Adaptive Optics Tutorial
Introduction
The purpose of this tutorial is to provide an introduction to
Adaptive Optics for engineers and other interested persons without going deep
into the theory and in a limited amount of time. The formulas which are useful
for estimating the system characteristics and/or to guide the design tradeoff
will be given. The relevant terminology will be introduced. Examples and
problems will be provided to help understanding the basic principles.
All credit to Dr. Andrei A. Tokovinin at Cerro Tololo Inter-American
Observatory, La Serena, Chile
PLAN OF THE TUTORIAL
0. INTRODUCTION.
Basic principle of AO compensation, main parts of AO system.
1. ATMOSPHERIC TURBULENCE.
Imaging through the turbulence: image width and
Strehl ratio. The main parameters to describe phase perturbations and
seeing. Kolmogorov turbulence. Temporal evolution and isoplanatic angle.
Statistical properties of Zernike modes.
2. CORRECTING THE TURBULENCE.
Partially corrected images. Deformable mirrors,
their major types and characteristics. Dependence of correction quality on the
number of actuators. Control aspects: open- and closed-loop
bandwidth, loop frequency, stability.
3. WAVEFRONT SENSORS.
Principle and realization of Shack-Hartmann and
curvature sensors. Photon noise. Other types of sensors (shearing
interferometer, pyramid, etc.).
4. LASER GUIDE STARS.
Two types of LGS (Rayleigh and sodium). Cone effect.
Tilt indetermination and solutions. Lasers for sodium GS. Operational
problems.
5. MULTI-CONJUGATE AO (optional).
Gain in the corrected FoV with more than
one DM. Atmospheric tomography. Tools to study MCAO. Gemini MCAO project.
The Basic Principle of Adaptive Optics
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The generic
AO system is presented in the Figure (credit: Imperial College). The
turbulence is corrected by a "rubber mirror" (or Deformable Mirror = DM)
located in a parallel beam in the exit pupil of a telescope. Signal to drive
the DM is obtained from the Wave-Front Sensor (WFS) which measures in real
time the optical aberrations that remain after corrections. A servo system
tries to obtain zero aberration by continously adjusting the DM shape.
The light used to probe the aberrations comes from a Guide Star (GS), which
can be either natural star (e,g, the observed object) or an artificial laser
guide star. The light from the scientific object is also corrected by DM, but
them directed to a scientific instrument (e.g. a camera). Often the light is
divided by wavelength: red part of the spectrum is directed to WFS, infrared
part - to the scientific instrument.
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Typical parameters of AO systems:
- Response time: 1 millisecond
- Size of corrected element: from 10 cm to 1 m
- Number of corrected elements: from 13 to 300 and more...
- Guide star magnitudes: brighter than 17
The stellar images obtained at the 10-m Keck telescope with and
without turbulence correction.
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NEXT:
Turbulence
Where to get more information?
AO tutorials on the WEB:
- at ESO.
- at The Imperial
College.
- at CFHT
(also a tour of AO systems worldwide)
- Claire Max AO class
Books:
- F. Roddier (ed), Adaptive optics in astronomy (Cambridge Univ. Press,
1999)
- John W. Hardy, Adaptive optics for astronomical telescopes (New York,
NY: Oxford University Press, 1998)
- Tyson, Robert K. Introduction to adaptive optics (Bellingham, WA: SPIE,
The International Society for Optical Engineering, 2000).
- Tyson, Robert K. Principles of adaptive optics (Boston, MA: Academic
Press, 1998)
