An Introduction to Adaptive An Introduction to Adaptive OpticsOptics

Mike HeinMike Hein

PH 464 – Applied OpticsPH 464 – Applied Optics

Winter 2005Winter 2005

Why do we need adaptive optics?Why do we need adaptive optics?

To Correct for Irregularities in the To Correct for Irregularities in the Transmission MediaTransmission Media– Atmospheric (Astronomical)Atmospheric (Astronomical)

Scintillation or Twinkling (Eddies ~ Scintillation or Twinkling (Eddies ~ √√λλLL)) Beam Wander or Quiver (Eddies > Beam Size)Beam Wander or Quiver (Eddies > Beam Size) Spreading (Eddies < Beam Size)Spreading (Eddies < Beam Size)

– Atmospheric (LASER Transmission)Atmospheric (LASER Transmission) Thermal BloomingThermal Blooming

– Ocular (Retinal Imaging)Ocular (Retinal Imaging) Defects in the Lens and CorneaDefects in the Lens and Cornea

How Turbulence Effects LightHow Turbulence Effects Light

Spherical waves emitted Spherical waves emitted by the source (star) by the source (star) are essentially plane are essentially plane waves when they waves when they arrive at the Earth’s arrive at the Earth’s atmosphere. The atmosphere. The waves are then waves are then randomly distorted by randomly distorted by turbulence in the turbulence in the atmosphere.atmosphere.

Checking the Figure of the Hale Checking the Figure of the Hale TelescopeTelescope

How does it work?How does it work?

Two Basic Functions:Two Basic Functions:– Wavefront sensingWavefront sensing

Shack-Hartmann wavefront sensorShack-Hartmann wavefront sensor Shearing interferometerShearing interferometer

– Wavefront correctionWavefront correction Tip-tilt optical elementTip-tilt optical element Deformable optical elementDeformable optical element

Wavefront SensingWavefront Sensing

Shack-Hartmann Shack-Hartmann SensorSensor– Has become the Has become the

standard type of sensorstandard type of sensor– Uses an array of Uses an array of

spherical “lenselets”spherical “lenselets”– Focuses wavefronts Focuses wavefronts

onto a CCD arrayonto a CCD array– (b) is a plane wave(b) is a plane wave– (c) is a distorted wave(c) is a distorted wave

Wavefront SensingWavefront Sensing

Shearing Shearing InterferometerInterferometer– Distorted wavefronts Distorted wavefronts

result in phase shifted result in phase shifted output signals between output signals between the detector arrays.the detector arrays.

– Two interferometers are Two interferometers are required for correction required for correction in the x-y plane.in the x-y plane.

Wavefront CorrectionWavefront Correction

High speed tip-tilt High speed tip-tilt mirror reduces overall mirror reduces overall wavefront tiltwavefront tilt

Deformable mirror Deformable mirror (monolithic or (monolithic or segmented) corrects segmented) corrects wavefront shapewavefront shape

Wavefront CorrectionWavefront Correction Monolithic Deformable MirrorMonolithic Deformable Mirror

– Reflective face cast on a solid piezoelectric backing (1.4um).Reflective face cast on a solid piezoelectric backing (1.4um).– Thin face supported by an array of electro-mechanical actuators (+/- 3.0um)Thin face supported by an array of electro-mechanical actuators (+/- 3.0um)– Thin face supported by an array of piezoelectric actuators (5.0um)Thin face supported by an array of piezoelectric actuators (5.0um)

Wavefront CorrectionWavefront Correction

Segmented MirrorsSegmented Mirrors– Capable of larger Capable of larger

corrections than corrections than monolithic types.monolithic types.

– Requires frequent Requires frequent calibrationcalibration

– Diffraction due to gaps Diffraction due to gaps between segmentsbetween segments

Some DefinitionsSome Definitions

Angular isoplanatism – the angular measure over Angular isoplanatism – the angular measure over which a compensated wavefront can be which a compensated wavefront can be considered planar. At visible wavelengths the considered planar. At visible wavelengths the isoplanatic angle is about 2 arc seconds isoplanatic angle is about 2 arc seconds increasing to 10 arc seconds in near infrared light.increasing to 10 arc seconds in near infrared light.

Fried Parameter (rFried Parameter (r00) – the turbulence coherence ) – the turbulence coherence

length. A measure of the size of turbulent cells in length. A measure of the size of turbulent cells in the atmosphere. Typically 5cm to 20cm. the atmosphere. Typically 5cm to 20cm.

Some DefinitionsSome Definitions

Apparent or Visual Apparent or Visual Magnitude – The relative Magnitude – The relative brightness of a star as brightness of a star as seen by an observer on seen by an observer on EarthEarth

Guide Star – A sufficiently Guide Star – A sufficiently bright star to use as a bright star to use as a reference for image reference for image compensation. The compensation. The required magnitude varies required magnitude varies as a function of the desired as a function of the desired observation wavelengthobservation wavelength

Limitations of Image CompensationLimitations of Image Compensation

Compensation in visible wavelengths (0.5um) Compensation in visible wavelengths (0.5um) requires a guide star of magnitude 10 or brighter. requires a guide star of magnitude 10 or brighter.

Compensation in infrared wavelengths (2.2um) Compensation in infrared wavelengths (2.2um) allows guide stars down to magnitude 14.allows guide stars down to magnitude 14.

