`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.
Useful links and references:
Center for Astronomical AO (CAAO)
Durham - Astronomical Instrumentation Group (AIG)
Edinburgh - Astronomy Technology Centre (ATC)
Imperial College - Applied Optics group
Laird Close AO lecture
Claire Max AO lecture
Andrei Tokovinin tutorial
Olivier Lai presentation
ESO introduction to adaptive-optics
- "Adaptive optics for astronomy". Beckers J.M., 1993, ARAA, 31, 13
- "Untwinkling the stars" parts 1 and 2. Wild W.J., Fugate R.Q., 1994,
Sky and Telescope, May 1994, 25 and Jun 1994, 20
- "Adaptive optics". Hardy J.W., 1994, Scientific American, Jun 1994, 40
- "Adaptive optics for astronomy: theoretical performance and
limitations". Wilson R.W., Jenkins C.R., 1996, MNRAS, 268, 39
CFHT tutorial (viewgraphs)
JHU (Lotz) AO pages
Keck bibliography of adaptive-optics papers
- Nature 26/8/99 (v400), p.807 (news article)
AO imaging of the eye