Adaptive Optics and its Applications Lecture 1

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Adaptive Optics and its ApplicationsLecture 1

• Claire Max
• UC Santa Cruz
• September 25, 2003

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Outline of lecture

• Introductions, goals of this course
• Overview of adaptive optics for astronomy
• Adaptive optics at UCSC
• How the course will work
• Homework for next week

Please remind me to stop for a break at 245
pm ice cream sundaes downstairs!
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Introductions who are we?
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Goals of this course

• To understand the main concepts behind adaptive
  optics systems
• To understand how to do astronomical observations
  with AO
• Planning, reducing, and interpreting data (your
  own data, but perhaps more importantly other
  peoples data)
• Some of this will apply to AO for vision science
  as well
• Opportunity to delve into engineering details if
  you are interested
• Brief introduction non-astronomical applications
  of AO
• I hope to interest a few of you in learning
  more AO, perhaps doing research

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Why is adaptive optics needed?
Turbulence in earths atmosphere makes stars
twinkle More importantly, turbulence spreads out
light makes it a blob rather than a point
Even the largest ground-based astronomical
telescopes have no better resolution than an 8"
telescope!
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Images of a bright star, Arcturus
Lick Observatory, 1 m telescope
? 1 arc sec
? l / D
Long exposure image
Short exposure image
Image with adaptive optics
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Turbulence changes rapidly with time
Image is spread out into speckles
Centroid jumps around (image motion)
Speckle images sequence of short snapshots of
a star, taken at Lick Observatory using the IRCAL
infra-red camera
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Turbulence arises in several places
stratosphere
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Vertical profile of turbulence
Measured from a balloon rising through various
atmospheric layers
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Optical consequences of turbulence

• Temperature fluctuations in small patches of air
  cause changes in index of refraction (like many
  little lenses)
• Light rays are refracted many times (by small
  amounts)
• When they reach telescope they are no longer
  parallel
• Hence rays cant be focused to a point

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Point focus
Light rays affected by turbulence
Parallel light rays
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Imaging through a perfect telescope

• With no turbulence, FWHM is diffraction limit
  of telescope, ? l / D
• Example
• l / D 0.02 arc sec for l 1 mm, D 10 m
• With turbulence, image size gets much larger
  (typically 0.5 - 2 arc sec)

FWHM l/D
1.22 l/D
in units of l/D
Point Spread Function (PSF) intensity profile
from point source
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Characterize turbulence strength by quantity r0
Primary mirror of telescope

• Coherence Length r0 distance over which
  optical phase distortion has mean square value of
  1 rad2 (r0 15 - 30 cm at good observing
  sites)
• Easy to remember r0 10cm ? FWHM 1 at l
  0.5?m

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Effect of turbulence on image size

• If telescope diameter D gtgt r0 , image size of a
  point source is (l / r0) gtgt (l / D)
• r0 is diameter of the circular pupil for which
  the diffraction limited image and the seeing
  limited image have the same angular resolution.
• r0 ? 10 inches at a good site. So any telescope
  larger than this has no better spatial
  resolution!

l / D
seeing disk
l / r0
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How does adaptive optics help?(cartoon
approximation)
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
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Infra-red images of a star, from Lick Observatory
adaptive optics system
With adaptive optics
No adaptive optics
Note colors (blue, red, yellow, white)
indicate increasing intensity
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AO produces point spread functions with a core
and halo

• When AO system performs well, more energy in core
• When AO system is stressed (poor seeing), halo
  contains larger fraction of energy (diameter
  r0)
• Ratio between core and halo varies during night

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Adaptive optics increases peak intensity of a
point source
Lick Observatory
No AO
With AO
Intensity
With AO
No AO
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Schematic of adaptive optics system
Feedback loop next cycle corrects the (small)
errors of the last cycle
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How to measure turbulent distortions (one method
among many)
Shack-Hartmann wavefront sensor
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Shack-Hartmann wavefront sensor measures local
tilt of wavefront

• Divide pupil into subapertures of size r0
• Number of subapertures ? (D / r0)2
• Lenslet in each subaperture focuses incoming
  light to a spot on the wavefront sensors CCD
• Deviation of spot position from a perfectly
  square grid measures shape of incoming wavefront
• Wavefront reconstructor computer uses positions
  of spots to calculate voltages to send to
  deformable mirror

