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1
Extreme Contrast Adaptive Optics with
Extremely Large Telescopes
Richard DekanyCaltech Optical Observatories
23 July 2004
2004 Michelson Summer School
Frontiers of High Contrast Imaging in Astrophysics
2
Outline
• Extremely large telescopes– AO Scaling Laws– Thirty Meter Telescope
• Current and near-term ExAO state-of-the-art– Palomar AO Coronagraph– ELT AO Technical Challenge
• ELT ExAO– Architectural Elements– Performance Model
• Menagerie of Worrisome Phenomena (10-6, 10-8, 10-10)
• High-leverage Component Technologies– Potential Science Reach– Reaching the Fundamental Limits
• Strategies for Low-Q Operation (e.g. IFU’s)• Passive v. Active Speckle Suppression
– Ground / space ExAO comparison summary
3
Extremely Large Telescopes
4
Lessons of History
• Plot of largest optical/IR telescope size vs. time reveals exponential growth
– Remarkable given various social, economic, and technical factors
• Extrapolating from Keck 10 m:• 10 m 1993• 25 m 2034• 50 m 2065• 100 m 2097
– History does not explain how future gains will be made
Log10 collectin
g area (meters2
)
Courtesy J. Nelson
Large telescope projects
1950-2020
Hale
Keck1
Keck2MMTHETGemini (x2)VLT (x4)Magellan….others
LBT (x2)GTC
TMT
HST
SIRTF
NGST
TPF
1949
1990
1995
2000
2005
2010
2015
2020OWL
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Adaptive Optics (AO) Scaling Laws
• AO significantly extends the science gain of large telescopes– Signal-to-noise
• AO off ~ D• AO on ~ D * (D/r0) ~ D2 r0
-1, for unresolved background limited target
• The AO gain, (D/r0) is typically 30 - 60 in the near-IR– With such promising return, it must be hard, right?…
• Required number of degrees of freedom ~ D2 -12/5
– Required closed-loop bandwidth ~ -6/5
– Required wavefront measurement photon flux ~-18/5
– Required level of control of systematics ~
• Note, scaling laws to reduce residual wavefront error (~) are typically steeper than for increasing aperture diameter
7
Thirty-Meter Telescope
• Public / private collaboration of ACURA, AURA, Caltech, and UC• First light ~2015 w/ general-purpose AO• ExAO is currently a top priority 2nd generation instrument
8
ExAO contrast metric
• Approximate smooth-halo contrast estimate– Collected planet flux grows S * D2; where S is the Strehl ratio
– Halo flux per AO diffraction-limited resolution element (1-S)
– Contrast within a resolutions element Q S D2 / (1-S)
• Practical contrast limits within today’s small working angles are usually dominated by speckle noise from quasi-static errors– Sources are typically non-common-path errors
• Thermal induced telescope/instrument changes
• Gravity gradients
• Chromatic errors
• Local turbulence effects
9
Photon-noise-limited ExAO contrast metric
TMT/MCAO 248 nm
TMTExAO
PALM-300085 nm
PALAO165 nm
Adapted from J. Graham
Gemini ExAOC
Keck248 nm
Keck / XAOPI
10
TMT draft science capabilities
Field Mode Spatial Spectral Wavelength (µm)
1. AO 10” FoV n-IFU /D R ~ 4,000 0.6 - 5
2. (---) 20’ FoR N-Slit r0/D(/2) 150 < R < 6,000 0.3 - 1.3
3. AO 10” FoV 1-Slit /D 5,000 < R < 100,000 5 - 28
4. AO 5’ FoR n-IFU ~/D 2,000 < R < 10,000 0.8 - 2.5
5. AO 2” FoV C = 108 - 2x1010 /D 50 < R < 300 0.8 - 2.5
6. AO 2” FoV 1-Slit /D 20,000 < R < 100,000 1 - 5
7. ---- 5” FoV 1-Slit r0/D 50,000 < R < 100,000 0.3 -1.3
8. AO 30” FoV Imaging /D 5 < R < 100 0.6 - 5
• Notes:– FoV = Field of View, FoR = Field of Regard (fields quoted by diameter)– N >> n >> 1– (/2) Indicates GLAO option - to be evaluated
11
Adaptive Optics Modes
