| VIII. Review Paper: ATMOSPHERIC LIMITATIONS TO ADAPTIVE IMAGE COMPENSATION Francois Roddier |
The properties of optical turbulence relevant
to real-time wave-front compensation are reviewed. Assuming Kolmogorov
turbulence, the performance of an adaptive optics (AO) system is essentially
determined by three parameters: the Fried parameter r_0, the Greenwood parameter
tau_0, and the isoplanatic patch size theta_0. Ideally, an AO system with N
degrees of freedom should perfectly compensate the N higher Karhunen-Loeve modes
of the random wave-front surface. A convenient criterion for image quality,
first introduced by Fried, is the normalized Strehl ratio. For an ideal AO
system with N degrees of freedom operating on axis, it is a function of the
ratio D/r_0 of the telescope diameter over the Fried parameter. The behavior of
an actual AO system is similar, although a higher number N* of degrees of
freedom is required to achieve the same performance. The ratio N/N* is a measure
of the compensation efficiency of the system. Actual systems have a finite
bandwidth, which further degrades the compensation. One can define a minimum
allowable bandwidth tau_c above which the degradation is deemed acceptable.
Similarly, there is a maximum distance theta_c to the guide star below which the
degradation is also acceptable. Both tau_c and theta_c are a function of N.
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| VIII.1 MULTI-CONJUGATE ADAPTIVE OPTICS: EXPERIMENT IN ATMOSPHERIC TOMOGRAPHY Jacques Maurice Beckers |
An essential component of Multi-Conjugate Adaptive Optics is the estimation of this wavefront distortion at different heights in the atmosphere. One technique to do so called "Atmospheric Tomography" uses the wavefront distortions at the telescope entrance pupil of objects in a number of different directions in the sky to infer the 3-D wavefront behavior. My experiment uses the small scale structure on the solar surface to do so. The Sun has the advantage that the wavefront can be observed in an large number of different directions using correlation wavefront sensing.
This paper describe the observation and analysis of the wavefronts over a 2.2 arcmin diameter area on the Sun at 411 nm wavelength. A Shack-Hartmann lenslet array covers the pupil of the 76 cm aperture Richard B. Dunn Solar Telescope at the National Solar Observatory with 69 subapertures, 9 across the pupil. The wavefront is measured at a large number of points on the solar surface within the 2.2' area. The observations will initially be used with a simple AT technique to obtain the 3-D wavefront. The quality of the associated hypothetical MCAO system will be evaluated as a function of the number of conjugates and their heights.
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| VIII.2 ADAPTIVE OPTICS CURVATURE SYSTEM FOR SEGMENTED PRIMARY TELESCOPES Salvador Cuevas |
Adaptive Optics is a mature technique which permits near to diffraction limited imaging on new generation monolithic primary telescopes.
AO systems for segmented telescopes is a real challenge because it seems
there are vibration among the segments introducing relative piston and tip-tilt among them. This impedes to reach the Strehl values obtained on monolithic primary telescopes for the same D/r0 values.
In this work we propose a new curvature AO system which
can measure the relative piston and tip-tilt among the segments besides the atmospheric wavefront distorsions and correct them using two mirrors: a segmented and a deformable bimorph mirror.
An extrapolation of this system for the cophasing of a diluted aperture interferometric arrays is analyzed
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| VIII.3 REFERENCE BEACON DESIGN AND MODELS OF ATMOSPHERIC TURBULENCE Vladimir Lukin |
Application of adaptive correction in a ground-based telescopes is possible to improve whole performance of telescope and to obtain object images with high resolution.
I am assuming to show new approaches for tip-tilt correction. Development of technical part of any project assumed scheme of creation special laser system if a guide star is formed through the atmosphere. It is difficult to correct general tilt of wavefront using signal only from a laser star because the required information could not be obtained directly from measurements of star dither. Some methods to solve this problem can be found in scientific publications, but all of them increase complexity of the system technical realization.
