Adaptive Optics. Dave Andersen NRC Herzberg.

Transcription

1 Adaptive Optics Dave Andersen NRC Herzberg

2 Resources Tokovinin Tutorial: - Excellent descriptions of many elements of AO systems Claire Max s AO course: - Full grad course on AO. Homework problems and lectures available on-line. Adaptive Optics in Astronomy, F. Roddier (2004) Best textbook Principals of Adaptive Optics, R. Tyson (2010) SPIE Field Guide to Adaptive Optics, R. Tyson (2012) All the equations you need in one place Adaptive Optics for Astronomical Telescopes, J. Hardy (1998) CfAO Summer School: - Taught every year in August in Santa Cruz. Many past year lectures available on-line. Adaptive Optics for Astronomy, 2012, ARA&A, Davies & Kasper Excellent review of AO with a focus on astronomical results.

3 Acronyms AO Adaptive Optics ASM Adaptive Secondary Mirror DL Diffraction-limited DM Deformable Mirror GS Guide Star MEMS Micro Electo-Mechanical System NGS - Natural Guide Star PSF Point Spread Function SH Shack-Hartmann WFE Wavefront Error WFS Wavefront Sensor

4 Outline of AO lectures Introduction Science Highlights Diffraction & the AO PSF The Atmosphere Future AO Flavors MCAO GLAO MOAO Wavefront Sensing Deformable Mirrors

5 Challenge! What are these objects observed with AO? Goode Sabesan, Roorda Betzig, Wang

6 Can monitor for volcanoes on Io Solar System K-band Galileo AO good for doing high spatial resolution observations that also require observations on a regular cadence Cheaper than going there! L-band No AO Marchis, Keck AO

7 Jupiter Without AO, amateur telescope Jupiter with AO, Gemini

8 Extrasolar planets AO+coronography + intense data processing can find planets ~10 8 contrast between planet and star HR Marois AO also good for finding circumstellar disks More will come soon HR 4796A - SPHERE

9 Science at the Galactic Center Astrometry of stars near Sag A* has constrained the mass of the BH (4x10 6 M ) Interesting stellar populations mostly old stars on random orbits in central 10, about half young star on warped elliptical orbits Ghez; Keck AO

10 High Redshift Galaxies IFUs+AO can probe the dynamics and star formation of galaxies at high redshift SINS survey has identified a progression of galaxies that are pressure dominated to rotationally dominated Forster-Schreiber; VLT

11 Diffraction & PSFs The Point Spread Function (PSF) is the image of a point source through your optical system PSF = F(Wavefront) 2 Wavefront = Aperture(r) exp(i Phase) A Diffraction-limited PSF is the PSF in the absence of aberrations (zero Phase)

12 Diffraction Limited PSFs More complicated apertures produce more complicated PSFs. FWHM of DL PSFs is ~ λ/ D (radians) FWHM 8m (λ=1.5 μm)= 39 mas FWHM 30m (λ=1.5 μm)= 10 mas Examples of LBT (left) and GMT (right). What will PSF look like? 23 meter baseline

13 PSFs and Phase Errors We ll stick with a circular aperture, and now introduce phase errors PSF = F[Aperture(r) exp(i Phase)] 2 Phase can be defined in nm, radians, waves Zernike Polynomials Sequence of polynomials Orthogonal on disk low order terms correspond to Seidel aberrations

14 Questions How does piston (Z 1 ) error affect the image? How does tip/tilt (Z 2, Z 3 ) affect the image? Focus Error (Z 4 ) H-band 0.5 arcsec Relative Flux

15 Astigmatism Astigmatism (Z 5, Z 6 ) is common in optical systems Easy to diagnose because of coupling to focus H-band 0.5 arcsec Relative Flux

16 Coma Coma (Z 7, Z 8 ) is another common aberration H-band 0.5 arcsec Relative Flux

17 Phase Diversity Determine the phase from out of focus images which allows you to reduce the phase errors and improve the PSF Original After correction using Phase Diversity M. Lamb

