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Adaptive Optics in the VLT and Adaptive Optics in the VLT and ELT eraELT era

François WildiObservatoire de Genève

Credit for most slides : Claire Max (UC Santa Cruz)

Basics of AOBasics of AO

Why is adaptive optics Why is adaptive optics needed?needed?

Even the largest ground-based astronomical telescopes have no better resolution than an 20cm

telescope

Turbulence in earth’s atmosphere makes stars twinkleMore importantly, turbulence spreads out light; makes it a blob rather than a point. This blob is a lot larger than the Point Spread Function (PSF) that would be limited by the size of the telescope only

Plane Wave Distorted Wavefront

Atmospheric Atmospheric perturbations cause perturbations cause distorted wavefrontsdistorted wavefronts

Index of refraction variations

Rays not parallel

Optical consequences of Optical consequences of turbulenceturbulence• Temperature fluctuations in small patches of Temperature fluctuations in small patches of

air cause changes in index of refraction (like air cause changes in index of refraction (like many little lenses)many little lenses)

• Light rays are refracted many times (by small Light rays are refracted many times (by small amounts) amounts)

• When they reach telescope they are no longer When they reach telescope they are no longer parallelparallel

• Hence rays can’t be focused to a point:Hence rays can’t be focused to a point:

Parallel light rays Light rays affected by turbulence

blurPoint focus

Images of a bright starImages of a bright star

1 m telescope

Speckles (each is at diffraction limit of telescope)

Turbulence changes rapidly with Turbulence changes rapidly with time time

“Speckle images”: sequence of short snapshots of a star, taken in H-band at 50Hz on the 6.5m MMT

Centroid jumps around(image motion)

Image is spread out

into speckles

Turbulence arises in many placesTurbulence arises in many places

stratosphere

tropopause

Heat sources w/in dome

boundary layer~ 1 km

10-12 km

wind flow over dome

Imaging through a perfect Imaging through a perfect telescope (circular pupil)telescope (circular pupil)

With With no turbulenceno turbulence, , FWHM is diffraction FWHM is diffraction limit of telescope, limit of telescope, ~ ~ / D / D

Example: Example:

/ D = 0.02 arc sec for / D = 0.02 arc sec for = 1 = 1 m, D = 10 mm, D = 10 m

With turbulenceWith turbulence, image , image size gets much larger size gets much larger ((typically 0.5 - 2 arc typically 0.5 - 2 arc secsec))

FWHM ~/D

in units of /D

1.22 /D

Point Spread Function (PSF): Point Spread Function (PSF): intensity profile from point intensity profile from point

sourcesource

Turbulence strength is Turbulence strength is characterized by quantity rcharacterized by quantity r00

• “ “Coherence Length” rCoherence Length” r0 0 : distance over which : distance over which optical phase distortion has mean square value of optical phase distortion has mean square value of 1 rad1 rad22 (r (r00 ~ 15 - 30 cm at good observing ~ 15 - 30 cm at good observing sites)sites)

• Easy to remember: rEasy to remember: r0 0 = 10 cm = 10 cm FWHM = 1 arc sec FWHM = 1 arc sec at at = 0.5 = 0.5mm

Primary mirror of telescope

r0 ““Fried’s parameter”Fried’s parameter”

Wavefront of light

Effect of turbulence on image Effect of turbulence on image sizesize• If telescope diameter D >> rIf telescope diameter D >> r00 , image size of a , image size of a

point source is point source is / r / r0 0 >> >> / D / D

• rr0 0 is diameter of the circular pupil for which the is diameter of the circular pupil for which the diffraction limited image and the seeing limited diffraction limited image and the seeing limited image have the same angular resolution. image have the same angular resolution.

• rr0 0 25cm at a good site. So any telescope 25cm at a good site. So any telescope larger than this has no better spatial resolution! larger than this has no better spatial resolution!

/ r0

“seeing disk” / D

How does adaptive optics help?How does adaptive optics help?

Measure details of blurring from “guide star” near the object you want to observe

Calculate (on a computer) the shape to apply to deformable mirror to correct blurring

Light from both guide star and astronomical object is reflected from deformable mirror; distortions are removed

Turning On the AO will change Turning On the AO will change the nature of the image:the nature of the image:

The state-of-the art in The state-of-the art in performance:performance:

Diffraction limit resolution LBT FLAO PSF in H band. Composition of Diffraction limit resolution LBT FLAO PSF in H band. Composition of two 10s integration images. It is possible to count 10diffraction two 10s integration images. It is possible to count 10diffraction rings. The measured H band SR was at least 80%. The guide star rings. The measured H band SR was at least 80%. The guide star has a mag of R =6.5, H=2.5 with a seeing of 0.9 arcsec V band has a mag of R =6.5, H=2.5 with a seeing of 0.9 arcsec V band correcting 400 KL modescorrecting 400 KL modes