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How a deformable mirror works (idealization)
BEFORE
AFTER
Deformable Mirror
Incoming Wave with Aberration
Corrected Wavefront
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Real deformable mirrors have continuous surfaces

• In practice, a small deformable mirror with a
  thin bendable face sheet is used
• Placed after the main telescope mirror

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Most deformable mirrors today have thin glass
face-sheets
Glass face-sheet
Light
Cables leading to mirrors power supply (where
voltage is applied)
PZT or PMN actuators get longer and shorter as
voltage is changed
Anti-reflection coating
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Deformable mirrors come in many sizes

• Range from 13 to gt 900 actuators (degrees of
  freedom)

About 12
A couple of inches
Xinetics
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New developments tiny deformable mirrors

• Potential for less cost per degree of freedom
• Liquid crystal devices
• Voltage applied to back of each pixel changes
  index of refraction locally
• MEMS devices (micro-electro-mechanical systems)

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If theres no close-by real star, create one
with a laser

• Use a laser beam to create artificial star at
  altitude of 100 km in atmosphere

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Laser is operating at Lick Observatory, being
commissioned at Keck
Keck Observatory
Lick Observatory
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Laser guide star at Lick Observatory is working
well
Uncorrected image of a star
Laser Guide Star correction
Images of a 15th magnitude star, l 2.2 microns
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Adaptive Optics World Tour
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Adaptive Optics World Tour (2nd try)
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Astronomical observatories with AO on 3-5 m
telescopes

• ESO 3.6 m telescope, Chile
• University of Hawaii
• Canada France Hawaii
• Mt. Wilson, CA
• Lick Observatory, CA
• Mt. Palomar, CA
• Calar Alto, Spain

Curvature sensing systems
gt 210 journal articles on AO astronomy, to date
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Adaptive optics system is usually behind main
telescope mirror

• Example AO system at Lick Observatorys 3 m
  telescope

Support for main telescope mirror
Adaptive optics package below main mirror
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Lick adaptive optics system at 3m Shane Telescope
DM
Off-axis parabola mirror
IRCAL infra-red camera
Wavefront sensor
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(No Transcript)
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Canada France Hawaii Telescope
Fifteen minute integration time 0.19 arc sec
resolution
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Palomar adaptive optics system
AO system is in Cassegrain cage
200 Hale telescope
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Adaptive optics makes it possible to find faint
companions around bright stars

• Two images from Palomar of a brown dwarf
  companion to GL 105

200 telescope
Credit David Golimowski
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The new generation adaptive optics on 8-10 m
telescopes
Summit of Mauna Kea volcano in Hawaii
Subaru
2 Kecks
Gemini North
And at other places MMT, VLT, LBT, Gemini South
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The Keck Telescope
Adaptive optics lives here
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Keck Telescopes primary mirror consists of 36
hexagonal segments
Nasmyth platform
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Keck AO system performance on bright stars is
spectacular!
A 9th magnitude star Imaged H band (1.6 mm)
Without AO FWHM 0.34 arc sec Strehl 0.6
With AO
FWHM 0.039 arc sec Strehl 34
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Neptune in infra-red light (1.65 microns)
With Keck adaptive optics
Without adaptive optics
2.3 arc sec
May 24, 1999
June 27, 1999
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Details of Neptunes bright storm at a scale of
400 - 500 km
Square root color map
Linear color map
Each pixel is 0.017 arc sec Dx 375 km at Neptune
H band (1.65 microns)
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How to relate IR and visible features?
Visible Voyager 2 fly-by, 1989
2 mm Keck adaptive optics, 2000
Compact features such as Great Dark Spot, smaller
southern features probably stable vortices
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Near-IR AO image of a volcano erupting on
Jupiters moon Io
Gas plume from a volcanic eruption
Credit Scott Acton
Visible-light image from Galileo spacecraft at
Io (every dark spot is a volcano)
Near-IR image from Keck adaptive optics
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Io volcanoes in infrared light

• Credit Franck Marchis and Team Keck

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European Southern Observatory 4 8-m
Telescopes in Chile
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NAOS - the AO system for the Very Large Telescope
in Chile
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VLT NAOS AO first light

• Cluster NGC 3603 IR AO on 8m ground-based
  telescope achieves same resolution as HST at 1/3
  the wavelength

NAOS AO on VLT ? 2.3 microns
Hubble Space Telescope WFPC2, ? 800 nm
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Some frontiers of adaptive optics

• Current systems (natural and laser guide stars)
• How can we monitor the PSF while we observe?
• How accurate can we make our photometry be?
• What methods will allow us to do high-precision
  spectroscopy with AO?
• Future systems
• Can we push new AO systems to achieve very high
  contrast ratios, to detect planets around nearby
  stars?
• How can we do AO with laser guide stars on 30-m
  telescopes of the future?