18 Real AO PSFs AO systems try to deliver diffraction-limited (DL) PSFs but there is always some residual error AO PSFs typically have a DL core and seeing-limited wings. The Strehl ratio is defined as the ratio of the peak flux and the peak flux that would be delivered by a DL PSF Intensity x

19 Maréchel approximation to Strehl Ratio The extended Maréchal approximation is an easy way to estimate Strehl ratio from a wavefront error σ is the RMS wavefront error in physical units (nm or microns) Valid for SR > ~10% SR ~ exp[-(2πσ/λ) 2 ] 50% Strehl in J-band corresponds to σ=150 nm σ=150 nm gives a Strehl of 83% in K-band and 16% in R-band

20 Point Source Sensitivity What is the Signal to Noise equation?

21 Point Source Sensitivity Compare background-limited exposure time equation for AO versus Seeing Limited Observations D 4 Advantage of AO! Orion Nebula, Gemini

22 What difference does Strehl make? HST LBT FLAO

23 Atmospheric Turbulence stratosphere tropopause km boundary layer ~ 1 km wind flow around dome Heat sources w/in dome Max

24 The Kolmogorov theory for turbulence Assumes energy is added at largest scales and cascades to smallest scales through eddies Statistical properties of Kolmogorov can be described by Zernike coefficients: <a i a j > = c ij (D/r 0 ) 5/3 c ij are nearly diagonal Anything wrong with this picture? Power k (m -1 ) Noll c ii Zernike Number

25 solar van Kármán turbulence, cartoon Outer scale L 0 Inner scale l 0 hν Wind shear convection hν ground Max

26 The Effect of Turbulence on Light Fluctuations in index of refraction are due to temperature fluctuations Refractivity of air where P = pressure in millibars, T = temp. in K, λ in microns n = index of refraction. Note VERY weak dependence on λ. Temperature fluctuations index fluctuations

27 Fried Parameter See Max s notes for a detailed derivation of the Fried Parameter r 0 is the Fried parameter a DL telescope of D=r 0 would have the same FWHM as a larger telescope imaging through this turbulence Convention: Usually quoted for λ= 500 nm at zenith Typically 5-30 cm (bad site / good site) (D/r 0 ) 2 is approximate # of speckles in short-exposure image and sets the rough order of the AO system k is (2π/λ) so r 0 α λ 6/5 ζis the zenith angle C 2 N gives the strength of turbulence [units: m -2/3 ] How does seeing FWHM vary with λ?

28 Turbulence Turbulence is usually concentrated in a few strong layers. Each layer moves at a different wind speed (and direction) Tokovinin CfAI Durham

29 Characteristic Turbulence Scales The strength of higher layer turbulence sets the correctable FOV for a classical AO system. θ 0 is the isoplanatic angle defined as 0.31 r 0 / h and sets the angle where the Strehl decreases by 1/e τ 0 is the atmospheric time const defined as 0.31 r 0 / V where V is the mean wind speed

30 What does a von Kármán (Kolmogorov) distribution of phase look like? Wavefront microns For Mauna Kea, typical atmospheric parameters (@ 500 nm) are: r 0 = 15.6 cm L 0 = 30 m θ 0 = 2.2 arcsec V = 20 m/s τ 0 = 7.8 ms f G = 1/τ 0 = 128 Hz 30 meters

31 What does turbulence look like imaged through different telescopes? Example 1: λ=500 nm; r 0 =15.6 cm; FWHM SEE = 0.66 ;θ 0 =2.2 ; f G =128 Hz Diameter D=2m D=8m D=30m FWHM DL (λ/d) 51 mas 13 mas 3 mas D/r long exposure short exposure