AO Mode(w/ corresponding science capability )
Wavelength range
Enabled science Components/
Instrument feed
Priority
MOAOa.Small-Field (#1, #6)
b.Multi-Objects on wide-field (#4)
a)0.65- 5b)1-2.5
•Galaxy chemistry•Star forming chemistry
•Multi Lasers•Deployable AO•MEMS•a) 0.005” IFU•b) 0.025-0.040” IFU
1st light, if successfully demonstrated
MIRAOMid IR (#3)
7-28 •Star forming regions, protoplanetary disks•Characterize planetary systems; AGNs
•Cryogenic DM or•Adaptive Secondary•NGS or multi-lasers•MidIR Echelle Spectrometer•MidIR Imager
1st light
GLAOWide Field (#2)(Ground Layer)
0.31-1.0 •Large sample galaxy spectra •Optical multiobject spectrograph
Option on 1st light wide-field instrument
ExAOExtreme (#5)
0.8-2.5 •Exo planet imaging•Protoplanetary disks
•MEMS•Coronagraph or Nulling Interferometer Planet Imager
Not yet known
MCAOMulticonjugate (#8)
0.8-5 •Dark ages•Early galaxies, AGNs•Nearby galaxies resolved star pop. and nuclei•Galactic Center•Star forming regions
•Multi Lasers•Tomography•Single or multi- DM•IFU (with imaging)
2nd light, assuming MOAO validation
12
TMT focal plane
13
ExAO state-of-the-art
14
ExAO state-of-the-art
Pal
omar
out
er w
orki
ng a
ngle
N=
16
Kec
k ou
ter
wor
king
ang
le N
=18
Pal
omar
oc
cult
ing
spot
PA
LM
-300
0 ou
ter
wor
king
ang
le N
=64
(20
07)
AE
OS
out
er w
orki
ng a
ngle
N=
32 (
2004
)
Gem
ini E
xAO
C o
uter
wor
king
ang
le N
=64
(20
09)
Courtesy B. Macintosh and S. Metchev
15
ExAO today
• ExAO today is saddled with the heavy yoke of general-use AO systems– Today’s AO:
• Is designed to optimize faint guide star Strehl ratio over wide FoV• Relies on non-common-path and non-common-wavelength wavefront sensing• Uses 70 yr-old coronagraph technology• Tolerates hysteretic and temperature dependent deformable mirrors• Is devoid of any real-time metrology• And Nyquist samples the focal plane
• One can hardly imagine setting out to design a worse ExAO system
• ELT ExAO systems are likely to:– Be highly specialized to the specific scientific requirements (ie, search young
systems for hot exo-Jupiters in emission; find water on exo-Earths at Eps Eri; etc.)– Pursue brand new architectures– Require successive generations of prototypes and demonstrations– Require large amounts of telescope time
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Open LoopFWHM ~0.70 arcsec
Strehl ~2% at K
Log Stretch
Closed LoopFWHM 0.090 arcsec
Strehl ~80% at K165nm Wavefront Error
Palomar Adaptive Optics
• Facility instrument at Palomar observatory for last ~4 years• The most requested instrument at Palomar• Natural guide star AO system
– 16x16 subapertures– Bright guide star Strehls as high as 80% at 2.2 m
• Maximum frame rate 2000Hz (<7e- read noise)– Limiting magnitude ~13.5mV, 10-15% Strehl at 2.2 m
• Read noise 3.5e- at < 500 fps• Science Camera
– J, H, and K imaging and 0.025 and 0.040 arcseconds/pixel– Coronagraph 0.41 and 0.91 arcsecond spot– J, H and K spectra at R~1500
17
PALAO High Strehl Images
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Palomar AO Comparison
’
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Palomar Performance
• Excellent agreement with simulation• High-Contrast imaging:
– AO corrected image is only a factor of 3 worse then perfect case for field angles greater then 0.5 arcseconds
• Spectra (and optical communication):– A factor of 2.4 improvement in 80% enclosed energy
20
Ks = 11.3 at 2.6″ (3.0*10-5)
Ks = 13.6 at 3.3″ 3.6*10-6)
6 x 60 sec on-source exposure with coronagraph;
Ks band (2.16µm);
V = 6.9; Strehl 65%.
