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Adaptive Optics and its Applications
Lecture 1
Claire Max
UC Santa Cruz
September 25, 2003
Page 1
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 2:45 pm:
ice cream sundaes downstairs!
Page 2
Introductions: who are we?
Page 3
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 people’s 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
Page 4
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 5
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)
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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 7
Turbulence arises in several places
stratosphere
tropopause
10-12 km
wind flow over dome
boundary layer
~ 1 km
Heat sources w/in dome
Page 8
Vertical profile of turbulence
Measured from a balloon rising
through various atmospheric layers
Page 9
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 10
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)
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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 12
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 13
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 14
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 15
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 16
Adaptive optics increases peak
intensity of a point source
Lick
Observatory
No AO
With AO
Intensity
No AO
With AO
Page 17
Schematic of adaptive optics system
Feedback loop:
next cycle
corrects the
(small) errors
of the last cycle
Page 18
How to measure turbulent distortions
(one method among many)
Shack-Hartmann wavefront sensor
Page 19
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
• 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 20
How a deformable mirror works
(idealization)
BEFORE
Incoming
Wave with
Aberration
Deformable
Mirror
AFTER
Corrected
Wavefront
Page 21
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 22
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 23
Deformable mirrors come in many sizes
• Range from 13 to > 900 actuators (degrees of freedom)
About 12”
A couple
of inches
Xinetics
Page 24
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
Mirror surface map
Page 25
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 26
Laser is operating at Lick Observatory,
being commissioned at Keck
Keck Observatory
Lick
Observatory
Page 27
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
Page 28
Adaptive Optics World Tour
Page 29
Adaptive Optics World Tour
(2nd try)
Page 30
Astronomical observatories with
AO on 3-5 m telescopes
• ESO 3.6 m telescope, Chile
• University of Hawaii
• Canada France Hawaii
}
“Curvature sensing” systems
• Mt. Wilson, CA
• Lick Observatory, CA
• Mt. Palomar, CA
• Calar Alto, Spain
> 210 journal articles on AO astronomy, to date
Page 31
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 32
Lick adaptive optics system at 3m
Shane Telescope
DM
Wavefront
sensor
Off-axis
parabola
mirror
IRCAL infrared camera
Page 33
Canada France Hawaii Telescope
Fifteen minute integration time
0.19 arc sec resolution
Palomar adaptive optics system
AO system is in
Cassegrain cage
200” Hale telescope
Page 36
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 37
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 38
The Keck Telescope
Adaptive
optics
lives here
Page 39
Keck Telescope’s primary mirror
consists of 36 hexagonal segments
Nasmyth
platform
Person!
Page 40
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%
Page 41
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 42
Details of Neptune’s 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)
Page 43
How to relate IR and visible features?
2 mm: Keck adaptive optics, 2000
Visible: Voyager 2 fly-by, 1989
Circumferential bands
Compact southern features
Compact features such as Great Dark Spot, smaller
southern features: probably stable vortices
Page 44
Near-IR AO image of a volcano
erupting on Jupiter’s 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
Page 45
Io volcanoes in infrared light
Credit: Franck Marchis and Team Keck
Quic kTime™ and a YUV420 codec decompress or are needed to s ee this picture.
Page 46
European Southern Observatory:
4 8-m Telescopes in Chile
Page 47
NAOS - the AO system for the Very
Large Telescope in Chile
Page 48
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 49
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?
Page 50
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
• Ways to make best use of information in
multiple laser guide stars
• …..
Page 51
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
Page 52
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
Page 53
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)
Page 54
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
Page 56
How people learn
• The traditional lecture is far from the ideal teaching tool
– Researchers on education study these things rigorously!
• I can’t “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
Page 57
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
– I’ll ask one person from each group to describe reasoning to
class as a whole
Page 58
Videoconference techniques
• Please identify yourself when you speak
– This is Mary Smith from Santa Cruz
• Report any technical problems
– This is UCLA; we’ve 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
Page 59
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”
– Don’t 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
Page 60
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
Page 61
• Enjoy!
Page 62