adaptive optics and giant telescopes - lick...

Adaptive Optics and Giant Telescopes

SACNASJerry Nelson

Center for Adaptive OpticsUC Santa Cruz2003 October 4

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How did I get here?

• One of those nerds who really liked math and science

• Bachelors degree in physics (Caltech)

• PhD in high energy physics (UC Berkeley)

• Migrated from elementary particle physics into astronomy

• Interests led me into “instrumentation” in astronomy and telescope design

• Currently director of Center for Adaptive Optics (CfAO) and Project Scientist for the Thirty Meter Telescope (TMT)

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Outline

• The Center for Adaptive Optics• What is Adaptive Optics (AO) and how does it work

• Giant Telescopes

• The role of AO in Giant Telescopes

• Conclusions

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Center for Adaptive Optics• The Center for Adaptive Optics (CfAO) is an NSF Science

and Technology Center. We began in 1999.

• Funded for 10 years at $4M/yr, the CfAO seeks to advance the use of adaptive optics (AO) in diverse areas including astronomical and vision science, health care, industry, and education

• CfAO is centered at UCSC and has active partners at 11 other locations, including UCB, UCI, UCLA, UCSD, Caltech, JPL, U Chicago, U Rochester, U Houston, Indiana University, LLNL

• Key AO theme areas are extremely large telescopes, extreme AO, vision science instrumentation, EHR

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The Optics Problem• In real life, optical systems often must look through

blurring media, thus they make poor images

• We can think about light as rays, and a good imaging system will converge all rays from the source (a star for example) (almost) onto a point on the light detector

• We also commonly refer to wavefronts, planes perpendicular to the rays

• Disturbing media divert the rays and distort the wavefronts

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Light rays and wavefronts

Light rays

lens focuswavefronts

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Plane Wave Distorted Wave

Atmospheric Perturbations

Density Variations

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Fundamental Limits

• One might think if the optics were perfect that we could make perfect point images of stars

• Unfortunately, diffraction (an effect where the waves of light interfere with each other) limits the angular resolution

• The fundamental limit is that the angular size of the image of a point source can be no smaller than λ/D, the ratio of the wavelength of the light to the diameter of the optical system

• For large telescopes the atmosphere limits can be 50 times worse than diffraction

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Examples of diffraction limited images

Aperture Image of distant point source (star)

Larger apertures give better image quality and resolution- this is important!

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What is Adaptive Optics

• AO is an optical system that senses wavefront error (optical aberrations) and cancels them with an equal and opposite wavefront error.

• This allows the optical system to make perfect images (limited only by diffraction)

• AO is designed to change the correction as the disturbance varies in time

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How a Deformable Mirror Works

BEFORE AFTER

Incoming Wave with Aberration

Deformable Mirror

Corrected Wavefront

Deformable Mirror

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Astronomical Adaptive Optics

• Atmospheric degradation– Temperature fluctuations in the atmosphere cause density variations– Density variations cause index of refraction variations– Index variations cause incident plane waves to become distorted– Distorted plane waves produce blurry images

• Adaptive Optics measures the wavefront errors and corrects them, thus restoring image quality to the diffraction limit of the telescope

• Natural stars can be used to measure the atmosphere, but with limited sky coverage (not enough bright stars)

• Artificial guide stars greatly improve sky coverage (laser beacons)

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Deformable mirrors now come in many shapes and sizes

• Example: commercial mirrors from Xinetics.• Range from 13 to > 900 actuators (degrees of freedom) • Future: new mirror technologies (MEMS, LCDs, ...)

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Neptune at 1.65 microns with and without Keck AO

Without adaptive optics With adaptive optics

June 28, 1999

2.3

arc

sec

May 24, 1999

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Interested in Adaptive Optics?

• Many research problems– High speed computing and control systems– Deformable mirror technologies– Development of high power lasers– Development of efficient optical and IR detectors– Systems and optical design

• Skill sets– Mathematics, physics, astronomy, optics– Electrical engineering– Mechanical engineering– Computer engineering

• If any of these subjects interest you, there are probably interesting opportunities for you in AO as an intern, in graduate school, and potential future jobs.

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Current ground based AO systems

• Lick 3m telescope– 70 actuators, Na laser beacon working

• CFHT 3.6m telescpe– 19 actuators natural guide stars, curvature

• Palomar 5.0m telescope– 250 actuators, natural guide stars , S-H

• Gemini 8m telescope– ~85 actuators, curvature, Na laser beacons planned, MCAO planned

• VLT 4x 8m telescopes– 150 actuators, just started working, Na laser beacon planned

• Keck 2 x 10m telescopes– 350 actuators, Na laser beacon working

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QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

Keck Observatory

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Making a sodium-layer laser guide star

• Sodium layer: region of increased density of atomic sodium at altitude of 95 to 105 km in earth’s atmosphere

• Excite resonance line of sodium (D2 line) by shining 589 nm light onto sodium layer

• Lick and Keck use dye lasers:– Use frequency-doubled Nd:YAG laser as pump light for a

dye laser tuned to 589 nm

Nd:YAG laser (frequency doubled)

Dye master

oscillator

95 to 105 km589 nm light

Glowing “pencil” of light in sodium layer

Dye amplifier

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Lick dye laser mounted on 3m telescope

• YAG pump lasers are in basement

• Fibers bring YAG light to dye laser on telescope

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Lick laser guide star

• Average power ~ 15 W

• Pulsed, rep rate 13 kHz

• Launched from side of telescope

• science observations

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Laser Guide Star correction Strehl = 0.6Uncorrected image of a star

Images of a 15th magnitude star, λ = 2.2 microns

Laser guide star at Lick Observatory has good Strehl performance

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Keck Laser is Being Commissioned Now

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The potential of AO

• Artificial guide stars to increase sky coverage

• AO on giant telescopes (CELT/GSMT)

• Multi-conjugate AO to achieve larger fields of correction

• Extreme AO to achieve very high Strehl’s and extremely high dynamic range (to detect faint objects near bright ones)

• AO working at short wavelengths (currently most work at ≥1 µm)

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California Extremely Large Telescope

• National Academy of Sciences’ decadal study on astronomy and astrophysics recommended a 30-m “Giant Segmented Mirror Telescope” as #1 priority for the next decade

• Adaptive optics for such a telescope will incorporate features significantly more sophisticated than those used today

• University of California and Caltech are planning to build a 30m ground based telescope with next-generation AO: CELT

Optical layout of CELT showing the primary, secondary and tertiary, and the Nasmyth focus.

The CELT conceptual design, shown inside of a cutaway enclosure -Note the large instrument platforms

A side view of CELT, showing the structure and place in the enclosure

CELT

Keck

The segmented primary mirrors for Keck and for CELT are shown side by side

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Conclusions

• Adaptive optics is an exciting new technology with compelling application in astronomy

• Giant telescopes are coming, requiring AO systems far beyond today’s state of the art

• Many opportunities for interns and graduate students interested in– Math– Physics– Astronomy– Many fields of engineering

• Photonics• Mechanical• Electrical• Structural• Computer