HD 166435
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RMS wavefront error vs. Telescope Diameter
0
50
100
150
200
250
300
350
400
1 10 100
Telescope Diameter (m)
RM
S w
avef
ron
t er
ror
mV < 7
mV = 13
mV > 16
mV < 7 (proposed)
mV = 13 (proposed)
mV = 16 (proposed)
AO challenge for ELTs
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D/sigma vs Year
10.0
100.0
1000.0
1980 1990 2000 2010 2020
Year
D/s
igm
a *
10
^6
mV < 7
mV = 13
mV > 16
mV < 7 (proposed)
mV = 13 (proposed)
mV > 16 (proposed)
AO development
23
ExAO architecture elements
• A respectable first stage AO system– Typically > 85% Strehl ratio (enables linearization of the residual
phase errors)
• Excellent diffraction suppression– Many techniques exist (e.g., occulting or phase-mask coronagraph,
nulling beamcombiner, Gaussian pupil apodization, cats-eye apodization)
• Dedicated system for nanometer wavefront control– Second stage high-order AO – Dark hole (Malbet), dark speckle (Layberie), black speckle (Dekany),
ripple sensor (Angel, Traub)
• New detection architectures– Polarization, multi-wavelength backends have make it to the telescope– IFU’s, interferometers, statistics engines et al. have not yet
• Calibration, calibration, calibration– Data analysis pipeline and algorithms directly drive the hardware
architecture (integrated experiment design)
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ExAO performance model
25
Key ExAO concepts
• Working region– Area of focal plane between inner and outer working angle, where
wavefront corrector exhibits beneficial control (a function of wavelength)
• Phase ripple– A single frequency component of a two-dimensional wavefront phase
power spectrum– Phase ripple variance, k
2, is integral of power spectrum from k to k+dk
• Q value– Planet photoflux [photons/m2/sec] divided by stellar photoflux within a
single focal plane resolution element• Q = 4 Cplanet/k
2
• Unpublished Palomar AO results (Boccaletti, 2002) hint thatQ ~ 1/60 detections possible with existing systems and careful calibration
26
Small phase error PSF
• Using a 2nd order complex wave expansion, the halo can be described as the sum of individual haloes for independent error processes having unique spatio-temporal behavior
• Example power spectra (following Rigaut et al. 1998)
i itotal Aap
22'
fcSpatial frequency, f
Wav
efro
nt v
aria
nce
rad
2/m
1/2
Fitting error
fcSpatial frequency, f
Wav
efro
nt v
aria
nce
rad
2 /m
1/2
Measurement error
1/r0
Spatial frequency, f [m-1]W
avef
ront
var
ianc
era
d2 /
m1/
2
Boiling wind errors
27
PSF halo widths
• The high-contrast point spread function (PSF) can be modeled as the superposition of haloes due to various error processes– Each halo has it’s own envelope, width (w), and speckle lifetime
• For frozen wind and boiling wind, w = /r0
• For scintillation, w = Sqrt(z); the Fresnel length
• For photon noise, WFS read noise, w = /dx; dx = actuator spacing (assumed same as WFS sensor spacing for pupil sensor)
• Interference effects average away over many speckle lifetimes
– Each halo contributes to reduce the SNR of planet detection
• The contribution of the total wavefront variance attributable to a single phase ripple of spatial frequency k is
2k = 2 / Nspeck in halo = 2
* (/w)2/(/D)2 = 2 * (w/D)2
28
Minimizing integration time
• Optimize AO system design for subaperture diameter, dx, and sensor sample time, dt, for different observation cases
)()( specklesvarphotonvar
signalPlanetSNR
where S is Strehl ratio, Fp is planet flux, A is telescope area, Fsky is sky background flux, Fs
sci is parent star flux at science wavelength, s2i is wavefront variance from ith error
process, w is ith halo width, ti is the coherence time of the ith process, and T is total integration time
T
N
F
N
FAFSAF
AFSSNR
iii
speckles
scis
ii
speckles
scis
psky
p
4
2
2
2
2
2
161616
16
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Speckle noise
• Speckle noise (e.g. Racine, et al. 1999)– Fundamentally different than photon noise
• Speckle noise variance based upon the square of speckle photoflux
• Smooth-halo photon noise variance based upon speckle photoflux
• PALAO PSF stability (Apr 04) over 1 minute
• 15 five-second K-band images taken on a 6th magnitude star in 0.9” (visible) seeing. The images are log stretched and 3 arcsec on a side.