32 What does turbulence look like imaged through different telescopes? Example 2: λ=1.5 μm; r 0 =58 cm; FWHM SEE = 0.53 ;θ 0 =8.2 ; f G =34 Hz Diameter 2m 8m 30m FWHM DL (λ/d) 150 mas 38 mas 10 mas D/r long exposure short exposure

33 Guide Stars Components of an AO system Adaptive Mirrors aka Deformable Mirrors Piezo-Stack MEMS ASM Interaction Matrices Command Matrices Wavefront Sensors Control System CfAO

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35 Rapidly Developing Technology Thin Mirror Actuators Placed in the pupil plane Must be able to change shape (by a few microns) rapidly

36 DM requirements Number of actuators: Several types of deformable mirrors (DMs), each has its own characteristic fitting error fitting 2 = μ ( d / r 0 ) 5/3 rad 2 Dynamic range: stroke (total up and down range) Typical stroke for astronomy depends on telescope diameter: several microns for 10 m telescope 10 microns for 30 m telescope Temporal frequency response: DM must respond faster than a fraction of the coherence time 0 Max

37 Surface quality Power dissipation: DM requirements (2) Don t want too much resistive loss in actuators, because heat is bad ( seeing, distorts mirror) Lower voltage is better (easier to use, less power dissipation) Hysteresis of actuators: Repeatability DM physical size: For 30-m telescope need big DMs: at least 30 cm across Consequence of the Lagrange invariant:

38 Types of deformable mirrors Segmented Made of separate segments with small gaps Continuous face-sheet Thin glass sheet with actuators glued to the back Bimorph 2 piezoelectric wafers bonded together with array of electrodes between them. Front surface acts as mirror. Max

39 Types of deformable mirrors Liquid crystal spatial light modulators Technology similar to LCDs Applied voltage orients long thin molecules, changes n Monochromatic and slow MEMS (micro-electro-mechanical systems) Fabricated using micro-fabrication methods of integrated circuit industry Potential to be inexpensive Max

40 Types of actuator: Piezoelectric Piezo from Greek for Pressure PZT (lead zirconate titanate) gets longer or shorter when you apply V Stack of PZT ceramic disks with integral electrodes Displacement linear in voltage Typically 150 Volts x ~ 10 microns 10-20% hysteresis (actuator doesn t go back to exactly where it started) Max

41 Palm 3000 High-Order Deformable Mirror: 4356 actuators Credit: A. Bouchez Xinetics Inc. for Mt. Palomar Palm 3000 AO system

42 Diagram from MicroGate s website MMT ASM

43 Components of an AO system Guide Stars Adaptive Mirrors aka Deformable Mirrors Wavefront Sensors Shack-Hartmann Curvature Pyramid Control System Real-time computer can be demanding For NFIRAOS, ~2x Hz CfAO

44 Wavefront Sensing Guide Star Optical Device Light detector Reconstructor Wavefront Phase WFSs get light from guide stars as faint as m R ~ 15 Optical devices have to transform phase aberrations into light intensity variations need to make ~(D/r 0 ) 2 measurements Detectors convert the light into electrical signals The reconstructor converts these signals into a wavefront estimate

45 Shack-Hartmann WFS Lenslet array in Pupil Plane images spots onto detector The displacement of the spots gives you the tangent slope of the phase in each subaperture Most AO systems use SH WFSs Tokovinin

46 SH WFS Trade-Offs Shack-Hartmann Design FOV how much can spots wander? # subap set by ~D/r 0 sensitivity (sampling) how many pixels/fhwm? sky coverage more subapertures & higher sampling mean fewer photons/pix Detector choices need fast readout, low RN, lots of pixels (set by FOV, # subap, sampling)

47 asas Tip-Tilt

48 Astigmatism

49 Focus What does Focus look like on a SH WFS?

50 Turbulence SH WFS are only sensitive to aberrations on scales greater than a subaperture Higher order aberrations can change measurements of lower order aberrations (aliasing) Phase Slopes WFS

