Title: Adaptive Optics in the VLT and ELT era Beyond Basic AO
1Adaptive Optics in the VLT and ELT era Beyond
Basic AO
François Wildi Observatoire de Genève
2Adaptive Optics wavefront errors reminder
- The residual wavefront error is the quality
criterion in AO - The wavefront error depends on
- The number of degrees do freedom (i.e. /- nb of
actuators) of the deformable mirror. - The lag (delay) in the control system
- The noise in the wavefront sensor which depends
on the guide star magnitude - The size of the field of view
- Side effects like WFS non-ideality, NCPA,
disturbances like vibrations
3Dependence of Strehl on l and number of DM
degrees of freedom
- Assume bright natural guide star
- No meast error or iso-planatism or bandwidth
error
Deformable mirror fitting error only
4Reminder 1 Dependence of Strehl on l and number
of DM degrees of freedom (fitting)
- Assume bright natural guide star
- No meast error or iso-planatism or bandwidth
error
Deformable mirror fitting error only
5Basics 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
Wavefront sensor
Computer
Transforms aberrations into intensity variations
6Types 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
7Shack-Hartmann wavefront sensor concept - measure
subaperture tilts
CCD
CCD
8WFS implementation
9How to reconstruct wavefront from measurements of
local tilt
10Effect 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
Assumes no fitting error or other error terms
11Effect of guide star magnitude (measurement
error)
Assumes no fitting error or other error terms
bright star
Decreaing measurement error
dim star
12Reminder 3 Strehl vs l and guide star angular
separation (anisoplanatism)
13Reminder 3 Strehl vs l and guide star angular
separation (anisoplanatism)
14Anisoplanatism side effect
- Because correction quality falls off rapidly
looking sideways from the guide star AND because
faint stars cannot be used as guide stars,Only
a very small part of the sky is accessible to
natural guide star AO systems!
15Sky coverage accounting for guide star densities
LGS coverage 80
Tip/tilt sensor magnitude limit
Hartmann sensor magnitude limit
Galactic latitude
NGS coverage 0.1
Isokinetic angle qk
Isoplanatic angle q0
16(Temporary) conclusion on isoplanatism
- 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.
17AOs great divide
Wide field LTAO (high coverage) GLAO MCAO
MOAO
18ExAO in a nutshell
- Like classical AO but more of the same
- The wavefront error minimized on axis
- Large number of degrees do freedom (i.e. /- nb
of actuators) of the deformable mirror. - Minimal lag (delay) in the control system
- Low noise in the wavefront sensor Bright guide
star - No field of view
- WFS non-ideality fought with spatial filter, NCPA
measured and corrected, disturbances like
vibrations countered with advanced signal
processing
19High contrast imaging
- Highest contrast observations require multiple
correction stages to correct for - Atmospheric turbulence
- Diffraction Pattern
- Quasi-static instrumental aberrations
20NCPA compensation
- Use of phase diversity for NCPA correction on
Vis. path
- Strong improvement of bench internal SR (45 -gt 85
in Vis) - various optimisations still to be performed
21NCPA compensation for IR path
Ghosts
NCPA compensation 320 modes estimated, 220
corrected
No compensation
22Implementation
CPI
Focus 1
HWP2
De-rotator
ITTM
HWP1
PTTM
Polar Cal
Focus 2
DM
Focus 4
NIR ADC
VIS ADC
DTTS
VIS corono
Focus 3
ZIMPOL
WFS
NIR corono
DTTP
IFS
IRDIS
23Sky coverage and Wide field in a nutshell
- To circumvent the sky coverage problem, several
ways have been devised and are actively pursued - Laser Tomography Adaptive Optics (LTAO)Laser
guide stars are used to probe the atmosphere and
project it in the science object direction - Ground Layer Adaptive Optics (GLAO)Laser guide
stars are used to probe the atmosphere but only
the ground layer is corrected - Multi-Conjugate Adaptive Optics (MCAO)Laser
guide stars are used to probe the atmosphere and
turbulence is projected and corrected in several
layers - Multi Object Adaptive Optics (MOAO)Laser guide
stars are used to probe the atmosphere and
turbulence is projected in several directions.
Each direction has one (or several DMs)
24Laser tomography AO
- In LTAO, the atmosphere is probed by multiple
Wave Front Sensors to form a model of the
atmosphere. This model is used to compute the
wavefront distorsion in a perticular direction
and therefore calculate a correction in that
direction. - It allows a good correction in a direction that
lacks a good natural guide star at the expense of
system complexity - Field is not increased!