• The average Strehl is 80% +- 2%, equivalent to165 nm +- 9nm of RMS wavefront error
• Coronagraph contrast (~ 5 x 10-4) dominated by speckle noise
30
ExAO issues I
Effect (10-6, 10-8, 10-10) Potential mitigation Mitigation maturity
Aliasing in the wavefront sensor Spatial filtering
Focal-plane WFSing
Moderate (simulations)Moderate (concepts)
Aliasing in the science array Spatial filtering Moderate (simulations)
Boiling wind (e.g. non-predictable phase errors)
Higher correction bandwidth Moderate
Complex occulter index of refraction
Better understand and/or materials Poor
Chromatism Meteorological monitoring
Common-band WFSing
Moderate
Moderate (concepts)
Deformable mirror fitting error Higher spatial bandwidth Moderate
Detector charge diffusion and amplifier glow
(science and/or WFS)
Improved detectors Moderate
Direct scintillation halo Active amplitude correctionTwo-conjugate correction
PoorModerate (simulations)
Space and Ground Ground only
31
ExAO issues II
Effect (10-6, 10-8, 10-10) Potential mitigation Mitigation maturity
Dispersion displacement Lateral dispersion correctorOptimized spectralwidth of WFS
Moderate (concepts)Moderate (concepts)
Flat-field stability Improved detectors Moderate
Fourth-order terms in the wavefront expansion
Higher Strehl
Contrast-optimizing amplitude and phase control laws
Poor
Poor
Frozen wind lag (e.g. predictable phase errors)
Predictive phase correction of multilayer atmosphere
Moderate (concepts)
Index of refraction inhomogeneities
More uniform materials, better pointing control
Poor
Multispectral error Common-band WFSing Good
Non-common path phase errors
Common-path WFSing
Improved calibration/metrology
Moderate (concepts)Poor
Non-common path polarization effects
Slow F/# telescopes, polarizers
Vector field AO coronagraph design
Poor
Space and Ground Ground only
32
ExAO issues III
Effect (10-6, 10-8, 10-10) Potential mitigation Mitigation maturity
Residual tip/tilt jitter Better control Good
Scintillation in WFS Amplitude correctionSensing of higher moments
Moderate, for clearing inner halo
Telescope pointing errors(Beam walk using a T/T mirror)
Better telescope pointing
Adaptive secondaries (to minimize beam walk)
Moderate
Uncorrectable dynamic telescope errors
Improved ACS, telescope stiffness, wind shielding
Moderate
WFS calibration instability WFS’s insensitive to seeing changes
Active thermal control
ModerateModerate
WFS star and background photon noise
Optimized system design Good
WFS read and dark current noise
Improved detectors Moderate
Space and Ground Ground only
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Cases to be considered
‘Fundamental’ errors ‘Expanded’ list of errors
Required innovation
Planet photon noiseSky photon noiseWFS photon noise
Boiling windResidual dispersion displacement
Fundamental errors +
Frozen windWFS read noise
ScintillationResidual chromatismMultispectral error
--
Predictive controlNoiseless detectors
Amplitude correctionCommon-band sensingCommon-band sensing
Sensing / Science mode Comment
R-band / H-band Traditional AO(chosen for maximum sky coverage)
R-band / R-band Limited by current deformable mirrors
H-band / H-band Limited by current detectors
34
Performance model example(with estimated H-band speckle coherence times)
• Planet photon noise• Sky photon noise (msky = 16)
• Extrasolar exozodical light well-resolved for D=30 m, so not significant
• WFS photon noise (phot = dt, the system update rate)• Atmospheric phase estimate imperfect due to WFS photon statistics
• Scintillation (scint = 0.024 sec)• Due to amplitude fluctuations arising from high-altitude turbulence• We will assume strong high-altitude turbulence
• Frozen wind (wind = 0.009 sec)• Correction is late due to finite AO correction bandwidth
– Solution: By definition, can be eliminated with predictive controller
• Boiling wind error (boil = 0.200 sec)• Component of error not predictable
• WFS detector read noise (phot = dt)• Includes dark current shot noise, etc.