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Frontiers in AO technology

• New kinds of deformable mirrors with gt 5000
  degree of freedom
• Wavefront sensors that can deal with this many
  degrees of freedom
• Innovative control algorithms
• Ways to make best use of information in multiple
  laser guide stars
• ..

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Adaptive optics at UCSC

• Center for Adaptive Optics
• This building is headquarters
• NSF Science and Technology Center (10 yrs, 40M)
• AO for astronomy and for looking into the living
  human eye
• 11 other universities (including Rochester) are
  members, as well as JPL and LLNL
• Laboratory for Adaptive Optics
• Funded last year by the Gordon and Betty Moore
  Foundation
• 6 years, 9M
• Two labs in Thimann
• Experiments on Extreme AO to search for
  planets, and on AO for Extremely Large Telescopes

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How the course will work

• Website http//www.ucolick.org/max/289C
• Lectures will be on web after each class
• (Hopefully before class)
• Textbooks
• Course requirements
• Videoconference techniques
• Homework

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Textbooks

• Main text
• Adaptive Optics for Astronomical Telescopes by
  John Hardy (Oxford Press, 98)
• Reference Texts
• "Principles of Adaptive Optics" by Robert K.
  Tyson (2nd edition) (Academic Press, 1998)
• "Adaptive Optics in Astronomy" edited by Francois
  Roddier (Cambridge University Press, 1999)
• "Adaptive Optics Engineering Handbook" edited by
  Robert K. Tyson (Marcel Dekker, 2000)

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Availability of texts

• Hardy out of print, but can buy on web
• Barnes and Noble www.bn.com
• www.bookfinder.com
• Supposed to be at Bay Tree Bookstore (call first)
• Should cost about 150
• In meantime, CfAO has some copies of Hardy
• Available on loan till you can get your own
• Sign out after class
• Reference texts CfAO has reference copies in its
  library. Do not remove from building.

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Course requirements

• Lectures
• Reading assignments
• Homework problems (due Tuesdays)
• Student group projects (presentations in class)
• Field trip to Lick Observatory
• Laboratory exercises (a few)
• Final exam

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How people learn

• The traditional lecture is far from the ideal
  teaching tool
• Researchers on education study these things
  rigorously!
• I cant pour knowledge into you
• It is you who must actively engage in the subject
  matter and assimilate it in a manner that makes
  it meaningful
• This course will emphasize active learning and an
  understanding of the unifying concepts of
  adaptive optics

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Concepts vs. plugging in numbers

• Lectures will emphasize concepts, challenge you
  to become critical thinkers
• It is important to know how to calculate things,
  but concepts are important too
• Difference between learning to plug numbers into
  equations and learning to analyze unfamiliar
  situations
• I will stop my lectures every once in a while,
  and ask a Concept Question.
• First think about the question by yourselves for
  a minute or two
• Then discuss with 2 other students, come to a
  consensus
• Ill ask one person from each group to describe
  reasoning to class as a whole

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Videoconference techniques

• Please identify yourself when you speak
• This is Mary Smith from Santa Cruz
• Report any technical problems
• This is UCLA weve lost our video of Santa Cruz
• Microphones are quite sensitive
• Try not to rustle papers in front of them
• Cover mic if you are making side-comments,
  sneezes, whatever

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Homework for Tuesday Sept 30

• Read Syllabus
• Do Homework 1 Tell me about yourself
• Read Chapter 1 of Hardy
• The Short, Eventful History of Adaptive Optics
• Dont sweat the details -- goal is to get a broad
  overview on where adaptive optics came from
• Be prepared to discuss your reactions to the
  history of military and civilian research in AO

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Homework for Thursday October 2

• Choose one astronomical (or vision science) issue
  that interests you. Be prepared to discuss
  whether, and how much, AO might help observations
  in this area.
• Reading Hardy sections 3.1 through 3.4

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• Enjoy!