Measurement of the size of the isoplanatic patch using a phase-correcting telescope

Widening the effective field of view of adaptive-optics telescopes by deconvolution from wave-front sensing: average and signal-to-noise ratio performance

A fundamental problem of adaptive-optics systems is the very narrow corrected field of view that can be obtained because turbulence is extended in altitude throughout the atmosphere. The correctable field of view is of the order of 5-10 µrad at visible wavelengths and increases as the wavelength increases. Previous concepts to broaden the corrected field of view have been hardware oriented, requiring multiple wave-front sensor (WFS) measurements to control multiple deformable mirrors. We analyze the average and the signal-to-noise-ratio performance of an image measurement and postprocessing technique that uses simultaneous measurements of a short-exposure compensated image measured in an off-axis direction; an additional WFS measurement is taken in the off-axis direction. Results are presented for infinite-altitude WFS beacons driving both the WFS for the adaptive optics and the WFS looking in the off-axis direction, a variety of seeing and WFS light-level conditions, and off-axis angles from two to six times the isoplanatic angle. This technique improves the average effective transfer function out to a field angle of at least six times the isoplanatic angle while providing a higher signal-to-noise ratio in the spatial frequency domain.

Improved compensation of atmospheric turbulence effects by multiple adaptive mirror systems

Optical wave-front propagation in a layered model for the atmosphere is analyzed by the use of diffraction theory, leading to a novel approach for utilizing artificial guide stars. Considering recent observations of layering in the atmospheric turbulence, the results of this paper indicate that, even for very large telescopes, a substantial enlargement of the compensated angular field of view is possible when two adaptive mirrors and four or five artificial guide stars are employed. The required number of guide stars increases as the thickness of the turbulent layers increases, converging to the conventional results at the limit of continuously turbulent atmosphere.

Use of the Hartman sensor to measure the unisoplanatic wavefront tilt

It is shown that the Hartman wavefront sensor can be used to measure the degree of unisoplanatism under atmospheric wavefront degradations. The measurements conform with theory and show the size of the isoplanatic patch in the object space and the coherent distance at the entrance of the pupil plane. These two measurements are unequal because of inhomogeneous turbulence. This inhomogeneity can be accounted for by using known theory and the height of the line of sight above the ground.

Isoplanatic degradation of tilt correction and short-term imaging systems

A MTF is derived that includes isoplanatic effects of tilt correction or short-term imaging systems. The MTF is used to predict degradation in Strehl ratio or resolution due to tilt decorrelation on offset paths for long horizontal paths through constant turbulence and for space-to-earth paths through a path-dependent structure constant C(2)(n).