– Solution: Photon-counting detectors
35
Integration time by effect (30 m Sun-Jupiter-analogue)
Error termsTi [hours]
R/HExpanded list of errors
(dx = 0.33m, dt = 0.083 ms)
H/H Fundamental errors
(dx = 0.19m, dt = 0.17 ms)
Multispectral error 7.5 --
Residual chromatism 0.6 --
Dispersion displacement 0.4 0.0006
Scintillation 0.4 --
Detector read noise 0.4 --
Frozen wind 0.3 --
WFS photon noise 0.15 0.3
Boiling wind 0.01 0.1
Sky photon noise 0.002 0.004
Planet photon noise 0.0002 0.0003
Total integration time 9.8 0.4
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Tint and Q / Expanded error terms
Expanded error terms
Integration time [hours] and Q value (Tint / Q)
to achieve SNR = 5 for various ground-based aperture diameters and contrast levels
Sensing band / Science band
10 meter@ 10-8
10 meter@ 10-9
30 meter@ 10-9
30 meter@ 1.7 x 10-10
R / H
dxopt = 32 cm
dtopt = 0.083 ms
7.4 / 0.001 740 / 0.0001 9.8 / 0.001 340 / 0.0001
R / R
dxopt= 31 cm
dtopt= 0.12 ms 2.4/ 0.001 240 / 0.0001 3.1 / 0.001 110 / 0.0002
H / Hdxopt = 125 cm
dtopt = 0.11 ms(May violate quadratic expansion assumption)
4.6 / 0.001 460 / 0.0001 5.7 / 0.0001 200 / 0.0001
37
Tint and Q / Fundamental error terms
Fundamental error terms
Integration time [hours] and Q value (Tint / Q)
to achieve SNR = 5 for various ground-based aperture diameters and contrast levels
Sensing band / Science band
10 meter@ 10-8
10 meter@ 10-9
30 meter@ 10-9
30 meter@ 1.7 x 10-10
R / H
dxopt = 16 cm
dtopt = 0.15 ms
6.1 / 0.002 610 / 0.0002 8.1 / 0.002 280 / 0.0003
R / R
dxopt(10m) = 8.8 cm
dtopt(10m) = 0.23 ms
dxopt(30m) = 11 cm
dtopt(30m) = 0.11 ms
0.10 / 0.008 9.5 / 0.0008 0.24 / 0.003 8.2 / 0.0006
H / H
dxopt = 19 cm
dtopt = 0.17 ms
0.35 / 0.003 35 / 0.0003 0.4 / 0.002 15 / 0.0004
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Scope of target list
0.01
0.1
1
10
100
0 1 2 3 4 5 6 7 8
Guide star magnitude [mV]
Inte
grat
ion
time
[hou
rs]
R/H Expanded
H/H fundamental
Integration time vs. guide star magnitude for R/H expanded and H/H fundamental error cases, using optimized dx and dt pairs. We again consider a Sun-Jupiter analogue at 10 pc, Cplanet = 10-9, D = 30m, 45 degree zenith angle.
For most cases, systems optimized for solar analogue good to mv = 6.
39
Better NIR detectors?
Integration time as a function of WFS read noise for H/H operation and the expanded error list. For each value of read noise, an optimal dx and dt were determined. For zero read noise, the optimal dx = 0.20 m and dt = 0.075 msec, growing for read noise = 50 e- rms to dx = 1.7 m and dt = 0.290 msec. Note, for large values of dx, the Strehl ratio in practice falls due to wavefront fitting error, violating the assumption that the quadratic phase used here.