Adaptive optics increases Adaptive optics increases peak intensity of a point peak intensity of a point sourcesource

No AO With AO

No AO With AO

Intensity

AO produces point spread AO produces point spread functions with a “core” and functions with a “core” and “halo”“halo”

• When AO system performs well, more energy in coreWhen AO system performs well, more energy in core

• When AO system is stressed (poor seeing), halo When AO system is stressed (poor seeing), halo contains larger fraction of energy (diameter ~ rcontains larger fraction of energy (diameter ~ r00))

• Ratio between core and halo varies during nightRatio between core and halo varies during night

Inte

nsity

x

Definition of “Strehl”:

Ratio of peak intensity to that of “perfect” optical

system

Schematic of adaptive optics Schematic of adaptive optics systemsystem

Feedback loop: next

cycle corrects the

(small) errors of the

last cycle

Cartoon time!Cartoon time!

Real deformable mirrors Real deformable mirrors have smooth surfaceshave smooth surfaces

• In practice, a small deformable In practice, a small deformable mirror with a thin bendable face mirror with a thin bendable face sheet is usedsheet is used

• Placed Placed afterafter the main telescope the main telescope mirrormirror

Astronomical observatories Astronomical observatories with AO on 6 - 10 m with AO on 6 - 10 m telescopestelescopes• European Southern Observatory (Chile)European Southern Observatory (Chile)

– 4 telescopes (MACAO, NAOS, CRIRES, SPIFFI, 4 telescopes (MACAO, NAOS, CRIRES, SPIFFI, MAD)MAD)

• Keck Observatory, (Hawaii) Keck Observatory, (Hawaii) – 2 telescopes2 telescopes

• Gemini North Telescope (Hawaii), ALTAIR + Gemini North Telescope (Hawaii), ALTAIR + LGSLGS

• Subaru Telescope, HawaiiSubaru Telescope, Hawaii• MMT Telescope, ArizonaMMT Telescope, Arizona• Large Binocular Telescope, ArizonaLarge Binocular Telescope, Arizona

• Soon:Soon:– Gemini South Telescope, Chile (MCAO, GPI)Gemini South Telescope, Chile (MCAO, GPI)

Adaptive optics makes it possible Adaptive optics makes it possible to find faint companions around to find faint companions around bright starsbright stars

Two images from Palomar of a brown Two images from Palomar of a brown dwarf companion to GL 105dwarf companion to GL 105

Credit: David Golimowski Credit: David Golimowski

200” telescope

No AONo AO With AOWith AO

Neptune in infra-red light (1.65 Neptune in infra-red light (1.65 microns)microns)

Without adaptive optics With adaptive optics

June 27, 1999

2.3

arc

sec

May 24, 1999

AO resolution in the IR can be AO resolution in the IR can be better than space telescopes in better than space telescopes in the visiblethe visible

HST, Visible Keck AO, IR

L. Sromovsky

Lesson: Keck in near IR has ~ same resolution as Hubble in visible

Some frontiers of Some frontiers of astronomical adaptive opticsastronomical adaptive optics

• Current systems (natural and laser guide stars):Current systems (natural and laser guide stars):– How can we measure the Point Spread Function while we How can we measure the Point Spread Function while we

observe?observe?– How accurate can we make our photometry? astrometry?How accurate can we make our photometry? astrometry?– What methods will allow us to do high-precision What methods will allow us to do high-precision

spectroscopy?spectroscopy?

• Future systems:Future systems:– Can we push new AO systems to achieve very high contrast Can we push new AO systems to achieve very high contrast

ratios, to detect planets around nearby stars?ratios, to detect planets around nearby stars?– How can we achieve a wider AO field of view?How can we achieve a wider AO field of view?– How can we do AO for visible light (replace Hubble on the How can we do AO for visible light (replace Hubble on the

ground)?ground)?– How can we do laser guide star AO on future 30-m How can we do laser guide star AO on future 30-m

telescopes?telescopes?

Frontiers in AO technologyFrontiers in AO technology

• New kinds of deformable mirrors with > New kinds of deformable mirrors with > 5000 degrees of freedom5000 degrees of freedom

• Wavefront sensors that can Wavefront sensors that can dealdeal with with this many degrees of freedomthis many degrees of freedom

• (ultra) Fast computers(ultra) Fast computers• Innovative control algorithmsInnovative control algorithms• ““Tomographic wavefront reconstuction” Tomographic wavefront reconstuction”

using multiple laser guide starsusing multiple laser guide stars• New approaches to doing visible-light AONew approaches to doing visible-light AO