Transcript An Introduction to Adaptive Optics

An Introduction to Adaptive
Optics
Mike Hein
PH 464 – Applied Optics
Winter 2005
Why do we need adaptive optics?
 To Correct for Irregularities in the
Transmission Media
– Atmospheric (Astronomical)
 Scintillation or Twinkling (Eddies ~ √λL)
 Beam Wander or Quiver (Eddies > Beam Size)
 Spreading (Eddies < Beam Size)
– Atmospheric (LASER Transmission)
 Thermal Blooming
– Ocular (Retinal Imaging)
 Defects in the Lens and Cornea
How Turbulence Effects Light
Spherical waves emitted
by the source (star)
are essentially plane
waves when they
arrive at the Earth’s
atmosphere. The
waves are then
randomly distorted by
turbulence in the
atmosphere.
Checking the Figure of the Hale
Telescope
How does it work?
 Two Basic Functions:
– Wavefront sensing
 Shack-Hartmann wavefront sensor
 Shearing interferometer
– Wavefront correction
 Tip-tilt optical element
 Deformable optical element
Wavefront Sensing
 Shack-Hartmann
Sensor
– Has become the
standard type of sensor
– Uses an array of
spherical “lenselets”
– Focuses wavefronts
onto a CCD array
– (b) is a plane wave
– (c) is a distorted wave
Wavefront Sensing
 Shearing
Interferometer
– Distorted wavefronts
result in phase shifted
output signals between
the detector arrays.
– Two interferometers are
required for correction
in the x-y plane.
Wavefront Correction
 High speed tip-tilt
mirror reduces overall
wavefront tilt
 Deformable mirror
(monolithic or
segmented) corrects
wavefront shape
Wavefront Correction

Monolithic Deformable Mirror
– Reflective face cast on a solid piezoelectric backing (1.4um).
– Thin face supported by an array of electro-mechanical actuators (+/- 3.0um)
– Thin face supported by an array of piezoelectric actuators (5.0um)
Wavefront Correction
 Segmented Mirrors
– Capable of larger
corrections than
monolithic types.
– Requires frequent
calibration
– Diffraction due to gaps
between segments
Some Definitions
Angular isoplanatism – the angular measure over
which a compensated wavefront can be
considered planar. At visible wavelengths the
isoplanatic angle is about 2 arc seconds
increasing to 10 arc seconds in near infrared light.
Fried Parameter (r0) – the turbulence coherence
length. A measure of the size of turbulent cells in
the atmosphere. Typically 5cm to 20cm.
Some Definitions
 Apparent or Visual
Magnitude – The relative
brightness of a star as
seen by an observer on
Earth
 Guide Star – A sufficiently
bright star to use as a
reference for image
compensation. The
required magnitude varies
as a function of the desired
observation wavelength
Limitations of Image Compensation
 Compensation in visible wavelengths (0.5um)
requires a guide star of magnitude 10 or brighter.
 Compensation in infrared wavelengths (2.2um)
allows guide stars down to magnitude 14.
 Small isoplanatic region around the guide star
limits total observation area:
– Visual – 1/100,000 of the sky
– IR – 1/1000 of the sky
Guide Stars
Artificial Guide Stars
 The area of sky coverage can be
expanded using artificial guide stars.
 Created using a LASER aligned with
the telescope optics.
 Rayleigh beacons take advantage of
Rayleigh scattering in the
atmosphere.
– Useful for telescopes with apertures < 2m.
– Creates a 1 to 2 arc second guide star 5 to
10km in altitude.
– 100W LASER
– LASER is pulsed so that backscatter can be
eliminated by range gating.
Artificial Guide Stars
 Sodium Beacons
exploit a layer of
sodium vapor in the
upper atmosphere
– 10W LASER at 589nm
– Excites sodium atoms
90 to 92 km in altitude.

Photo - The Sodium laser launching
from the side of the Lick
Observatory 120" Shane Telescope.
This is a 10 minute time exposure note the star trails
Artificial Guide Stars
 Natural guide star
required for tilt
correction.
 Limiting magnitude for
guide star is about 20.
 Result is nearly 100%
sky coverage.
IR Stellar imaging from Lick
Observatory




Upper Left: Io image
taken with Keck
adaptive optics; Kband, 2.2 micron.
Upper Right: Io image
based on visible light
taken with Galileo
spacecraft orbiter.
Lower Left: Io image
taken with Keck
adaptive optics; L-band,
3.5micron.
Lower Right: Io image
taken without Keck
adaptive optics.
The Lick Observatory CIS
Non-Astronomical Applications
 Most of the current AO
technology was
developed by DARPA
during the Cold War to
image and shoot down
satellites with LASER
weapons.
Retinal Imaging
References









Carroll, Joseph, Daniel C. Gray, Austin Roorda, David R. Williams, “Recent Advances in
Retinal Imaging with Adaptive Optics,” Optics and Photonics News, Jan 2005: p. 36-42
Chaisson, Eric and Steve McMillan, Astronomy Today 4th Edition, Prentice Hall, Upper
Saddle River, NJ, 2002
Florence, Ronald, The Perfect Machine, Harper Collins, New York, NY, 1994
Hardy, John W., Adaptive Optics for Astronomical Telescopes, Oxford University Press,
New York, NY, 1998
Hardy, John W., “Adaptive Optics,” Scientific American, June 1994: p. 60-65
Hecht, Eugene, Optics 4th Edition, Addison-Wesley, Reading, MA, 2002
Tyson, Robert K., Principles of Adaptive Optics 2nd Edition, Academic Press, San Diego,
CA, 1998
AdaptiveOptics.com, 2002, Adaptive Optics Associates Inc., http://www.aoainc.com/
Center for Adaptive Optics, 2005, University of California Santa Cruz,
http://cfao.ucolick.org/