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50
H-band WFS read noise [electrons, rms]
Inte
gra
tion
tim
e [
hr]
40
Larger telescopes
0.010
0.100
1.000
10.000
100.000
1000.000
0 10 20 30 40 50 60 70 80 90 100
Telescope diameter [m]
Inte
grat
ion
time
[hr]
R/H expanded
H/H fundamental
Integration time vs. telescope diameter for R-band sensing/H-band science (expanded list of errors) and for H-band sensing/ H-band science (fundamental errors). The target system is a Sun-Jupiter analogue at 10 pc (Cplanet = 10-9) and the desired SNR = 5. Each case has been
separately optimized (R/H has dx = 0.33 m and dt = 0.083 msec, H/H has dx = 0.19 m and dt = 0.16 msec).
Exoearth times typically 50x greater, but use similar architectures w/ similar D dependence.
Note, 8-10m’s can’t reach mature 5AU exojupiters at 10pc in reflection
41
42
Exoplanet astrometry and photometryat 30 m fundamental limits
• With repeated imaging observations, we can deduce– From orbital characteristics
• Equilibrium temperature• Tidal locking• Resonances among sibling planets
– From phase function• Presence of a cloudy atmosphere• Albedo rotation rates
• Mean radius of habitable zone at 15 pc– 31 /D (R-band) and 13/D (H-band)– Aggressive apodization possible due to large collector and high angular resolution
• For nearest few stars, binary exoearths or exoearths ‘moons’ of exojupiters could be resolved (but SNR still low)– @ 3 pc, resolution of 0.01 AU 25 Rjupiter = orbital radius of Callisto
• Within 15 pc, there are hundreds of plausible candidate stars for TMT-based exoearth search at R = 5 (down to mV = 6)
43
ELT ExAO Potential Science Reach
44
45
Ground-based exoearth spectroscopyat 30 m fundamental limits
• Photon-noise limited R = 5 spectroscopy (visible and near-IR) would enable:– The presence of a clear atmosphere (e.g. Earth via Rayleigh scatter), a
deeply clouded atmosphere (e.g. Venus via Mie scatter)
• R = 20 spectroscopy might be reachable– Require long integrations and careful calibration of Telluric effects– Notable exceptions possible in sub-classes of exoearths
• e.g. H2O steam lines (seen in brown dwarfs from the Earth’s surface)
• Other plausible, unearthly atmospheres can be imagined
• Biomarkers (e.g. O2, O3, CH4) are likely not available with 30 m SNR from Earth’s surface– High spectral resolution (R=70) implies prohibitive integration times– Telluric confusion may not be soluble at such small SNR
• Technique using orbital Doppler shifting of narrow lines, used to study brown dwarfs, generally not available due to low R
46
Ultimate science reach for 30m
• Fundamental limits for AO allow direct detection of exoearths with TMT but not biomarker studies
• Potential number of systems – Thousands for hot, young exojupiters (R = 10-1000)– Hundreds for mature exojupiters (R = 10-100) – Scores for exoearths (R = 5)
• Each requires tens of hours of observation– Observations favor R = 2 R planets, e.g. waterworlds
• High-resolution spectroscopy is very difficult except for non-terrestrial atmospheres (e.g. steam lines or severe pressure broadening)
47
Reaching the Fundamental Limits
48
Reaching the ground fundamental limits
• New techniques– Speckle noise suppression
• Post-processing– Chromatic techniques– Photon statistical techniques
• Active– Higher-Strehl ratio operation– Complex amplitude optimizing control laws (not phase conjugation)
• New components– Deformable mirrors with 104 - 105 actuators– Stable back-end instruments– Focal plane wavefront sensors
• Prototype systems– Develop H/H or R/R band AO systems optimized for high contrast– Many currently uncontrolled error processes must be addressed by design
(partial list follows)– Typical development cycle for 8-10m telescope is 5 years and $10M
• Likely to need several generations to get from 2 x 10-4 to 10-8
49
Speckle discrimination in post-processing
• Published techniques for PSF subtraction– Achromatic techniques
• PSF calibrator star – COME-ON Plus (c. 1997)
• Multiple “roll angles”– Field (Keck) and pupil (Palomar) rotation (c. 2000)
• Centro-symmetric PSF subtraction (2002)– Chromatic techniques
• Discreet multispectral discrimination– TRIDENT – 3 channel (2001)
– Several discrete channel successors (2004+)
50
Active speckle suppression
• The large penalty for speckle noise arises when bright focal plane speckles are allowed to build up (typ. 1000’s photons)– This suggests one strategy: avoid speckle noise by running closed-loop
