page_title: Adaptive optics
section_title: Other uses
hierarchical_section_title: Adaptive optics / Other uses
caption_attribution_description: English: This image shows some of the GRAAL instrument team, inspecting GRAAL’s mechanical assembly in the integration hall of ESO’s Headquarters in Garching bei Munchen, Germany. GRAAL, which will be installed on ESO’s Very Large Telescope (VLT) on Cerro Paranal in Chile, is designed to improve the vision of the VLT’s already excellent HAWK-I camera even further. GRAAL stands for GRound layer Adaptive optics Assisted by Lasers. It will use the technique of adaptive optics to improve the quality of images by compensating for turbulence in the lower layers of the atmosphere, up to an altitude of 1 kilometre. GRAAL will form part of the observatory’s next generation Adaptive Optics Facility (AOF). The VLT already uses a powerful laser beam to create an artificial guide star, 90 kilometres up in the atmosphere. The current adaptive optics systems use this guide star as a reference to remove the effect of turbulence in the atmosphere, giving sharper observations, almost as though the telescope were in space. The next generation AOF, however, will have no fewer than four laser guide stars, launched from Yepun, the VLT’s fourth Unit Telescope. GRAAL captures their light with four sensors, and then adjusts the shape of a deformable mirror up to 1000 times per second to compensate for the blurring effect of the atmosphere. This mirror — part of the AOF upgrade — is in fact a complete replacement for the 1.1-metre secondary mirror of the telescope, and will be the largest deformable mirror yet made. Combined with the multiple guide stars of the laser launching facility, it allows for better corrections over a wider field of view. GRAAL will be attached to the High Acuity Wide field K-band Imager (HAWK-I), already installed on Yepun. Currently, HAWK-I operates without adaptive optics. Installing GRAAL will improve the sharpness of HAWK-I’s images, and reduce the exposure times needed by up to a factor of two. After recent successful testing of the main parts of its mechanical assembly, GRAAL’s optics are now being assembled at ESO’s Headquarters. The instrument is expected to reach Paranal at the end of 2013.
context_page_description: Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems to reduce optical aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a device that corrects those errors such as a deformable mirror or a liquid crystal array.
  Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the primary mirror geometry.
  Other methods can achieve resolving power exceeding the limit imposed by atmospheric distortion, such as speckle imaging, aperture synthesis, and lucky imaging, or by moving outside the atmosphere with space telescopes, such as the Hubble Space Telescope.
context_section_description: Besides its use for improving nighttime astronomical imaging and retinal imaging, adaptive optics technology has also been used in other settings. Adaptive optics is used for solar astronomy at observatories such as the Swedish 1-m Solar Telescope and Big Bear Solar Observatory. It is also expected to play a military role by allowing ground-based and airborne laser weapons to reach and destroy targets at a distance including satellites in orbit. The Missile Defense Agency Airborne Laser program is the principal example of this.
  Adaptive optics has been used to enhance the performance of free-space optical communication systems 
    
  and to control the spatial output of optical fibers.
  Medical applications include imaging of the retina, where it has been combined with optical coherence tomography. Also the development of Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) has enabled correcting for the aberrations of the wavefront that is reflected from the human retina and to take diffraction limited images of the human rods and cones. Development of an Adaptive Scanning Optical Microscope (ASOM) was announced by Thorlabs in April 2007. Adaptive and active optics are also being developed for use in glasses to achieve better than 20/20 vision, initially for military applications.
  After propagation of a wavefront, parts of it may overlap leading to interference and preventing adaptive optics from correcting it. Propagation of a curved wavefront always leads to amplitude variation. This needs to be considered if a good beam profile is to be achieved in laser applications. In material processing using lasers, adjustments can be made on the fly to allow for variation of focus-depth during piercing for changes in focal length across the working surface. Beam width can also be adjusted to switch between piercing and cutting mode. This eliminates the need for optic of the laser head to be switched, cutting down on overall processing time for more dynamic modifications.
  Adaptive optics, especially wavefront-coding spatial light modulators, are frequently used in optical trapping applications to multiplex and dynamically reconfigure laser foci that are used to micro-manipulate biological specimens.
caption: GRAAL is a ground layer adaptive optics instrument assisted by lasers.[15]
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