Twinkle, Twinkle Little Star:An Introduction to Adaptive OpticsMt. Hamilton Visitors NightJuly 28, 2001

Turbulence in the atmosphere limits the performance of astronomical telescopesEven the largest ground-based astronomical telescopes have no better resolution than an 8" backyard telescope!Turbulence is the reason why stars twinkle

More important for astronomy, turbulence spreads out the light from a star; makes it a blob rather than a point

Images of a bright star, ArcturusDistant stars should resemble points, if it werent for turbulence in Earths atmosphere

Turbulence changes rapidly with time Sequence of very short snapshots of a star.Movie is much slower than "real time."

Measure details of blurring from guide star near the object you want to observeCalculate (on a computer) the shape to apply to deformable mirror to correct blurringLight from both guide star and astronomical object is reflected from deformable mirror; distortions are removed

How to correct for atmospheric blurring

Basic idea of AOAberratedwavefrontWavefrontsensorWavefront controlcomputerCorrectedwavefrontWavefrontcorrector

Adaptive optics in actionStar with adaptive opticsStar without adaptive opticsLick Observatory adaptive optics system

The Deformable Mirror

Deformable mirrors come in many shapes and sizesToday: mirrors from Xinetics. From 13 to 900 actuators (degrees of freedom); 3 - 15 inches in diameter. Future: very small mirrors (MEMS, LCDs); very large mirrors (replace secondary mirror of the telescope)

Xinetics Inc.Devens, MA

Adaptive optics system is usually behind main telescope mirrorExample: AO system at Lick Observatorys 3 m telescopeSupport for maintelescope mirrorAdaptive optics package under main mirror

What does a real adaptive optics system look like?Wavefront sensorInfra-red cameraDeformable mirrorLight from telescope

If there is no nearby star, make your own star using a laserConceptImplementationLick Obs.

Laser in 120-inch dome

Laser guide star adaptive optics at Lick ObservatoryLaser Guide Star correction of a star: Strehl = 0.6Uncorrected image of a starIrcal1129.fitsRX J0258.3+194710/20/00 2:04KsV=15K=~13.3220sS=0.6 LGS

AO at the Keck 10 m TelescopeAdaptive optics lives here

Adaptive optics on 10-m Keck II Telescope: Factor of 10 increase in spatial resolutionWithout AOwidth = 0.34 arc secWithout AOWith AOwidth = 0.039 arc sec9th magnitude star imaged in infrared light (1.6 mm)

Neptune in Infrared LightWithout adaptive opticsWith Keck adaptive opticsJune 27, 19992.3 arc secMay 24, 1999l = 1.65 microns

Neptune:Ground-based AO vs. Voyager SpacecraftInfrared: Keck adaptive optics, 2000Visible: Voyager 2 fly-by, 1989

Saturns moon Titan: Shrouded by hazeas seen by Hubble Space TelescopeLimb Brightening due to hazeHints of surface detailImage at 0.85 microns

Titan at Keck: with and without adaptive optics Titan with adaptive optics Titan without adaptive optics Typical @ wavelength 1.65 mmFebruary 26-27, 1999

Uranus as seen by Hubble Space Telescope and Keck AO Hubble Space Telescopefalse-color image (1.1, 1.6, 1.9 mm)Keck adaptive optics image (2.1 mm)

Keck AO Can See the Faintest RingsDiscovered by Voyager Voyager: 4 groups of ringsed g 4 5 6b aKeck AO: outer e ring plus 3 inner groups (individual rings unresolved)

Infrared image (2 microns)A volcano erupting on Io: Jupiter's largest moon

Io with adaptive optics sees most of the volcanic features seen by GalileoSame volcanoesSame volcanoes

Other Uses for AO

High-speed communications with laser beamsCheaper and lighter telescopes in spaceHigh-powered lasers for fusion powerVision science research

Perfect EyeAberrated EyeWhy Correct the Eyes Optics?

pupil images

followed by

psfs for changing pupil sizeVisual Acuity Is Worse at Night When Pupils Dilate1 mm2 mm3 mm4 mm5 mm6 mm7 mm

The Rochester Adaptive Optics Ophthalmoscope

Adaptive optics provides a clear improvement in retinal image qualityWave AberrationPoint Spread FunctionRetinal Image at 550nmRetinal Image inWhite Light6.8 mm pupilBefore adaptive optics:After adaptive optics:1 degYY

Adaptive optics provides highest resolution images of living human retinaWithout AOWith AO:Resolve individual conesWilliams, Roorda et al. (U Rochester)

Looking Inside the Eye with AO

JW right eye1 deg field 1 deg eccentricity

Photoreceptors

Capillaries

10 arcmin(48.6 m)

This pair of images shows the whole 1 degree field that we take. The image on the previous slide is an average of the central part of the right slide.Left figure is capillaries which is obtained by focusing in front of (about 100 m) the photoreceptorsWhen you focus back into the retina, photoreceptors come into focus and the blood vessel appear as diffuse shadows.

Diameter of the smallest capillaries of about 6 microns can be measured. These capillaries define the edge of the foveal avascular zone. Fovea is to the left.These are single frames images.Images like this might be used to detect early changes in the retinal vasculature that are precursors to new vessel growth or development of microanyeurisms. A regular-sized microanyeurism would cover about 1/3 of this field of view.

Real time imaging could allow visualization of the flow of single blood cells.

It follows that if the image can be seen with such clarity, then light can be delivered into the eye with the same control, which may allow precise, localized retinal laser surgery near the fovea.

View of Lunar Eclipse

Retinal Imaging Basic ScienceFirst images of the trichromatic photoreceptor mosaic in the human eye(Roorda and Williams, Nature, 1999)Scale bar = 5 m

5 arc min

human (JW)

human (AN)

macaque

Primary Mirrors: CELT vs. KeckCELTKeck

CELT and StonehengeKeck

CELT in PacBell Park

How to measure turbulent distortions(one method among many)

Applications and Results

So now that we know the wavefront, or aberrations of the eye we are measuring we can shape a mirror appropriately so that upon reflection, the wavefront is rendered parallel. The appropriate shape of the mirror to compensate for each aberration is simply the same shape as the wavefront with half the amplitude.

Recall that the aberrations of each eye are different so each eye requires a unique correction.So, why would want to improve the eyes optics?

Two reasons1: To improve human vision. 2. To improve images of the retina.

Both possibilities will give us a better understanding of the visual system but will have clinical and real-world applications as well.This slide illustrates both diffraction and monochromatic aberrations in the eye. Each image shows the calculated point spread function for a range of pupil sizes in the same eye. Ill explain how we compute these patterns later.

The eye has fairly good central optics. For small pupils, aberrations are low. So, you might think that the problem is solved by going to a small pupil. But the problem is that smaller pupils give more diffraction. So for a small pupil, the point spread function is very round, but it is also quite large.

Diffraction theory tells us that larger pupils should give us smaller point spread functions but near the margins, aberrations start to dominate so that we can not take advantage of the higher numerical aperture offered by a larger pupil.

In this image, the optimal pupil size lies somewhere between 2 and 4 mm, depending on your criteria.

If you can correct for the aberrations, then the best image quality is through the largest possible pupil.This is a typical AO telescope that uses the star being imaged as the source for wavefront sensing.