“Twinkle, Twinkle Little Star”:An Introduction to Adaptive Optics

Mt. Hamilton Visitor’s Night

July 28, 2001

Turbulence in the atmosphere limits the performance of astronomical telescopes

Even the largest ground-based astronomical telescopes have no better resolution than an 8" backyard telescope!Even 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

Distant stars should resemble “points,”

if it weren’t for turbulence in Earth’s atmosphere

Images of a bright star, Arcturus

Lick Observatory, 1 m telescope

Long exposureimage

Short exposureimage

“Perfect” image: diffraction limit

of telescope

Turbulence changes rapidly with time

Sequence of very short snapshots of a star.Movie is much slower than "real 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 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

How to correct for atmospheric blurring

Basic idea of AO

Aberratedwavefront

Wavefront

sensor

Wavefront control

computer

Corrected

wavefront

Wavefrontcorrector

Adaptive optics in action

Star with adaptive opticsStar without adaptive optics

Lick Observatory adaptive optics system

The Deformable MirrorThe Deformable Mirror

Deformable mirrors come in many shapes and sizes

• Today: 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 mirror

• Example: AO system at Lick Observatory’s 3 m telescope

Support for maintelescope mirror

Adaptive optics package under main mirror

What does a “real” adaptive optics system look like?

Wavefront sensor

Infra-red camera

Deformable mirror

Light from telescope

If there is no nearby star, make your own “star” using a laser

ConceptImplementation

Lick Obs.

Laser in 120-inch dome

Laser guide star adaptive optics at Lick Observatory

Laser Guide Star correction of a star: Strehl = 0.6

Uncorrected image of a star

Ircal1129.fits RX J0258.3+1947 10/20/00 2:04 Ks V=15 K=~13.32 20s S=0.6 LGS

AO at the Keck 10 m Telescope

Adaptive optics lives here

Adaptive optics on 10-m Keck II Telescope: Factor of 10 increase in spatial resolution

Without AOwidth = 0.34 arc sec

Without AO

With AOwidth = 0.039 arc sec

9th magnitude star imaged in infrared light (1.6 m)

Neptune in Infrared Light

Without adaptive optics With Keck adaptive optics

June 27, 1999

2.3

arc

sec

May 24, 1999 = 1.65 microns

Neptune:Ground-based AO vs. Voyager Spacecraft

Infrared: Keck adaptive optics, 2000 Visible: Voyager 2 fly-by, 1989

Circumferential bands

Compact southern features

Saturn’s moon Titan: Shrouded by hazeas seen by Hubble Space Telescope

Limb Brightening due to haze

Hints of surface detail

Image at 0.85 microns

Titan at Keck: with and without adaptive optics

Titan with adaptive optics Titan without adaptive optics

Typical @ wavelength 1.65 m February 26-27, 1999

Uranus as seen by Hubble Space Telescope and Keck AO

Hubble Space Telescopefalse-color image (1.1, 1.6, 1.9 m)

Keck adaptive optics image (2.1 m)

Keck AO Can See the Faintest RingsDiscovered by Voyager

Voyager: 4 groups of rings

4 5 6

Keck AO: outer ring plus 3 inner groups

(individual rings unresolved)

Infrared image (2 microns)

1 a

rc se

c

A volcano erupting on Io: Jupiter's largest moon

Volcano erupting on limb

Io with adaptive optics sees most of the volcanic features seen by Galileo

Keck AO: three IR "colors" Galileo: visible CCD camera

Same volcanoes Same volcanoes

Other Uses for AO

• High-speed communications with laser beams

• Cheaper and lighter telescopes in space

• High-powered lasers for fusion power

• Vision science research

Perfect Eye

Aberrated Eye

Why Correct the Eye’s Why Correct the Eye’s Optics?Optics?

pupil images

followed by

psfs for changing pupil size

Visual Acuity Is Worse at Night When Pupils DilateVisual Acuity Is Worse at Night When Pupils Dilate

1 mm 2 mm 3 mm 4 mm

5 mm 6 mm 7 mm

The Rochester Adaptive Optics OphthalmoscopeThe Rochester Adaptive Optics Ophthalmoscope

Adaptive optics provides a clear improvement in retinal image quality

Wave Aberration Point Spread Function

Retinal Image at 550nm

Retinal Image inWhite Light

6.8 mm pupil

Before adaptive optics:

After adaptive optics: 1 deg

YY

Adaptive optics provides highest resolution images of living human retina

Without AOWith AO:

Resolve individual cones

Williams, Roorda et al. (U Rochester)

JW right eye1 deg field

1 deg eccentricity

PhotoreceptorsPhotoreceptorsCapillariesCapillaries

10 arcmin(48.6 )m

Looking Inside the Eye with AO

View of Lunar EclipseView of Lunar Eclipse

Retinal Imaging – Basic Science

First images of the trichromatic photoreceptor mosaic in the human eye(Roorda and Williams, Nature, 1999)

Scale bar = 5 µmScale bar = 5 µm

5 arc min

human (JW)human (JW) human (AN)human (AN) macaquemacaque

L 75.8%M 20%S 4.2%

L/M = 3.79

L 50.6%M 44.2%S 5.2%

L/M = 1.14

L 53.4%M 38%S 8.6%

L/M = 1.40

Primary Mirrors: CELT vs. Keck

CELT Keck

CELT and Stonehenge

Keck

CELT in PacBell Park

How to measure turbulent distortions(one method among many)

Applications and ResultsApplications and Results