51 Pros and Cons of SH WFS Pros Linear Achromatic Works with extended objects Cons Doesn t use the DL of the telescope Less sensitive to low order modes Fixed WFS geometry

52 Calibrating AO Need to establish a relationship between WFS slopes and DM commands Can do this by poking actuators 1 by 1 or by poking actuators in set patterns Hadamard pattern DM poke

53 Interaction Matrices Need to determine the interaction between the DM and WFS The poke matrix measurements can be used to construct the Interaction Matrix Want to know the inverse to run the AO system:

54 Applying the AO correction Once we have, D -1, the matrix that converts slopes to DM commands (aka the command matrix or reconstructor ), we can close the loop on our simple AO system. Output wavefront Input wavefront + + Reconstructor Gain Delay t DM Shape Slopes WFS

55 Diameter 2m 8m FWHM DL (λ/d) 150 mas 38 mas D/r short exposure long exposure Example 2 from slide 40: λ=1.5 μm r 0 =58 cm FWHM SEE = 0.53 θ 0 =8.2 f G =34 Hz d 0 =50 cm AO image

56 Errors have to be balanced when designing an AO system 1. How does each part contribute to achieving the overall performance goal 2. In a new project: Start with top down performance requirements from the science that will be done. Allocate errors to each component to satisfy overall requirements.

57 Elements of an adaptive optics system DM fitting error Not shown: tiptilt error, anisoplanatism error temporal delay, noise propagation Noncommon path errors Measurement error, aliasing Max

58 Error budget concept (sum of 2 s) Not much to be gained by making any particular term much smaller than all the others: try to roughly equalize all the terms Individual terms we know so far: Anisoplanatism Temporal error Fitting error 2

59 NFIRAOS AO error budget example High Order Errors Baseline - Center Tomography + DM fitting + WFS aliasing Servo lag 18.6 LGS WFS noise 38.6 DM hysteresis 25.0 Na layer altitude tracking error 53.0 RTC numerical precision & round-off errors 20.0 Residual telescope aberrations 88.0 Residual science instrument aberrations 50.0 NCPA calibration errors 35.0 Residual NFIRAOS optics errors 35.0 Obscuration from M2 support structure 30.0 TOTAL RSS % Strehl in H-band One of many NFIRAOS error budgets. Also worked out for different FOVs, different zenith angles, different atmospheres In addition, there exist low order error budgets/sky coverage estimates as well

60 Guide Stars Components of an AO system NGS Natural Guide Stars LGS Laser Guide Stars Sky Coverage Does AO work in the part of the sky that it is needed Adaptive Mirrors Wavefront Sensors Control System CfAO

61 Artificial Bright Stars: Laser Guide Stars (LGSs) Rayleigh beacons 10 km star produced just by scattering Sodium beacons 90 km star produced by exciting Na atoms deposited by meteorites

62 Two types of laser guide stars in use today: Rayleigh and Sodium Sodium guide stars: excite atoms in sodium layer at altitude of ~ 95 km Rayleigh guide stars: Rayleigh scattering from air molecules sends light back into telescope, h ~ 10 km Higher altitude of sodium layer is closer to sampling the same turbulence that a star from infinity passes through ~ 95 km 8-12 km Turbulence Telescope

63 LGSs on Mauna Kea

64 LGS WFEs Cone effect error can be substantial if: LGS altitude is low Telescope is BIG Turbulence is strong in high layers Spot Elongation also a problem: Side projection Big telescopes Thick LGS beacon Need more pixels on LGS WFSs

65 So why do we need NGSs for LGS AO systems? Lasers are propogated up through the atmosphere Location of beacon depends on this turbulence Need tip-tilt reference star can be several magnitudes fainter can be ~3x further out (tip-tilt isoplanatic angle is larger) Tokovinin

66 Véran & Trujillo Sky Coverage with LGS Because NGSs can be fainter and further away, sky coverage increases by ~10x Still low sky coverage at Galactic Poles

