For many astronomers, Adaptive Optics is something like a dream coming true. Since 1609 and the first observations of celestial bodies performed with the help of an optical telescope, astronomers have always fought to improve the resolving power of their instruments. For a long time, engineers have trimmed the optical quality of the telescopes, until they finally reached the barrier set by the atmospheric turbulence.
At that point, the intrinsic quality of the site became a major issue to establish new observatories with modern telescopes, and astronomers started to desert the urban skies and to migrate toward mountains and deserts (Alloin and Mariotti 2004 9). The recent emergence Adaptive optics aims at providing diffraction limited long exposure images at large telescopes, which is greatly considered as one of astronomy’s breakthrough (Alloin and Mariotti 2004 9; Espinosa 1997 12). By far, the largest limitation to the application of adaptive optics to astronomy is very limited sky coverage when using natural guide stars for wavefront sensing.
Similar limitations existed for many military applications of adaptive optics (Espinosa 1997 12). Adaptive optics systems provide a real time correction of the distorted wavefronts; they restore all the properties of light prior to the final part of its travel through the atmosphere (Alloin and Mariotti 2004 9). Discussion Adaptive Optics: Wavefront and Laser Guide Star (LGS) Adaptive optic systems for atmospheric turbulence compensation require a reference star for correcting wavefront distortion.
The sodium LGS relies on resonant backscattering of a laser tuned into the D2 sodium absorption line to provide a reference, but LGS must sufficiently bright to correct high order wavefront aberration (Zamorano, Gorgas and Gallego 2001 317). The key concept in adaptive optics is the wave front, which is found by tracing out an equal optical path (distance x refractive index) from a source to the region of interest. For a point source and free space, wavefronts are spherical, and for starlight, the distance is so large that for all practical purposes the wavefronts entering the Earth’s atmosphere are plane.
After propagating through the random refractive index of the atmosphere, the wavefront entering the telescope pupil is random, and its statistics determine the image quality, and govern how an adaptive optical system might be used to compensate for the distortion (Agerorges 2000 4). Such effect greatly depends on the laser lunch power, polarization’s beam, atmospheric transmission and the sodium column density, which, if obtained appropriately, might even enhance the imagery, increase the scope of telescopic observatory, and improve the image details of pictures obtained from astronomical studies (Zamorano, Gorgas and Gallego 2001 317).
The use of LGS AO has caught on quickly within the astronomical community in large part because, equipped with adaptive optics operating at the diffraction limit in the near-infrared, the new 6- to 10-m telescopes possess the capability to match the angular resolution in the Hubble Space Telescope (HST) in the visible and to exceed its resolution in the near infrared (Rodier 256). Laser Guide Star LGS, basically, is a technology that utilizes AO imaging in order to enhance graphical imagery of telescopes and view astronomical images with additional quality.
The technology uses an artificial star in order to act as a wavefront reference source, which consequently corrects light distortion (Zamorano, Gorgas and Gallego 2001 317). The need for a bright stellar was always a concern for astronomical applications of adaptive optics. To operate, a wavefront sensor must have sufficient light to overcome photon noise and background noise with enough light left over to form the image. In astronomy, few stars of scientific interest are sufficiently bright. For imaging uncooperative satellites, reflected light is often too dim or nonexistent.
In 1985, French astronomers Foy and Labeyrie published work detailing how one might use backscatter from a laser focused to a point in the atmosphere as an artificial beacon (a guide star) for astronomical adaptive optics. As work progressed in the astronomy community to build and test a laser powerful enough to have sufficient backscatter for the Foy-Labeyrie method (Tyson 2000 5). Since the beginning of the 1980s, classified U. S military work was addressing the problems of projecting high-energy laser beams from the ground to space for missile defense and secure communications.
The research from 1982 at the U. S Air Force Starfire Optical Range (USAFSOR) directed by Robert Fugate advanced the laser guide star concept and produced a wealth information about laser performance requirements, adaptive optics system operation, atmospheric physics, and closed loop image of space borne objects. By 1991, the bulk of military work on laser guide starts was declassified and made available to astronomers around the world (Tyson 2000 5). Currently, LGS is being developed by various nations, setting up their own laser beacon and extensively enhancing research through the said technology.
The use of a laser beacon as the reference source enables faint objects to be compensated by adaptive optics, at the expense of greater hardware complexity. The laser beacon must be directed within the isoplanatic angle of the science object at the observation wavelength; although, this procedure provide only short-exposure correction. On the other hand, a fixed natural guide star possesses the ability to further stabilized the image during long exposures so that sky coverage depends on the distribution of stars, which consequently enhances the space imagery (Hardy 1998 309).
There are many cases where the object itself, such as a sunlit satellite, is insufficient for wavefront sensing. In astronomy, there are billions of stellar objects too dim for sensing and not near enough in the sky to bright objects. For high-energy laser propagation to uncooperative targets or satellite tracking and imaging, an artificial source must be placed above the atmosphere to provide photons for the wavefront sensor and subsequent compensation.
Lasers actually provide only partial correction, because a natural star still is required and opposite tilt upon traveling up into the atmosphere and returning (Tyson 2000 6-7). Particularly exciting development is the use of adaptive telescope mirrors to compensate for the distortion of stellar images produced by atmospheric turbulence. Using adaptive optics, ground-based telescopes are now demonstrating diffraction-limited performance, albeit over relatively small fields of view.
It can be expected that large ground-based telescopes will have higher resolution and light-gathering power than space-based telescopes, since both of these performance metrics depend on aperture size. Moreover, ground-based telescopes can be larger than those space-based ones; hence, enabling higher development and scope for stellar imagery. The adaptive optical system of LGS displays 0. 07-arcsecond resolution, which is almost a hundred times better than past ground-based telescope systems but uses a telescope built approximately 80 years ago (national Research Council 1998 137-138).
Conclusion LGS AO is one of the breakthroughs of current astronomical imagery, which provides enhanced images and display features that are usually not present in the space-based telescopes, such as HCT. The mechanism involved utilizes a guide star or an artificial star that facilitates the graphical imagery. LGS works as distortion corrector enabling enhanced imaging through AO ground-based telescopes. Conclusion Ageorges, N. Laser Guide Star Adaptive Optics for Astronomy. Springer, 2000. Alloin, Danielle M. , and Jean-Marie Mariotti. Adaptive Optics for Astronomy. Springer, 2004.