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

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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

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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

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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

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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

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TMT focal plane

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ExAO state-of-the-art

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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)

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ELT ExAO Potential Science Reach

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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

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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)

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Reaching the Fundamental Limits

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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

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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+)

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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

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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)