25Proper use of the system requires several
wavefront sensors to perform Tomography
Altitude Layer (phase aberration )
Ground Layer Pupil (phase aberration O)
Tomography Stereoscopy
WFS1
WFS2
26WFS Set-up and LTAO reconstruction
Telescope
Turb. Layers
2
1
DM corrects 1 2 in red direction
WFS
Atmosphere
UP
27Ground layer AO
- In GLAO, the atmosphere is probed by multiple
Wave Front Sensors to form a model of the
atmosphere. Only the ground layer is extracted
form the model and used to feed back a correction
mirror conjugated to the ground. - It allows a correction of the atmospheric
wavefront error that happens in the common path
of all objects at the expense of system
complexity - Field is very large but performance is limited
28Performance expected from GLAO (Gemini)
29WFS Set-up and GLAO reconstruction
Turb. Layers
2
1
Telescope
WFS
DM corrects 1
Atmosphere
UP
30Laser guide stars vs natural guide stars
- Tomography can also be performed with natural
guide stars BUT - Requires planning the NGS for each observation
- Quality is not constant due to NGS geometry and
flux distribution - Requires movable wave front sensors
- Solution unanimously discarded today
31Multi Conjugate Adaptive Optics
- 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 layerEach 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).
32What is multiconjugate? Case without
Deformable mirror
33What is multiconjugate? Case with it
34Multiconjugate AO Set-up
Telescope
Turb. Layers
WFS
2
1
DM1
DM2
Atmosphere
UP
35Effectiveness 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
36Effectiveness 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
37Effectiveness 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
38MCAO Performance SummaryEarly NGS results, MK
Profile
2 DMs / 5 NGS
1 DM / 1 NGS
165
320 stars / K band / 0.7 seeing
Stars magnified for clarity
39The reality GEMINI MCAO Module
40Example 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 R18 TT GS 30 off axis (MCAO)
- One R18 TT GS on axis(AO)
41MCAO Performance
MCAO
Classical LGS AO
1
1
Strehl
0
- Surface plots of Strehl ratio over a 1.5 arc min
FoV. - 13x13 actuator system, K band, CP turbulence.
42Other nice features of MCAO
- Robustness
- Sensitivity to noise is fairly better than with
AO - Prop noise AO / Prop noise MCAO ? sqrt( NGS )
- Predictive algorithms possible ?
- Robustness
- Sensitivity to noise is fairly better than with
AO - Prop noise AO / Prop noise MCAO ? sqrt( NGS )
43Generalized Fitting (Finite number of DMs)
44Generalized 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.
45Generalized Fitting (Finite number of DMs)
Error rd2 ? (?.?h)5/3
Design Criteria e.g. Error balanced ?
?hmax(?,dact) DM Spacing 2 x ?hmax
46Generalized Anisoplanatism (Finite number of
Guide Star)
- Turbulence altitude estimation error
- OK toward GS, but error in between GS Strehl
dips - Maximum FoV depends upon DM pitch.
- Example for 7x7 system
-
47Generalized Anisoplanatism goes down with
increasing apertures
48MCAO 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, aliasing
49Multi objects adaptive optics
50- 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.
51(No Transcript)
52- MOAO
- Up to 20 IFUs each with a DM
- 8-9 LGS
- 3-5 TTS
53MOAO for TiPi (TMT)
54Key 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.
55Laser guide stars
56Laser guide star AO needs to use a faint tip-tilt
star to stabilize laser spot on sky
from A. Tokovinin
57Effective isoplanatic angle for image motion
isokinetic angle
- Image motion is due to low order modes of
turbulence - Measurement is integrated over whole telescope
aperture, so only modes with the largest
wavelengths contribute (others are averaged out) - Low order modes change more slowly in both time
and in angle on the sky - Isokinetic angle
- Analogue of isoplanatic angle, but for tip-tilt
only - Typical values in infrared of order 1 arc min
58Sky coverage is determined by distribution of
(faint) tip-tilt stars
From Keck AO book
59LGS Related Problems Null modes
- Tilt Anisoplanatism Low order modes gt 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 b30o
60(No Transcript)
61- 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
62LGS 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
63Concept Overview
TMT MIRES (proposal) LGS trombone system
643. NGS WFS
- RadialLinear stages with encoders offer flexile
design with min. vignetting - 6 probe arms operating in Meatlocker just
before focal plane - 2x2 lenslets
- 6 FOV - 60x60 0.1 pix
EEV CCD60
Flamingos2 OIWFS
65Issues 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
66Effects of laser guide star on overall AO error
budget
- The good news
- Laser is brighter than your average natural guide
star - Reduces measurement error
- Can point it right at your target
- Reduces anisoplanatism
- The bad news
- Still have tilt anisoplanatism stilt2
( ? / ?tilt )5/3 - New focus anisoplanatism sFA2 ( D /
d0 )5/3 - Laser spot larger than NGS smeas2 (
?b / SNR )2