eso8908 — Organisation Release
Catching a Twinkling Star: Successful Tests of Adaptive Optics Herald New Era
26 October 1989
An old dream of ground-based astronomers has finally come true, thanks to the joint development of a revolutionary new technique, adaptive optics, by ESO and ONERA , LdM  and Observatoire de Paris in France.
It effectively eliminates the adverse influence of atmospheric turbulence on images of astronomical objects, yielding images almost as sharp as if the telescope were situated in space. An editorial appraisal of this important break-through appears in today's issue of the scientific journal Nature.
Why adaptive optics?
Ever since the invention of the telescope in the early 17th century, astronomers have had to accept that the sharpness of astronomical images obtained with ground-based instruments is severely limited by a factor which is beyond their control, that is the turbulence in the Earth's atmosphere.
This turbulence is perceived by the eye as the twinkling of stars. High above the observer, mostly at altitudes between 5 and 10 kilometres, there are many small, moving cells of air, each of which produces a "sub-image" of the same star; the result is a swarm of moving sub-images. (Compare with the air above a toaster or a hot radiator.)
To a naked-eye observer, the number of sub-images which fall within the periphery of his eye pupil changes all the time. The perceived intensity of the star varies; the star twinkles.
In a telescope, the size (that is, the sharpness) of a stellar image, is equal to the area within which this swarm of sub-images moves. The greater the air turbulence, the larger is this area and the less sharp are the resulting images. Because of this effect, an increase of the size of a telescope does not improve its ability to resolve details of astronomical objects, once the aperture of the telescope exceeds 10 or 20 cm; the best achievable image sharpness, even by high-quality, large astronomical telescopes, is effectively determined by the state of the atmosphere, and is referred to as "astronomical seeing" during the exposure. For this reason, large telescopes are placed at sites where the atmospheric turbulence is as small as possible, for instance La Silla.
For a long time it was thought impossible to avoid this natural limit. Now, for the first time, this old problem has been demonstrably solved.
A break-through in optical technology
In a major technological break-through in ground-based astronomy, a new device, known as the VLT Adaptive Optics Prototype (see eso8717, eso8808, and eso8707), has now proved its ability to overcome this natural barrier during a series of successful tests in the period 12 - 23 October 1989. They were performed at the coudé focus of the 1.52 m telescope at the Observatoire de Haute Provence (OHP), France (see photo).
The extensive tests showed that it was possible effectively to “neutralize'' the atmospherically induced smearing of a stellar image by continuously monitoring the motion of the sharp sub-images and then focussing them into one spot by means of a deformable mirror. In this way stellar images were obtained at infrared wavelengths whose sharpness was only limited by the telescope aperture (this is referred to as diffraction limited imaging).
On each of ten nights, exposures were made of about 10 bright stars through 4 or 5 infrared filters. Several integrations were made through each filter without the adaptive device, immediately followed by an equal number with the device activated. Depending on the brightness of the observed star, each exposure lasted between 10 and 100 seconds. For wavelengths of 3.5 μm and longer, the diffraction limit was always reached, irrespective of the atmospheric turbulence; it was often reached at 2.2 μm (see the figure) and a noticeable improvement was seen at 1.2 μm.
How does it work ?
The adaptive optics technique can also be described in terms of correcting the atmospherically introduced distortions of the light wavefront from the star.
It is based on a feed-back loop, and the optical system contains a deformable mirror which can change its surface profile in a way that exactly compensates for the distortions of the light wavefront after it has passed through the atmosphere. The information about how to deform the mirror comes from a wavefront sensor which allows to measure the shape of the distorted light wavefront. It requires a very fast and powerful computer to calculate how the actuators located behind the deformable mirror have to push and pull the mirror surface.
The present prototype system has a mirror with 19 actuators. The mirror is deformed, hence the wavefront is corrected, 100 times per second.
This prototype system will soon be installed at the ESO 3.6 m telescope at La Silla. The encouraging results represent a first, major step on the way towards an adaptive system for the 16 m Very Large Telescope (VLT).
The new technology makes it possible to achieve the theoretical limits for optical imaging in the infrared wavelength range by means of a medium-sized telescope. Further developments will aim at perfecting the technique for larger telescopes and at shorter wavelengths. Not only will present day telescopes benefit, but this technique will revolutionize the exploitation of the next generation telescopes, such as the ESO VLT, and, in many cases, compete with observations carried out by telescopes deployed in space.
Note that the technique of adaptive optics, as described here, is complementary to active optics, a system that allows to keep large astronomical mirrors in optimal shape when gravity, wind and heat distort them, and which has recently been successfully installed at the ESO New Technology Telescope (eso8903).
A scientific-technical paper, describing the first adaptive optics results, is expected to appear soon in the European journal Astronomy & Astrophysics.
The design and construction of this system is the product of a three year effort, involving a collaboration between the European Southern Observatory (ESO), the Office National d'Etudes et de Recherches Aérospatiales, the Laboratories de Marcoussis (formerly CGE, now Aérospatiale) and the Observatoire de Paris.
The project received support from ESO, the Ministère de la Recherche et de la Technologie (France), Ministère de l'Education Nationale, Direction de la Recherche (France), Université Paris VII, Centre National de la Recherche Scientifique (CNRS) and Institut National des Sciences de l'Univers (INSU).
The early development of critical optical components of the system has been independently supported by La Direction des Recherches et Etudes Techniques (DRET), Ministère de la Défense, France.
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