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Adaptive Optics and its Applications
Lecture 1
Neptune
Claire Max
UC Santa Cruz
January 5, 2006
Page 1
Outline of lecture
• Introductions, goals of this course
• How the course will work
• Homework for next week
• Overview of adaptive optics for astronomy
Please remind me to stop for a break at 2:45 pm!
Page 2
Introductions: who are we?
• Via video:
– AEOS, Maui: Ben Wheeler
– Indiana U. Optometry School: Weihua Gao, Yan Zhang
– JPL: Ian Crossfield
– Keck Observatory: Eric Johansson, Roger Sumner
– UCLA: Tuan Do, Jessica Lu, Jon Mauerhon, Emily
Rice, Shelley Wright
– UC Irvine: Lianqi Wang
– UCSC Mt. Hamilton: Bryant Grigsby
• In the CfAO conference room at UCSC
Page 3
Videoconference techniques
• Please identify yourself when you speak
– “This is Mary Smith from Santa Cruz”
• Report technical problems to the UCOP contact
person (see your email)
• Microphones are quite sensitive
– Do not to rustle papers in front of them
– Mute your microphone if you are making sidecomments, sneezes, eating lunch, whatever
Page 4
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
just as importantly other people’s data)
– Some of this will apply to AO for vision science as well
• Get acquainted with AO components in the Lab.
– Delve into engineering details if you are interested.
• Brief introduction to non-astronomical applications
• I hope to interest a few of you in learning
more AO, and doing research in the field
Page 5
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 components
• Homework
Page 6
Required Textbooks
• Reader containing key articles and excerpts
for this class. Available at Slug Books Coop,
next to 7-11 Store. http://www.slugbooks.com/
– We can arrange to buy copies on behalf of folks in
video land
– PDF versions will be available on restricted website
• Field Guide to Adaptive Optics by Robert K.
Tyson and Benjamin W. Frazier, SPIE Press.
Available from Bay Tree Bookstore.
Page 7
Course components
• Lectures
• Reading assignments
• Homework problems
• Student group projects (presentations in class)
• Laboratory exercises
• Field trip to Lick Observatory
• Web discussions (perhaps)
• Final exam
Page 8
Next Week
• Next Tuesday I will be at the American
Astronomical Society meeting in Washington
DC
• Instead of regular lecture class, there will be a
tour of the Laboratory for Adaptive Optics
• Meet here in CfAO Conference Room at 2 pm
• Next regular class is Thursday January 12th
Page 9
Homework for Thursday January 12
• Read Syllabus carefully (on web)
• Do Homework # 1: “Tell me about yourself”
• Reading: in Reader
–Chapter 1 of Roggeman (pages 59-66)
– Excerpt from Hardy’s Chapter 2 (pages 5-15)
– Don’t sweat the details -- goal is to get a broad
overview on where adaptive optics came from
Page 10
Why is adaptive optics needed?
Turbulence in earth’s
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!
Page 11
Images of a bright star, Arcturus
Lick Observatory, 1 m telescope
 ~ 1 arc sec
Long exposure
image
~ l/D
Short exposure
image
Image with
adaptive optics
Speckles (each is at diffraction limit of telescope)
Page 12
Turbulence changes rapidly with time
QuickTime™ and a YUV420 codec decompressor are needed t o see this picture.
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
Page 13
Turbulence arises in several places
stratosphere
tropopause
10-12 km
wind flow over dome
boundary layer
~ 1 km
Heat sources w/in dome
Page 14
Atmospheric perturbations
cause distorted wavefronts
Rays not parallel
Plane Wave
Index of refraction
variations
Distorted
Wavefront
Page 15
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 can’t be focused to a point:
Point
 focus
Parallel light rays
 blur
Light rays affected by turbulence
Page 16
Imaging through a perfect telescope
With no turbulence,
FWHM is diffraction limit
of telescope,  ~ l / D
FWHM ~l/D
Example:
1.22 l/D
in units of l/D
Point Spread Function (PSF):
intensity profile from point source
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)
Page 17
Characterize turbulence strength
by quantity r0
Wavefront
of light
r0
“Fried’s parameter”
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 = 0.5mm
Page 18
Effect of turbulence on image size
• If telescope diameter D >> r0 , image size of a point
source is l / r0 >> l / D
l/D
“seeing disk”
l / r0
• 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!
Page 19
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
Page 20
Infra-red images of a star, from Lick
Observatory adaptive optics system
QuickTime™ and a YUV420 codec decompressor are needed to see this picture.
