Extreme Contrast Adaptive Optics with Extremely Large Telescopes

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					Extreme Contrast Adaptive Optics
              with
  Extremely Large Telescopes

                   Richard Dekany
             Caltech Optical Observatories

                     23 July 2004


           2004 Michelson Summer School
  Frontiers of High Contrast Imaging in Astrophysics



                                                       1
                              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

                                                               2
Extremely Large Telescopes




                             3
                     Lessons of History



•   Plot of largest optical/IR
    telescope size vs. time reveals
    exponential growth
     – Remarkable given various
       social, economic, and technical




                                         Log10 collecting area (meters2)
       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


                                                                           Courtesy J. Nelson

                                                                                                4
              Hale                  1949


                                    1990

  Large       Keck1
                            HST



telescope     Keck2
                                    1995
              MMT

 projects     HET
              Gemini (x2)
              VLT (x4)              2000

1950-2020     Magellan
              ….others
                            SIRTF

              LBT (x2)
              GTC                   2005


                            NGST
                                    2010
              TMT


                            TPF     2015


        OWL
                                    2020
     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 l-12/5
    – Required closed-loop bandwidth ~ l-6/5
    – Required wavefront measurement photon flux ~l-18/5
    – Required level of control of systematics ~l

• Note, scaling laws to reduce residual wavefront error (~l) are
  typically steeper than for increasing aperture diameter



                                                                           6
              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




                                                                   7
                  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




                                                                          8
         Photon-noise-limited ExAO contrast metric




                              TMT/MCAO                                  TMT
                               248 nm
                                                                        ExAO


                                Keck             Keck / XAOPI
                               248 nm                                   Gemini
                                                                        ExAOC
                                        PALAO
                                        165 nm

                                                          PALM-3000
                                                                85 nm




Adapted from J. Graham                                                       9
                      TMT draft science capabilities

              Field     Mode                Spatial    Spectral                  Wavelength (µm)
1.   AO       10” FoV   n-IFU               l/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              l/D        5,000 < R < 100,000         5 - 28
4.   AO       5‟ FoR    n-IFU               ~l/D       2,000 < R < 10,000          0.8 - 2.5
5.   AO       2” FoV    C = 108 - 2x1010    l/D        50 < R < 300                0.8 - 2.5
6.   AO       2” FoV    1-Slit              l/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             l/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


                                                                                              10
                            Adaptive Optics Modes
AO Mode                  Wavelength   Enabled science                                  Components/                    Priority
(w/ corresponding        range                                                         Instrument feed
science capability )
                                      •Galaxy chemistry                                •Multi Lasers                  1st light, if
MOAO                     a)0.65- 5m
                                      •Star forming chemistry                          •Deployable AO                 successfully
a.Small-Field (#1, #6)   b)1-2.5 m                                                     •MEMS                          demonstrated
b.Multi-Objects on                                                                     •a) 0.005” IFU
wide-field (#4)                                                                        •b) 0.025-0.040” IFU


                                      •Star forming regions, protoplanetary disks      •Cryogenic DM or               1st light
MIRAO                    7-28m
                                      •Characterize planetary systems; AGNs            •Adaptive Secondary
Mid IR (#3)
                                                                                       •NGS or multi-lasers
                                                                                       •MidIR Echelle Spectrometer
                                                                                       •MidIR Imager


                                      •Large sample galaxy spectra                     •Optical multiobject           Option on 1st
GLAO                     0.31-1.0m
                                                                                       spectrograph                   light wide-field
Wide Field (#2)
(Ground Layer)
                                                                                                                      instrument

                                      •Exo planet imaging                              •MEMS                          Not yet known
ExAO                     0.8-2.5m
                                      •Protoplanetary disks                            •Coronagraph or Nulling
Extreme (#5)                                                                           Interferometer Planet Imager