67 Laser Guide Stars LGS AO NGS AO A. Ghez

68 Special Topic Wide Field AO Multi-Conjugate Adaptive Optics GEMS on Gemini NFIRAOS for TMT Multi-Object Adaptive Optics Raven on Subaru IRMOS hopefully! for TMT

69 MCAO Use multiple WFSs with multiple NGSs (e.g. MAD on VLT) or LGSs (e.g. GEMS on Gemini and NFIRAOS on TMT) Use 2-3 DMs to correct different turbulence layers Use Tomography to build model of atmosphere

70 Simulation of Tomographic Reconstruction FOV set by generalized isoplanatic angle θ N (N = # of DMs) θ 2 = 9.6 for NFIRAOS (θ 0 =2.2 )

71 GEMS GEMS MCAO system on Gemini South long time in coming 5 LGSs 3 DM system (only using 2 right now) Now delivering some fantastic results.

72 NGC 1851 r h = 61 K S (2.15 μm) GeMS (K S ) FWHM = 0.09 FOV = 85 J (1.25 μm) ACS@HST (F606W) FWHM = 0.06 P. I. Sarajedini, 2006 ACS Survey of GGCs

73 Precision Photometry AGB HB RGB SGB MSTO MSK MS Turri, McConachie, Stetson, Andersen

74 TMT MCAO - NFIRAOS NFIRAOS is the first light Adaptive Optics system for TMT MCAO gives wider field of correction and higher sky coverage Uses 6 LGS over 70 FOV

75 NFIRAOS

76 NFIRAOS with IRIS Size of 2 large fixed body trucks Cooled to -30 C Designed at NRC Herzberg

77 NFIRAOS design 60x60 DM 0 75x75 DM LGS WFS 50% Strehl in H-band >50% sky coverage at North Galactic Pole

78 MCAO in the lab HeNOS Herzberg NFIRAOS Optical Simulator TEAM: Paolo Turri Matthias Rosensteiner Dave Andersen Jean-Pierre Véran Glen Herriot

79 MOAO

80 The Case for MOAO With SINFONI / OSIRIS / NIFS can observe one object and do it very well But we re wasting photons gathered by the large (expensive) telescopes Can gain a multiplex advantage for ~$30-50M (compare to $1B cost of TMT) UDF

81 Envisioning a MOAO instrument: IRMOS for TMT 5arcmin FOR 8 LGS WFS 6 NGS WFS ~20 objects ~20 MEMS DMs ~20 Spectrographs ~20 2K NIR Arrays Eikenberry, Andersen et al. 2006

82 Raven Block Diagram

83 Raven and Canary on WHT should motivate next MOAO instruments on ELTs

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85 Raven on Subaru NIR Nasmyth Platform Raven IRCS 85

86 Raven on sky May 2014 August 2014 Hurricane! June 2015 Earthquake! Protests! 13 total nights on Subaru

87 On-Sky Results Observed Engineering Fields with 5 bright stars within Field of Regard Acquisition Camera allowed us to start within 5 minutes of slew Tested SCAO, GLAO, and MOAO w/ different Used SLODAR to measure turbulence for Tomography In example below, 2 Science Objects were 40 apart on sky, NGS asterism was about 2.5 across 87

88 Performance Shows 30 s exposures in J-band Good conditions r 0 =23 cm Peak flux for 2 science objects scaled for comparison GLAO PSF much more uniform than no AO case Significant image quality improvement using MOAO, even for wide asterism with relatively faint stars (R=13.4) SCAO works the best on bright stars (R=10.8, 12.5) FWHM (mas) No AO GLAO MOAO SCAO Pred Strehl - - 6% 18% 11% 88

89 Raven observations of Saturn Used moons for NGSs Challenging to acquire due to scattered light 89

90 First Raven MOAO Science Spectra Observed 2 stars in M71 to compare NIR and optical metallicity measurements Used ABBA nodding with MOAO Venn 90