No adaptive optics
With adaptive optics
Note: “colors” (blue, red, yellow, white) indicate increasing intensity
Page 21
AO produces point spread functions
with a “core” and “halo”
Intensity
Definition of “Strehl”:
Ratio of peak intensity to
that of “perfect” optical
system
x
• 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
Page 22
Adaptive optics increases peak
intensity of a point source
Lick
Observatory
No AO
With AO
Intensity
No AO
With AO
Page 23
Schematic of adaptive optics system
Feedback loop:
next cycle
corrects the
(small) errors
of the last cycle
Page 24
How to measure turbulent distortions
(one method among many)
Shack-Hartmann wavefront sensor
Page 25
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 sensor’s CCD detector
• 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
Page 26
How a deformable mirror works
(idealization)
BEFORE
Incoming
Wave with
Aberration
Deformable
Mirror
AFTER
Corrected
Wavefront
Page 27
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
Page 28
Deformable Mirror for real wavefronts
Most deformable mirrors today
have thin glass face-sheets
Glass face-sheet
Light
Cables leading to
mirror’s power
supply (where
voltage is applied)
PZT or PMN actuators:
get longer and shorter
as voltage is changed
Anti-reflection coating
Page 30
Deformable mirrors come in many sizes
• Range from 13 to > 900 actuators (degrees of freedom)
About 12”
A couple
of inches
Xinetics
Page 31
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)
Ele ctrostati cal ly
Me mbrane
actuate d
Attachm e nt mi rror
diaphragm
post
Conti nuous mirror
Page 32
If there’s 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
Page 33
Laser is operating at Lick Observatory,
being commissioned at Keck
Keck Observatory
Lick
Observatory
Page 34
Galactic Center with Keck laser guide star
Keck laser guide star AO
Best natural guide star AO
Page 35
Adaptive Optics World Tour
Page 36
Adaptive Optics World Tour
(2nd try)
Page 37
Steady growth in AO astronomy
publications since 1995
Page 38
Citations for AO papers are
equal to astrophysics average
Page 39
Astronomical observatories with
AO on 3-5 m telescopes
• ESO 3.6 m telescope, Chile
• Canada France Hawaii
• William Herschel Telescope, Canary Islands
• Mt. Wilson, CA
• Lick Observatory, CA
• Mt. Palomar, CA
• Calar Alto, Spain
Page 40
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 below
main mirror
Page 41
Lick adaptive optics system at 3m
Shane Telescope
DM
Wavefront
sensor
Off-axis
parabola
mirror
IRCAL infrared camera
Page 42
Palomar adaptive optics system
AO system is in
Cassegrain cage
200” Hale telescope
Page 45
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
Page 46
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
Page 47
The Keck Telescope
Adaptive
optics
lives here
Page 48
Keck Telescope’s primary mirror
consists of 36 hexagonal segments
Nasmyth
platform
Person!
Page 49
Neptune in infra-red light (1.65 microns)
With Keck
adaptive optics
2.3 arc sec
Without adaptive optics
May 24, 1999
June 27, 1999
Page 50
Neptune at 1.6 mm: Keck AO exceeds
resolution of Hubble Space Telescope
HST - NICMOS
Keck AO
~2 arc sec
2.4 meter telescope
10 meter telescope
(Two different dates and times)
Page 51
Uranus with Hubble Space
Telescope and Keck AO
L. Sromovsky
HST, Visible
Keck AO, IR
Lesson: Keck in near IR has same resolution as Hubble in visible
Page 52
Uranus with Hubble Space
Telescope and Keck AO
de Pater
HST, Visible
Keck AO, IR
Lesson: Keck in near IR has same resolution as Hubble in visible
Page 53
European Southern Observatory:
4 8-m Telescopes in Chile
Page 54
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
Hubble Space Telescope
WFPC2, l = 800 nm
NAOS AO on VLT
l = 2.3 microns
Page 55
Some frontiers of adaptive optics
• Current systems (natural and laser guide stars):
– How can we measure the Point Spread Function while we
observe?
– How accurate can we make our photometry? astrometry?
– 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 achieve a wider AO field of view?
– How can we do AO for visible light (replace Hubble on the
ground)?
– How can we do laser guide star AO on future 30-m telescopes?
Page 56
Frontiers in AO technology
• New kinds of deformable mirrors with > 5000
degree of freedom
• Wavefront sensors that can deal with this many
degrees of freedom
• Innovative control algorithms
• “Tomographic wavefront reconstuction” using
multiple laser guide stars
• New approaches to doing visible-light AO
Page 57
Adaptive optics at UCSC
• Center for Adaptive Optics
– This building is headquarters
– NSF Science and Technology Center
– AO for astronomy and for looking into the living
human eye
– 11 other universities are members, as well as national
labs and observatories
• Laboratory for Adaptive Optics
– Funded by the Gordon and Betty Moore Foundation
– Two labs in Thimann
– Experiments on “Extreme AO” to search for planets,
and on AO for Extremely Large Telescopes
Page 58
• Enjoy!
Page 59