“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 t urbulence in Earth’s atmosphere Images of a bright star, Arcturus Lick Observatory, 1 m telescope Long exposure image Short exposure image “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 Aberrated wavefront Wavefront sensor Wavefront control computer Corrected wavefront Wavefront corrector

Adaptive optics in action Star with adaptive optics Star without adaptive optics Lick Observatory adaptive optics system

The Deformable Mirror

Deformable mirrors come in many shapes and sizes Today: mirrors from Xinetics. From 13 to 900 actuators (degrees of freedom); 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 main telescope 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 Concept Implementation 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.fitsRX J /20/00 2:04KsV=15K=~ sS=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 AO width = 0.34 arc sec Without AO With AO width = 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, 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 haze as 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 wavelength 1.65  m February 26-27, 1999

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

Keck AO Can See the Faintest Rings Discovered by Voyager Voyager: 4 groups of rings     Keck AO: outer  ring plus 3 inner groups (individual rings unresolved)

Infrared image (2 microns) 1 arc sec 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

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 Optics?

pupil images followed by psfs for changing pupil size Visual Acuity Is Worse at Night When Pupils Dilate 1 mm2 mm3 mm4 mm 5 mm6 mm7 mm

The 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 in White 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 AO With AO: Resolve individual cones Williams, Roorda et al. (U Rochester)

Looking Inside the Eye with AO

View 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 µm

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 Results