A Short Introduction A Short Introduction to Adaptive Opticsto Adaptive Optics

Presentation for NGAO Controls TeamPresentation for NGAO Controls Team

Erik JohanssonErik JohanssonAugust 28, 2008August 28, 2008

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OverviewOverview

• Why we need AO• The basics of AO• Intro to wavefront sensing• Intro to tip-tilt correction• Intro to higher-order wavefront correction• LGS vs NGS AO• Limitations of AO• How NGAO will differ form our current AO

system• Q&A

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Why do we need AO?Why do we need AO?

Short exposure images of the stars Gamma Perseus and Alpha Orionis (Betelgeuse) demonstrate the effects of atmospheric turbulence

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Light from distant star

Telescope aperture

Focal Plane

Image

Spot size = 2.44 /D

Without atmosphere, the telescope Without atmosphere, the telescope forms a perfect “diffraction-limited” forms a perfect “diffraction-limited” spot in the focal planespot in the focal plane

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Light from distant star

Telescope aperture

Focal Plane

Image

Atmosphere (lens size = r0)

Spot size = 2.44 /r0

Freeze the speckles by using short exposures < ~0.1 sec

r0 is characteristic size of the atmosphere

Number of speckles ~ (D/r0)2

First characterized by Fried in 1966

What is D/r0 for Keck?

The atmosphere acts like many lenses The atmosphere acts like many lenses of size r0 to create moving “speckles” of size r0 to create moving “speckles” in the imagein the image

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Narrowband Broadband

(Credit C. Neyman, AMOS)

A broad optical bandwidth smears theA broad optical bandwidth smears thespeckles out in a radial fashionspeckles out in a radial fashion

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Details of diffraction Details of diffraction from circular aperturefrom circular aperture

1) Amplitude

2) Intensity

First zero at r = 1.22 / D

FWHM / D

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Diffraction pattern from Diffraction pattern from hexagonal Keck hexagonal Keck telescopetelescope

Ghez: Keck laser guide star AO

Stars at Galactic Center

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A sheet with a sinusoidal “wave” which can vary in frequency (wavelength) and orientation (direction)

A spatial frequency also has phase: its peaks and valleys have some kind of reference to a known point in the image

What is a spatial frequency?What is a spatial frequency?

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Telescope OTF

Seeing Limited TF

Tip-Tilt Compensated TF

For D/r0 = 15

How does the atmosphere affect How does the atmosphere affect system performance? system performance?

Normalized Spatial Frequency

The Basics of AOThe Basics of AO

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How does AO work?How does AO work?

AO corrects distorted wavefronts in real time to compensate for blurring effects of the atmosphere

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What do AO and flying What do AO and flying potato chips have in potato chips have in common?common?

Intro to Wavefront Intro to Wavefront SensingSensing

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How do we measure How do we measure wavefronts?wavefronts?

Detectors cannot measure the phase of the light, only the intensity.

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Intro to Tip-Tilt Intro to Tip-Tilt CorrectionCorrection

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TelemetryRecorder

(TRS)

Tip-Tilt Mirror Controller

Tip-TiltSensor

Closed-loopMirror

PositioningController(CLMP)

ResidualTip-Tilt

Error(arc-sec)

MirrorPosition

Commands(arc-sec)

Rotation(UTT only)

Variable RotationAngle

Angular Offset(DT Ctrl Offset)

Control lawParametersLoop cmd

Control lawServo

MirrorDisturbance

Vector

MirrorOffset

Tip-tilt correctionTip-tilt correction

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Closed-Loop Mirror Positioning Controller

AtmosphericTip-Tilt

Controller

PIDServo

High voltageAmplifier

Digital toAnalog

Converter

MirrorActuators

MirrorPosition

Commands

(arc-sec)

High voltageActuator

Signals

BridgeSensors

StrainGauge

Outputs

CurrentMirror

Position

Arc-sec toactuator space

conversion

ConversionMatrix

ServoParameters

Closed Loop Mirror Closed Loop Mirror PositioningPositioning

Intro to Wavefront Intro to Wavefront Reconstruction and Reconstruction and CorrectionCorrection

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Wave FrontSensor(WFS)

Camera

BackgroundCompensation

Flat FieldCompensation

Pixel threshold

CentroidComputation

Control lawServo

Matrix-VectorMultiply

DeformableMirror(DM)

Tip-tiltControllers(DTT/UTT)

Background imageFlat field

Intensity thresholdCentroid gainCentroid origin

ReconstructionMatrix

Control lawParameters

Loop commandActuator map

DM origin

WFSparameters

TelemetryRecorder

(TRS)

Raw framesCentroids

Subap intensity

Residual WF errorRSS Residual WF Error

Tip-tilt errorWFS focus error

Actuator vectorDM focus

DMHWIF

WFSHWIF

Tip-tilt

error

Wave Front Processor (WFP)

WFC main data flowWFC main data flow

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BEFORE AFTER

Incoming Wave with Aberration

Deformable Mirror

Corrected Wavefront

How a deformable mirror How a deformable mirror works (idealization)works (idealization)

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Deformable Mirror for real wavefronts

27Anti-reflection coating

Glass face-sheet

PZT or PMN actuators: get longer and shorter as voltage is changed

Cables leading to mirror’s power supply (where

voltage is applied)

Light

Most deformable mirrors today Most deformable mirrors today have thin glass face-sheetshave thin glass face-sheets

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(paper coasters)

349 degrees of freedom; ~250 in use at any one time

Front view of Xinetics DM Front view of Xinetics DM (Keck)(Keck)

What are What are MEMs MEMs deformable mirrors?deformable mirrors?

