Conferences

NAOS - Nasmyth Adaptive Optics System

Perspectives

Object

Uncorrected Image

AO Corrected Image

Toward Very High Resolution

• Improving Adaptive Optics Direct Detection of Extrasolar Planets

  This is one of the major stake of astrophysics. Observing directly the photons coming from planets orbiting around other stars should lead to precise measurements of masses and diameters of such planets and, above all, their chemical composition. This would contribute to a better understanding of formation and evolution of planetary systems.

  Direct detection requires high-contrast imaging, to distinguish the few photons coming from the planet from those coming from the star. The measuring device will be a high-performance coronograph - to block most of the light coming from the star - relying on a sophisticated imaging system.

  Both turbulence and defects of the instrument have to be corrected with unprecedented precision, so a high-performance adaptive optics system is needed. It will be much more rapid (more than 1500 corrections per second) and much more precise (with a mirror having 1600 correction points).

  Considering the last developments in adaptive optics - which made NAOS possible, a system for direct detection of extrasolar planets might exist on 8-10 meters diameter telescopes within 5 years.

  Onera collaborates with the Observatories of Grenoble, Marseille, Paris, Nice, Geneva and with the British Astronomical Technical Center on a feasibility study for the European Southern Observatory. This study might open the way to the building of a telescope able to detect photons by the end of the decade.

• Increasing the Field of View with Multi-Conjugate Adaptive Optics (MCAO)

  Today, adaptive optics is limited mainly by the very small field of view (a few arc seconds only) for which correction is possible.

  Indeed, when a large field is observed, opposite rays at the limits of the beam come to the mirror through different ways and thus pass through slightly different volumes of turbulent atmosphere. This means that each ray is not disturbed by the same atmospheric turbulence. A single deformable mirror can not correct simultaneously for defects generated in the corresponding areas of the picture. Today, adaptive optics permits significant correction, but only in the close surrounding of one observation direction.

  Future "advanced" adaptive optics systems will deal with several deformable mirrors and wavefront sensors to increase tenfold the field of view at affordable cost.

• Increasing Resolution with Interferometry

  The theoretical resolution of a telescope is the size of the smallest detail in the picture with no atmospheric turbulence. The larger the telescope, the greater the resolution. But it is today technically impossible to built a mirror with a diameter of more than 10 meters. On a mirror that would be the size of France, the residual impairment of the VLT's 8-meter mirror would correspond to 1-millimeter bumps!

  The solution: linking several telescopes and having them work in interferometric mode. The beams coming from the telescopes are re-combined, as if they were provided by a single fictitious instrument. The resolution depends no more on the size of the mirror, but on the distance between the telescopes. The latest large telescopes, for example the VLTI (Very Large Telescope Interferometer), work in interferometric recombination mode after each single telescope has been corrected through adaptive optics.

Other Applications

• Focusing Laser Beams

  • Focusing very precisely a powerful laser beam on a target is obviously useful in a military perspective.
  • New welding or cutting processes using laser beams should be developped, in metallurgy as well as in several other sectors.
  • Thermonuclear fusion thanks to a very high power laser beam (so-called "megajoule laser") makes it necessary to focuse rays as precisely as possible. This method would allow to study fundamental physics without carring out nuclear tests.

• Imaging in Turbulent or Blurring Environments

  In ophtalmology, you have to observe very small details on the retina through the crystalline lens and the vitreous body. The use of adaptive optics should improve the resolution with which the back of the eye is seen, allowing to detect pathologies much earlier than it is now possible.

Last Update: July 13, 2004 - © ONERA 2009 - Terms of use