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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 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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