A prom ising new class of deform able m irrors, called M EM s DM s, has em erged in the past few years.

Devices fabricated using sem iconductor batch processing technology and low pow er electrostatic actuation.

Potential to be very inexpensive ($10/actuator instead of $1000/actuator)

MEMS: micro-

electro-mechanical

systems

MEMS: MEMS: micromicro--

electroelectro--mechanical mechanical

systemssystems

Boston University MEMS ConceptBoston University MEMS Concept

Electrostatically actuated diaphragm

Attachment post

M embrane mirror

Electrostatically actuated diaphragm

Attachment post

M embrane mirror

Continuous mirror

• Fabrication: Silicon micromachining (structural silicon and sacrificial oxide)

• Actuation: Electrostatic parallel plates

Boston University Boston MicroMachines

NGS vs LGS AONGS vs LGS AO

DMScienceCamera

TTM

WFS

NGS

WavefrontController

Tip/tilt

IR transmissivedichroic

beamSplitter

Telescope pointing offload

Offload focus to

telescope

Light fromTelescope

NGS AO Control

NGS Reconstructor FluxRot & pupil angleWhen TT closedCentroid Origins

DMScienceCamera

STRAP

LBWFS

TTM

WFS

NGS

WavefrontController

Focus

Optimized centroids offsets

Tip/tilt

IR transmissivedichroic

LGS

Sodium transmissive

dichroic

Lenslets

Telescope pointing offload

Offload focus to

telescope

Tip/tilt to Laser

Light fromTelescope

LGS AO Control

LGS Reconstructor

TSS x,y,z stage

Laser TT mirror Laser pointing offload

Laser OrientationSpot size & fluxRot & pupil angleWhen DM closed

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Limitations of AOLimitations of AO

• Isoplanatism– Tip-Tilt Isoplanatism– Focus isoplanatism

• Sky coverage– WFS sensitivity– TT sensor sensitivity

• Imaging wavelength• Controller bandwidth• Error budgets, and more…

What determines how close the What determines how close the reference star has to be?reference star has to be?

Turbulence has to be similar on path to reference star and to science object

Common path has to be large

Anisoplanatism sets a limit to distance of reference star from the science object

Reference StarReference Star ScienceObjectScienceObject

Telescope

Turbulence

Telescope

Turbulence

zz

Common Atmospheric

Path

Common Atmospheric

Path

Common Atmospheric

Path

Expression for Expression for isoplanatic isoplanatic angle angle 00

• Strehl = 0.38 at = 0

0 is isoplanatic angle

0 is weighted by high-altitudeturbulence (z5/3)

• If turbulence is only at low altitude, overlap is very high.

• If there is strong turbulence at high altitude, not much is in common path

0 2.914 k 2(sec )8 / 3 dz CN2 (z) z5 / 3

0

3 / 5

Telescope

Common Path

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• Composite J, H, K band image, 30 second exposure in each band

• Field of view is 40”x40” (at 0.04 arc sec/pixel)• On-axis K-band Strehl ~ 40%, falling to 25% at field corner

credit: R. Dekany, Caltech

Anisoplanatism: how does AO image Anisoplanatism: how does AO image degrade as you move farther from degrade as you move farther from guide star?guide star?

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AO image of sun in visible light:

11 second exposure

Fair Seeing

Poor high altitude conditions

From T. Rimmele

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AO image of sun in visible light:

11 second exposure

Good seeing

Good high altitude conditions

From T. Rimmele

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Focus Anisoplanatism: The laser Focus Anisoplanatism: The laser doesn’t sample all the turbulencedoesn’t sample all the turbulence

Additional slides from Additional slides from Claire Max’s UCSC ClassClaire Max’s UCSC Class

NGWFC ResultsNGWFC Results

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Successes: Old vs. newSuccesses: Old vs. new

Some of the best images of a 7th magnitude star taken with the old WFC (left) and the NGWFC (right). The images have K-band Strehls of 58% and 66% respectively.

Strehl record: 71% at K-bandLimiting magnitude: R=16

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NGS performance exceeds NGS performance exceeds expectationsexpectations

Requirement was to meet or exceed 30% Strehl for 14th magnitude guide star in good seeing (r0 ≥ 20 cm).

60+% Strehl for R=14 guide star

Strehl record: 71% at K-bandLimiting magnitude: R=16

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LGS performance has improved as LGS performance has improved as wellwell

• LGS AO results during especially good seeing.• Best performance increased from 44% to 51% Strehl in

K.• Limiting magnitude R=19

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Improved performance on Brown Improved performance on Brown DwarfsDwarfs

J-band image of a brown dwarf binary pair with separation of 80 mas (Michael Liu, 26 March 2007).

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Best LGS AO images of the Best LGS AO images of the galactic centergalactic center

K-band image of the Galactic Center in LGS AO (left) and NGS AO (right). Credit: Andrea Ghez, Jessica Lu.

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Extended ObjectsExtended Objects

• J, H and K’ color composite o Uranus (left). The inset on the top left is an enlarged image of Miranda at K’.

• H and K’ color composite of Neptune (middle) • K’ image of Titan (right).

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Uranus ring crossingUranus ring crossing

The rings of Uranus as observed with the Keck AO system since 2004. Optically-thick rings like disappear due to inter-particle shadowing; optically-thin rings like brighten. Credit: Imke de Pater.

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Improved NGS/LGS crossover Improved NGS/LGS crossover pointpoint

• We are now able to use NGS in observing scenarios where we used LGS before and get better performance

NGS perf

LGS perf