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Adaptive Optics

`Active optics' and `Adaptive optics' are techniques for improving image quality, by correcting some of the distortions imposed on the wavefront during its passage through the atmosphere and the telescope. `Active optics' corrects for slow (<~ 0.01 Hz), low-order (few elements across the telescope diameter) distortions, such as those caused by sagging of optics when the telescope is tipped. Active optics is essential for telescopes with primary-mirror diameters > 4 m, which cannot be made rigid at reasonable cost. Active optics maintain a thin deformable primary mirror (e.g. Galileo, Gemini, Subaru, VLT) or a segmented one (e.g. Keck) in the correct form at any elevation.

Adaptive optics techniques improve seeing by ironing out some of the wrinkles imposed on the wavefront as it passes through turbulent layers in the atmosphere. Typically, the wavefront is corrected by making independent tip, tilt and piston movements to each of ~ 10 - 100 elements of a continuous-facesheet mirror in the light path, at ~ 100 - 1000 Hz. The corrections required are obtained by analysing the wavefront (e.g. with a Shack-Hartmann wavefront sensor, curvature sensor or shearing interferometry) from a bright star < 1 arcmin from the object of interest. Most of the distortions imposed by the atmosphere are in phase (typically a few wavelengths across the diameter of a large telescope) rather than in amplitude. In a Shack-Hartmann wavefront sensor, a rectangular array of lenslets is placed at an image of the telescope pupil. Each lenslet creates an image of the star using light from one segment of the primary mirror, and the positions of each image changes with the slope of the wavefront impinging on that portion of the mirror. In curvature sensing, the curvature of the wavefront is measured from the difference between images of a star taken either side of focus.

The improvement in image quality is usually characterised by the Strehl ratio = the ratio between the peak intensities of the corrected and unaberrated (diffraction-limited) point-spread functions, maximum value 1.0. FWHM can be a misleading statistic, because partial correction (e.g. tip-tilt only) can yield an image with a narrow core superimposed on a diffuse plateau of emission, the latter including most of the light.

The quality of the seeing is typically parametrised by r0, the atmospheric coherence length, which is proportional to wavelength^(6/5). Without adaptive optics, the seeing FWHM is ~ the resolution of a telescope of diameter = r0, e.g. r0 = 10 cm yields optical seeing ~ 1 arcsec, r0 = 20 cm yields seeing ~ 0.5 arcsec.
For telescope diameter D < r0, diffraction dominates.
For D ~ r0, image motion dominates, and in this regime, tip-tilt provides useful correction.
For D > 4 r0, speckle dominates.
The quality of a site for adaptive optics is determined not only by the median seeing, but also by the height and thickness of the turbulent layers which cause it (often a few thin layers dominate). Some adptive-optics systems can be conjugated to correct for turbulence originating at a particular height.

Compared with Hubble Space Telescope, ground-based adaptive optics can deliver better resolution and larger collecting area, but the dynamic range is poorer.

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Last modified: 17 December 2010
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