                                      •Dark ages                                       •Multi Lasers                  2nd light,
MCAO                     0.8-5m
                                      •Early galaxies, AGNs                            •Tomography                    assuming
Multiconjugate (#8)                   •Nearby galaxies resolved star pop. and nuclei   •Single or multi- DM           MOAO
                                      •Galactic Center
                                                                                       •IFU (with imaging)            validation
                                      •Star forming regions
                                                                                                                                         11
TMT focal plane




                  12
ExAO state-of-the-art




                        13
                                       Keck outer working angle N=18
                                                                                       Palomar
                                       Palomar outer working angle N=16               occulting
                                                                                           spot




                                       Gemini ExAOC outer working angle N=64 (2009)
                                       AEOS outer working angle N=32 (2004)




                                       PALM-3000 outer working angle N=64 (2007)
                                                                                                  ExAO state-of-the-art




Courtesy B. Macintosh and S. Metchev
                        14
                               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                                         15
                     Palomar Adaptive Optics


       Open Loop                                                        Closed Loop
    FWHM ~0.70 arcsec                                               FWHM 0.090 arcsec
     Strehl ~2% at K                                                  Strehl ~80% at K
                                                                   165nm Wavefront Error
       Log Stretch


•   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 mm
         • Maximum frame rate 2000Hz (<7e- read noise)
     – Limiting magnitude ~13.5mV, 10-15% Strehl at 2.2 mm
         • 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                                                        16
                PALAO High Strehl Images

 Ten
 5 sec images
 K
 s (2.145 mm)
 r
 0(0.5 mm) ~ 11 cm
Strehl (Mean/sample Stdev): 75% +/- 2%
RMS Wavefront error: 185 +/- 10 nm
Wavefront error Max/Min: 198/168 nm

Strehl estimates are a lower estimate of
truth
    Ignore 2% Strehl loss to reflection
    Ignore spiders an ~3% effect
     Mean Wavefront 165 nm!
     




                                            17
                               Palomar AO Comparison
3 arcseconds




                                                                                                                Log
                                                                                                              Stretch




                 Perfect Image                 Simulated AO Image                     Measured image on sky
                 (simulated wave diffraction   RMS wavefront = 130nm                  RMS wavefront=165nm
                 w/ spiders and 1.5% ghost)
                                               Strehl=0.86                                Strehl= 0.80
                                               (using Caltechs Arroyo C++ lirary)


               Simulations performed using Arroyo (Caltech)
               Wavelength Ks (2.145 mm, width=0.3mm)
                5
                second exposures
               Excellent agreement with simulations!
                   Difference of 100nm is consistent with AO caliration
                  errors
                                                                                                                   18
                      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



                                                                                           19
                                Ks = 11.3 at 2.6″ (3.0*10-5)
                    HD 166435




6 x 60 sec on-source exposure
with coronagraph;
                                     Ks = 13.6 at 3.3″ (3.6*10-6)
Ks band (2.16µm);
                                   Slide courtesy of Stan Metchev
V = 6.9; Strehl  65%.
                                                                     20
                                      AO challenge for ELTs


                                RMS wavefront error vs. Telescope Diameter

                      400
                            AO experience
RMS wavefront error




                      350                                          mV < 7
                      300                      ELT LGS
                                               Trajectory          mV = 13
                      250
                                                                   mV > 16
                      200
                                                                   mV < 7 (proposed)
                      150
                      100                                          mV = 13 (proposed)
                       50                                          mV = 16 (proposed)
                        0
                            1                10              100
                                    Telescope Diameter (m)
                                                                                       21
                                     AO development


                                         D/sigma vs Year

                 1000.0
                                                                  mV < 7
D/sigma * 10^6




                                   ELT LGS                        mV = 13
                                   Trajectory
                                                                  mV > 16
                  100.0
                             AO experience                        mV < 7 (proposed)
                                                                  mV = 13 (proposed)
                                                                  mV > 16 (proposed)
                   10.0
                      1980      1990         2000   2010   2020
                                             Year

                                                                                      22
            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)
                                                                            23
ExAO performance model