Small isoplanatic region around the guide star Small isoplanatic region around the guide star limits total observation area:limits total observation area:– Visual – 1/100,000 of the skyVisual – 1/100,000 of the sky– IR – 1/1000 of the skyIR – 1/1000 of the sky

Guide StarsGuide Stars

Artificial Guide StarsArtificial Guide Stars

The area of sky coverage can be The area of sky coverage can be expanded using artificial guide stars.expanded using artificial guide stars.

Created using a LASER aligned with Created using a LASER aligned with the telescope optics.the telescope optics.

Rayleigh beacons take advantage of Rayleigh beacons take advantage of Rayleigh scattering in the Rayleigh scattering in the atmosphere.atmosphere.– Useful for telescopes with apertures < 2m.Useful for telescopes with apertures < 2m.– Creates a 1 to 2 arc second guide star 5 to Creates a 1 to 2 arc second guide star 5 to

10km in altitude.10km in altitude.– 100W LASER100W LASER– LASER is pulsed so that backscatter can be LASER is pulsed so that backscatter can be

eliminated by range gating.eliminated by range gating.

Artificial Guide StarsArtificial Guide Stars

Sodium Beacons Sodium Beacons exploit a layer of exploit a layer of sodium vapor in the sodium vapor in the upper atmosphereupper atmosphere– 10W LASER at 589nm10W LASER at 589nm– Excites sodium atoms Excites sodium atoms

90 to 92 km in altitude.90 to 92 km in altitude. Photo - The Sodium laser launching Photo - The Sodium laser launching

from the side of the Lick from the side of the Lick Observatory 120" Shane Telescope. Observatory 120" Shane Telescope. This is a 10 minute time exposure - This is a 10 minute time exposure - note the star trailsnote the star trails

Artificial Guide StarsArtificial Guide Stars

Natural guide star Natural guide star required for tilt required for tilt correction.correction.

Limiting magnitude for Limiting magnitude for guide star is about 20.guide star is about 20.

Result is nearly 100% Result is nearly 100% sky coverage.sky coverage.

IR Stellar imaging from Lick IR Stellar imaging from Lick ObservatoryObservatory

Upper Left: Io image Upper Left: Io image taken with Keck taken with Keck adaptive optics; K-band, adaptive optics; K-band, 2.2 micron. 2.2 micron.

Upper Right: Io image Upper Right: Io image based on visible light based on visible light taken with Galileo taken with Galileo spacecraft orbiter.spacecraft orbiter.

Lower Left: Io image Lower Left: Io image taken with Keck taken with Keck adaptive optics; L-band, adaptive optics; L-band, 3.5micron. 3.5micron.

Lower Right: Io image Lower Right: Io image taken without Keck taken without Keck adaptive optics.adaptive optics.

The Lick Observatory CISThe Lick Observatory CIS

Non-Astronomical ApplicationsNon-Astronomical Applications

Most of the current AO Most of the current AO technology was technology was developed by DARPA developed by DARPA during the Cold War to during the Cold War to image and shoot down image and shoot down satellites with LASER satellites with LASER weapons.weapons.

Retinal ImagingRetinal Imaging

ReferencesReferences Carroll, Joseph, Daniel C. Gray, Austin Roorda, David R. Williams, “Recent Advances in Carroll, Joseph, Daniel C. Gray, Austin Roorda, David R. Williams, “Recent Advances in

Retinal Imaging with Adaptive Optics,” Retinal Imaging with Adaptive Optics,” Optics and Photonics NewsOptics and Photonics News, Jan 2005: p. 36-42, Jan 2005: p. 36-42 Chaisson, Eric and Steve McMillan, Astronomy Today 4Chaisson, Eric and Steve McMillan, Astronomy Today 4 thth Edition, Prentice Hall, Upper Edition, Prentice Hall, Upper

Saddle River, NJ, 2002Saddle River, NJ, 2002 Florence, Ronald, The Perfect Machine, Harper Collins, New York, NY, 1994Florence, Ronald, The Perfect Machine, Harper Collins, New York, NY, 1994 Hardy, John W., Adaptive Optics for Astronomical Telescopes, Oxford University Press, Hardy, John W., Adaptive Optics for Astronomical Telescopes, Oxford University Press,

New York, NY, 1998New York, NY, 1998 Hardy, John W., “Adaptive Optics,” Hardy, John W., “Adaptive Optics,” Scientific AmericanScientific American, June 1994: p. 60-65, June 1994: p. 60-65 Hecht, Eugene, Optics 4Hecht, Eugene, Optics 4thth Edition, Addison-Wesley, Reading, MA, 2002 Edition, Addison-Wesley, Reading, MA, 2002 Tyson, Robert K., Principles of Adaptive Optics 2Tyson, Robert K., Principles of Adaptive Optics 2ndnd Edition, Academic Press, San Diego, Edition, Academic Press, San Diego,

CA, 1998CA, 1998 AdaptiveOptics.com, 2002, Adaptive Optics Associates Inc., AdaptiveOptics.com, 2002, Adaptive Optics Associates Inc., http://www.aoainc.com/http://www.aoainc.com/ Center for Adaptive Optics, 2005, University of California Santa Cruz, Center for Adaptive Optics, 2005, University of California Santa Cruz,

http://cfao.ucolick.org/http://cfao.ucolick.org/