correction so fast that speckles typically contain only a few photons
• Wavefront sensing in the focal plane– Minimizes speckle noise (as well as many other error sources)– Decoupling of wavefront sensing (into the focal plane) allows more
flexibility in DM technology (at the pupil plane)• Phase and wavelength information are both needed• Concepts:
– An interferometric technique has been suggested by Angel (2002)– Superconducting tunnel junctions (STJ’s) appear well-suited, but remain
small format– New field, open to new architectures
– While good Strehl is needed to sharpen planet light, modest DM formats (typ. N = 128) allow exploration of habitable zones on exoearth target list
51
IFU’s for exoplanet study
• The logical extension of multichannel coronagraphic imagers– R = 20-100 spectroscopic speckle discrimination– An intermediate step toward spectroscopic focal-plane wavefront sensors
• ExAO is new application for IFU’s– Requires development of new observational techniques and data analysis– Requires a professional group of exoplanet IFU researchers– We need to learn how to use these things
• Near-term integral field coronagraph (IFC) prototype options– Lab tests
• Rapid prototyping, but does not engage scientific community– Existing AO systems
• All require new (presumably, warm) coronagraph relay and have larger than necessary spectral resolution
– Slit spectrographs (most existing AO systems)– AO-fed IFU’s (e.g. Keck/OSIRIS, 2004)
– Gemini Extreme AO Coronagraph (ExAOC) (2009)• Instrument call includes consideration of an IFU-mode
52
IFU’s for exoplanet study (cont.)
• Demand for low-Q operation drives ground ExAO concepts– A photon-counting IFU can be used to determine wavefront
amplitude and phase and drive the ‘sharp-end’ of an optimized ExAO system (e.g. hierarchical control)
• IFU technology in the path of TMT and other ELT ExAO development
• Ground-based experience with ExAO IFU’s could be extremely useful for TPF coronagraph mission design– Similar sub-component requirements (Detectors, fibers/slicers, etc.)
– Similar data sets– Similar analysis techniques (Implies similar humans)
– Difference is only bandwidth of wavefront control loop
53
Why must TPF work at Q=1?
• Low-Q operation aka speckle discrimination is fundamental to all techniques of high-contrast direct detection, and is stock in trade for ground systems
• Ground-based observers only have just a few years experience in PSF calibration, but no one on the ground is planning Q=1 instruments– Q = 0.25 published PALAO (Boccaletti, 2002)– Q < 0.05 reported MMT (Close, private communication, 2003)– Q = 0.016 unpublished PALAO (Boccaletti, 2002)
• Working Q value for ground-based exoplanet study will be < 0.1 for next 20 years– We don’t fret about about this, but seek to develop new techniques
• Significant relaxation of TPF coronagraph requirements possible if tightest contrast (1/2 exoearth) requirements planned for Q = 0.1
54
Exoearth study comparison
• Ground 30m – Cons
• Biomarkers not accessible• R < 20 (integration time limited)• Must master Q = 0.001• One hemisphere accessible• Not top ELT priority• Many new technologies required
to reach fundamental limits– Speckle suppression
– Pros• Rapid instrument development • Smaller inner working angle• Higher spatial resolution• Science possible in red and
near-IR• Typically 10 nm rms WFE
• Space– Pros
• Biomarkers accessible • R > 20• Could work at Q = 0.1 – 1.0• Science in visible or mid-IR• Whole sky accessible• Enterprise mission – top priority
– Cons• Slow mission development • Typically 0.02 nm rms WFE• Highly stable• Many new technologies required
to reach fundamental limits• Speckle suppression
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EAO key concepts
• Phase ripple– A single frequency component of a two-dimensional wavefront phase
power spectrum
– Phase ripple variance, k2, is integral of power spectrum from k to k+dk
• Q value– Planet photoflux [photons/m2/sec] divided by stellar photoflux within a
single focal plane resolution element
• Q = 4 Cplanet/k2
• Unpublished Palomar AO results (Boccaletti, 2002) hint that Q ~ 1/60 detections possible with careful calibration
• Working region– Area of focal plane between inner and outer working angle, where
wavefront corrector exhibits beneficial control (a function of wavelength)