                         24
                    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, sk2, 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/sk2
       • Unpublished Palomar AO results (Boccaletti, 2002) hint that
         Q ~ 1/60 detections possible with existing systems and careful
         calibration

                                                                                  25
                                       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

                                  ptotal = a                       (    i
                                                                                Fi
                                                                                            2
                                                                                                + A'
                                                                                                            2
                                                                                                                        )
• Example power spectra (following Rigaut et al. 1998)
                           Fitting error                            Measurement error                                   Boiling wind errors
                                               Wavefront variance




                                                                                                   Wavefront variance
   Wavefront variance




                                                   rad2/m1/2




                                                                                                       rad2/m1/2
       rad2/m1/2




                                  fc                                            fc                                                   1/r0
                        Spatial frequency, f                         Spatial frequency, f                               Spatial frequency, f [m-1]
                                                                                                                                                     26
                      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 = l/r0
       • For scintillation, w = Sqrt(lz); the Fresnel length
       • For photon noise, WFS read noise, w = l/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

        s2k = s2 / Nspeck in halo = s2 * (l/w)2/(l/D)2 = s2 * (w/D)2


                                                                         27
                   Minimizing integration time

• Optimize AO system design for subaperture diameter, dx,
  and sensor sample time, dt, for different observation cases

                                            Planet signal
                     SNR =
                                  var( photon) + var( speckles)

                                            16 (S F p A)
                                                           2

   SNR 2 =                                                                                       T
                                          F          
                                                                              2
                                                sci 2
                                                                Fssci 
             16 Fsky A + 16 S F p A + 16  s          
                                          N speckles    s +  N
                                                               
                                                               i
                                                                2         
                                                                                  (s   iti )
                                                                                          4


                                                        i     speckles        i



where S is Strehl ratio, Fp is planet flux, A is telescope area, Fsky is sky background flux,
   Fssci 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

                                                                                                     28
                         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 to
                                                    165 nm +- 9nm of RMS
                                                    wavefront error
                                            •   Coronagraph contrast (~ 5 x 10-4)
                                                dominated by speckle noise
                                                                                  29
                                     ExAO issues I

Effect (10-6, 10-8, 10-10)           Potential mitigation                 Mitigation
                                                                          maturity
Aliasing in the wavefront sensor     Spatial filtering                    Moderate (simulations)
                                     Focal-plane WFSing                   Moderate (concepts)
Aliasing in the science array        Spatial filtering                    Moderate (simulations)
Boiling wind (e.g. non-              Higher correction bandwidth          Moderate
predictable phase errors)
Complex occulter index of            Better understand and/or materials   Poor
refraction
Chromatism                           Meteorological monitoring            Moderate
                                     Common-band WFSing                   Moderate (concepts)
Deformable mirror fitting error      Higher spatial bandwidth             Moderate

Detector charge diffusion and        Improved detectors                   Moderate
amplifier glow
(science and/or WFS)
Direct scintillation halo            Active amplitude correction          Poor
                                     Two-conjugate correction             Moderate (simulations)


                                   Space and Ground       Ground only
                                                                                                   30
                                ExAO issues II

Effect (10-6, 10-8, 10-10)      Potential mitigation                              Mitigation
                                                                                  maturity
Dispersion displacement         Lateral dispersion corrector                      Moderate (concepts)
                                Optimized spectralwidth of WFS                    Moderate (concepts)
Flat-field stability            Improved detectors                                Moderate
Fourth-order terms in the       Higher Strehl                                     Poor
wavefront expansion             Contrast-optimizing amplitude and phase           Poor
                                control laws
Frozen wind lag (e.g.           Predictive phase correction of multilayer         Moderate (concepts)
predictable phase errors)       atmosphere
Index of refraction             More uniform materials, better pointing control   Poor
inhomogeneities
Multispectral error             Common-band WFSing                                Good
Non-common path phase           Common-path WFSing                                Moderate (concepts)
errors                          Improved calibration/metrology                    Poor
Non-common path polarization    Slow F/# telescopes, polarizers                   Poor
effects                         Vector field AO coronagraph design

