific locations (i.e.) objects
corrected in the field.
• An AO system designed to provide this kind of
correction is called a Multi Objects Adaptive Optics
system
• MOAO are the systems of choice to feed spectrographs
and Integral Field Units in the ELT era.
–MOAO
• Up to 20 IFUs each with a DM
• 8-9 LGS
• 3-5 TTS
MOAO for TiPi (TMT)
MEMSDMs
Flat 3-axis
steering mirrors
OAPs
Tiled
MOAO
focalplane
4 of 16 d-IFU
spectrograph
units
Key Design Points for AO
Key points:
• 30x30 piezo DM placed at M6, providing partial
turbulence compensation over the 5’ field.
• All LGS picked off by a dichroic and directed back to
fixed LGS WFS behind M7. Dichroic moves to
accommodate variable LGS range.
• The OSM is used to select TT NGS and PSF reference
targets.
• MEMS devices placed downstream of the OSM to
provide independent compensation for each object:
16 science targets, 3 TT NGS, PSF reference targets.
LASER GUIDE STARS
LGS Related Problems: Null modes
• Tilt Anisoplanatism : Low order modes > Tip-Tilt at
altitude
–  Dynamic Plate Scale changes
• Within these modes, 5 “Null Modes” not seen by LGS
(Tilt indetermination problem)
 Need 3 well spread NGSs to control these modes
• Detailed Sky Coverage calculations (null modes
modal control, stellar statistics) lead to
approximately 15% at GP and 80% at b=30o
• Additional error terms are necessary to represent laser
guide star MCAO. Tomography error arises
• from the finite number and placement of guide stars on
the sky. Generalized anisoplanatism error results from
the correction of the continuous atmosphere at only a
finite number of conjugate layer altitudes
LGS Related Problems
90 km
min=D/2hNa
D
min
8
10’’
50
1’
LGS WFS Subsystem needs constant refocussing!
• Trombone design accomodates LGS altitudes between
85-210 km (Zenith to 65 degrees)
• Astigmatism corrector present / Will study Coma
corrector
TMT.IAO.PRE.06.03
54
TMT MIRES (proposal)
Concept
Overview
LGS
trombone
system
TMT.INS.PRE.06.02
55
Why Multiple Tip/Tilt NGS’s?
– Consider a turbulence profile with a
focus aberrations at two ranges (blue)
– LGS measurements (yellow) cannot
determine range of the aberration
» Tip/tilt information lost
» Equal focus measurement from each
LGS, regardless of aberration range
– Tip/tilt NGS measurements can
determine range from the differential
tilt between stars
– Three tip/tilt NGS’s needed for all three
quadratic modes
– Alternate approaches: Rayleigh LGS’s,
or a solution to the LGS tilt
indeterminacy problem
r)=a(cr+d)2
=ac2r2+2acdr+ad2
~ ac2r2
After tilt removal
r)=ar2
3. NGS WFS
• Radial+Linear stages with encoders
offer flexile design with min.
vignetting
• 6 probe arms operating in
“Meatlocker” just before focal plane
• 2x2 lenslets
EEV CCD60
• 6” FOV - 60x60 0.1” pix
Flamingos2 OIWFS
TMT.IAO.PRE.06.03
57
What is Tomography ?
90 km
2. Multiple guide star
Large DM’s are on every ELT
technological roadmap
Existing MEMS Device
(sufficient for Hybrid-MOAO)
Boston
Micromachines
32x32 actuator,
1.5 um MEMS
device.
(In Stock)
60