Adaptive optics tutorial at CTIO

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.

Where to get more information?
AO tutorials on the WEB:
at ESO [DEAD].
at The Imperial College [DEAD].
at CFHT (also a tour of AO systems worldwide) [ACTIVE]
Claire Max AO class [DEAD]
Books:
 

PLAN OF THE TUTORIAL

0. INTRODUCTION. 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.

Principle of adaptive optics


 

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.


 
 
 
 
 

Typical parameters of AO systems:
AO images The stellar images obtained at the 10-m Keck telescope with and without turbulence correction.
 
NEXT: Turbulence

Last modified: May 28, 2001 A. Tokovinin