                               Space and Ground       Ground only
                                                                                                        31
                              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 correction                      Moderate, for clearing
                               Sensing of higher moments                 inner halo
Telescope pointing errors      Better telescope pointing                 Moderate
(Beam walk using a T/T         Adaptive secondaries (to minimize beam
mirror)                        walk)
Uncorrectable dynamic          Improved ACS, telescope stiffness, wind   Moderate
telescope errors               shielding
WFS calibration instability    WFS‟s insensitive to seeing changes       Moderate
                               Active thermal control                    Moderate
WFS star and background        Optimized system design                   Good
photon noise
WFS read and dark current      Improved detectors                        Moderate
noise


                              Space and Ground      Ground only
                                                                                                  32
               Cases to be considered

  ‘Fundamental’ errors             ‘Expanded’ list                Required
                                      of errors                  innovation
      Planet photon noise          Fundamental errors                  --
       Sky photon noise                      +
      WFS photon noise                 Frozen wind             Predictive control
          Boiling wind               WFS read noise            Noiseless detectors
Residual dispersion displacement       Scintillation          Amplitude correction
                                   Residual chromatism       Common-band sensing
                                    Multispectral error      Common-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
                                                                                     33
             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 (tphot = dt, the system update rate)
        • Atmospheric phase estimate imperfect due to WFS photon statistics
• Scintillation (tscint = 0.024 sec)
        • Due to amplitude fluctuations arising from high-altitude turbulence
        • We will assume strong high-altitude turbulence
• Frozen wind (twind = 0.009 sec)
        • Correction is late due to finite AO correction bandwidth
             – Solution: By definition, can be eliminated with predictive controller
• Boiling wind error (tboil = 0.200 sec)
        • Component of error not predictable
• WFS detector read noise (tphot = dt)
        • Includes dark current shot noise, etc.
             – Solution: Photon-counting detectors
                                                                                       34
Integration time by effect (30 m Sun-Jupiter-analogue)
                                                     Tsi [hours]
       Error terms
                                      R/H                                    H/H
                             Expanded list of errors                 Fundamental errors
                           (dx = 0.33m, dt = 0.083 ms)             (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
                                                                                                35
                    Tint and Q / Expanded error terms


  Expanded                           Integration time [hours] and Q value (Tint / Q)
 error terms             to achieve SNR = 5 for various ground-based aperture diameters and
                                                     contrast levels


Sensing band /             10 meter          10 meter         30 meter         30 meter
Science band                @ 10-8            @ 10-9           @ 10-9         @ 1.7 x 10-10


       R/H
  dxopt = 32 cm            7.4 / 0.001      740 / 0.0001      9.8 / 0.001      340 / 0.0001
 dtopt = 0.083 ms


       R/R
   dxopt= 31 cm
   dtopt= 0.12 ms
                           2.4/ 0.001       240 / 0.0001      3.1 / 0.001      110 / 0.0002




       H/H
 dxopt = 125 cm
 dtopt = 0.11 ms           4.6 / 0.001      460 / 0.0001      5.7 / 0.0001     200 / 0.0001
(May violate quadratic
expansion assumption)                                                                         36
              Tint and Q / Fundamental error terms


Fundamental                        Integration time [hours] and Q value (Tint / Q)
 error terms           to achieve SNR = 5 for various ground-based aperture diameters and
                                                   contrast levels


Sensing band /           10 meter          10 meter         30 meter         30 meter
Science band              @ 10-8            @ 10-9           @ 10-9         @ 1.7 x 10-10


      R/H
 dxopt = 16 cm           6.1 / 0.002      610 / 0.0002      8.1 / 0.002      280 / 0.0003
 dtopt = 0.15 ms