The following methods can listed as example: simultaneous measurements of general angular dither of bright natural star and guide star, employment of two-color laser guide stars, usage of auxiliary telescopes, and additional laser sources (illuminators).
I am creating some new approaches for tip-tilt correction under knowledge of models of atmospheric turbulence: structure parameters and outer scale of turbulence. First of all, I have offered a new scheme for laser guide star formation: LGS have view as a two croosing lines - laser reference croos. Secondly, I am developing some approaches for angular nonisoplanarity minimizing.
And, finally, I am using an algorithm of "optimal" correction, insuring the lowest possible level of residual errors and partially giving focal nonisoplanarity correction.
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| VIII.4 MULTI SCINTILLATION LAYER ORIENTED SEEING MONITOR Roberto Ragazzoni |
The feasibility of Multi Conjugated Adaptive Optics (MCAO)
is actually the subject of a great number of studies.
The three-dimensional reconstruction of the atmospheric turbulence is one of the main goals to achieve optimum correction with MCAO.
To obtain the optimal correction it is necessary to properly
choose the layer to which conjugate the deformable mirrors (DM).
In the framework of dividing the turbulence strength into a small number of layer, but containing most of the turbulence power, we have conceived a seeing monitor based on scintillation measurements, able to optically-separate the turbulence induced by two or more conjugated layers. Using a single star it is shown how one can derive the residual aberrations for an optimum MCAO system with two, three or more DMs conjugated to a specific layer. This is accomplished by conjugating some stop-down to the specific ranges and by means of numerical correlation between the various scintillation measurements.
In this way one can estimate the goodness of the MCAO correction for a fixed or variable DMs configuration using a single small telescope and a single bright star. Together with the theoretical framework we present here the results of a simulation code.
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| VIII.5 WHAT KIND OF ADAPTIVE OPTICS FOR THE ROTATING SLIT APERTURE TELESCOPE? Hamid Tourma |
The interest of a ground-based Slit Aperture Telescope (S.A.T.) which operates with a long thin pupil of size Dxro, ro being the Fried's parameter, has been underlined for applications in the field of high angular resolution observations by speckle-interferometry techniques.
In the absence of atmospheric turbulence, the methods of image reconstruction with a S.A.T., which can rotate around its optical axis, are analogous to those used in computerised tomography and make use of the inverse Radon transformation. First I shall present briefly, in this communication, the principal of an image processing algorithm which could be used associated with a space-S.A.T., for astronomical image reconstruction.
In the presence of atmospheric turbulence, image reconstruction with a S.A.T. presents advantages for both passive and active techniques. In the present communication, I briefly describe these possible advantages with regards to speckle imaging techniques, and I develop in more detail the advantages of a S.A.T. for adaptive optics technology. I will present a numerical simulation developed to test the concept of partial adaptive correction with the S.A.T. Simulations of image reconstruction are given when a partial atmospheric correction is performed in one dimension only, along the long axis of the S.A.T.
It is shown that a S.A.T. can use a small adaptive optical system with much fewer actuators than conventional two dimensional systems, but a relatively high degree of one-dimensional correction in needed to recover a satisfactory two-dimensional image.
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| VIII.6 SITE AND ADAPTIVE PHASING FOR 1-10km HYPERTELESCOPES Antoine Labeyrie |
The hypertelescope approach to long-baseline interferometry announces direct imaging with a high dynamic range.
Pending space versions, terrestrial hypertelescopes are of interest if suitable sites can be found. A near-spherical basin of 5 km diameter and 800m depth would be suitable for a diluted optical version of Arecibo's radio dish, affording up to one kilometer of effective aperture size and collecting area similar to OWL . Steps are taken for optical tests at Arecibo in the coming months.
A different design , the Optical Very Large Array, can also operate as a hypertelescope. With its mobile telescopes, it can use a moderately flat site spanning up to 10 km.
Critical for the efficient use of such large arrays is adaptive phasing on faint sources . Whether multi-conjugate techniques using natural stars are applicable to diluted apertures is a central issue .
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