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Proceedings Paper

Adaptive Optical System For Astronomical Applications
Author(s): F. Merkle; J. Bille; K. Freischlad; M. Frieben; G. Jahn; H.-L. Reischmann
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Paper Abstract

Light propagation through the atmosphere is disturbed by atmospheric turbulence. This limits the high angular resolution in astronomical imaging. Active optics is a method to overcome this problem. It allows a real-time optimization of the resolving power. An active mirror was developed which consists of an electrostatically deformable membrane. The polymer foil is actuated by an electrode array with 63 hexagonal elements arranged in a ring structure. The sensitivity is 0,05 microns per volt. It works up to 4 kHz without resonances. The maximum local tilt of the membrane is 3 microns per 5 millimeter. For an atmospheric tilt compensation of the wavefront the mirror housing is in a gimbal mount. Piezo-electric actuators provide a total mirror angular movement up to 20 Hz and angular sensitivity of 77 arcsecs per kV. With this active mirror device and a multi-microprocessor control unit the stabilisation of the star-speckle pattern positions and the deconvolution of the speckle patterns are possible. Two control methods are in development and analyzed in comparison. One is working with a modified shearing interferometer as a wave-front sensor and a feedback with cross-talk compensation. The other is sensing the optical information in the image plane by a diode-array and computing the wave-front from its Fourier modulus. The compensating phase distribution is generated by an expansion of the turbulence phase distortions into modes of a set of basis functions, i.e. Zernike-polynomials and Karhunen-Loeve-functions. By applying a modal control concept to the adaptive optical system, the electrodes of a membrane mirror are controlled in parallel with compensated cross-talk. The coefficients of the approximating functions, each of which corresponds to a mode on the mirror surface, are fed back to the actuators by a modal control matrix. The system presented in this paper is designed for the 0,75 m RC-telescope with alt-az mount at the Landessternwarte in Heidelberg, F.R.G..

Paper Details

Date Published: 16 June 1983
PDF: 9 pages
Proc. SPIE 0369, Max Born Centenary Conf, (16 June 1983); doi: 10.1117/12.934382
Show Author Affiliations
F. Merkle, University of Heidelberg (Germany)
J. Bille, University of Heidelberg (Germany)
K. Freischlad, University of Heidelberg (Germany)
M. Frieben, University of Heidelberg (Germany)
G. Jahn, University of Heidelberg (Germany)
H.-L. Reischmann, University of Heidelberg (Germany)

Published in SPIE Proceedings Vol. 0369:
Max Born Centenary Conf
M. John Colles; D. William Swift, Editor(s)

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