      R/R
dxopt(10m) = 8.8 cm
dtopt(10m) = 0.23 ms
dxopt(30m) = 11 cm      0.10 / 0.008      9.5 / 0.0008      0.24 / 0.003     8.2 / 0.0006
dtopt(30m) = 0.11 ms




      H/H
 dxopt = 19 cm          0.35 / 0.003      35 / 0.0003       0.4 / 0.002       15 / 0.0004
 dtopt = 0.17 ms
                                                                                            37
                                                   Scope of target list

                                    100
         Integration time [hours]


                                     10


                                                                                             R/H Expanded
                                      1
                                                                                             H/H fundamental


                                     0.1



                                    0.01
                                           0   1    2     3      4     5         6   7   8
                                                     Guide star magnitude [mV]



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.
                                                                                                               38
                                             Better NIR detectors?

                                        40
                                        35

                Integration time [hr]
                                        30
                                        25
                                        20
                                        15
                                        10
                                        5
                                        0
                                             0    10         20         30        40      50
                                                 H-band WFS read noise [electrons, rms]




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.                                             39
                                                     Larger telescopes
                                     1000.000

                                      100.000
             Integration time [hr]


                                       10.000
                                                                                     R/H expanded
                                                                                     H/H fundamental
                                        1.000

                                        0.100

                                        0.010
                                                0   10 20 30 40 50 60 70 80 90 100
                                                        Telescope diameter [m]


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                                 40
                                Integration time vs. telescope diameter
                                 for solar analogue exoearth @ 10 pc

                   (l = 0.85 microns, r0 = 0.47 m, t0 = 10 ms, vwind = 15 m/s, sc2 = 0.006, tscint =
                     20 ms, Z = 20 km, toil = 90 ms, sread = 3 e-, n = 4 pix, q = 0.1, msky = 21.5 /
                                    asec2, R = 5, SNR = 5, Cplanet = 1.7 x 10-10)




Total
Scintillation
WFS read noise
WFS photon
noise
Frozen wind
Boiling wind
Planet photon
noise
Sky photon
noise



                          dt = 0.00007 sec,                             dt = 0.00014 sec,
                          dx = 0.16 m                                   dx = 0.20 m

                 30 m telescope requires 70 hours                30 m telescope requires 5.7 hours
                                                                                                        41
       Exoplanet astrometry and photometry
                       at 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 l/D (R-band) and 13 l/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)                   42
ELT ExAO Potential Science Reach




                                   43
Bright stars in 15   44
         Ground-based exoearth spectroscopy
                        at 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

                                                                               45
          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)




                                                                         46
Reaching the Fundamental Limits




                                  47
       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
                                                                                 48
     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+)




                                                                  49
                  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

                                                                                         50
                   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                  51
             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


                                                                              52
           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
                                                                           53
                   Exoearth study comparison


• Ground 30m                                  • Space
   – Cons                                        – Pros
      •   Biomarkers not accessible                 •   Biomarkers accessible
      •   R < 20 (integration time limited)         •   R > 20
      •   Must master Q = 0.001                     •   Could work at Q = 0.1 – 1.0
      •   One hemisphere accessible                 •   Science in visible or mid-IR
      •   Not top ELT priority                      •   Whole sky accessible
      •   Many new technologies required            •   Enterprise mission – top priority
          to reach fundamental limits            – Cons
            – Speckle suppression                   •   Slow mission development
   – Pros                                           •   Typically 0.02 nm rms WFE
      • Rapid instrument development                •   Highly stable
      • Smaller inner working angle                 •   Many new technologies required
      • Higher spatial resolution                       to reach fundamental limits
      • Science possible in red and                       • Speckle suppression
        near-IR
      • Typically 10 nm rms WFE


                                                                                            54
                      EAO key concepts

• Phase ripple
   – A single frequency component of a two-dimensional wavefront phase
     power spectrum
   – Phase ripple variance, sk2, 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/sk2
       • 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)



                                                                                  55

				
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Lingjuan Ma Lingjuan Ma MS
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