page_title: Astronomical seeing
section_title: Overcoming atmospheric seeing
hierarchical_section_title: Astronomical seeing / Overcoming atmospheric seeing
caption_attribution_description: English: ESO’s Paranal Observatory is one of the best places on Earth to take in the night sky. This panorama of ESO’s Very Large Telescope (VLT) atop the platform of Chile’s Cerro Paranal was taken by Babak Tafreshi, one of the ESO Photo Ambassadors. A mix of colours sweeps across the sky in this image. From the fiery red sunset up to the dusty purple band of the Milky Way. Although the VLT site is one of the best places on Earth for astronomy, the telescope must still overcome the natural distortions caused by our atmosphere. This is done through a process known as adaptive optics. Astronomers can make use of an artificial star by shining a powerful laser — seen here projected from the telescope on the far right — to correct for the blurring caused by the atmosphere. The VLT, as a flagship facility for European astronomy, has opened up a new golden age of discovery in astronomy by making use of the most state-of-the art technologies.
context_page_description: Astronomical seeing refers to the amount of apparent blurring and twinkling of astronomical objects like stars due to turbulent mixing in the atmosphere of Earth, causing variations of the optical refractive index. The seeing conditions on a given night at a given location describe how much Earth's atmosphere perturbs the images of stars as seen through a telescope.
  The most common seeing measurement is the full width at half maximum of the optical intensity across the seeing disc. The FWHM of the point spread function is the best possible angular resolution that can be achieved by an optical telescope in a long-exposure image, and corresponds to the FWHM of the fuzzy blob seen when observing a point-like source through the atmosphere. The size of the seeing disc is determined by the seeing conditions at the time of the observation. The best conditions give a seeing disk diameter of ~0.4 arcseconds and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma.
  Seeing is one of the biggest problems for Earth-based astronomy.
context_section_description: The first answer to this problem was speckle imaging, which allowed bright objects with simple morphology to be observed with diffraction-limited angular resolution. Later came NASA's Hubble Space Telescope, working outside the atmosphere and thus not having any seeing problems and allowing observations of faint targets for the first time (although with poorer resolution than speckle observations of bright sources from ground-based telescopes because of Hubble's smaller telescope diameter). The highest resolution visible and infrared images currently come from imaging optical interferometers such as the Navy Prototype Optical Interferometer or Cambridge Optical Aperture Synthesis Telescope, but those can only be used on very bright stars.
  Starting in the 1990s, many telescopes have developed adaptive optics systems that partially solve the seeing problem. The best systems so far built, such as SPHERE on the ESO VLT and GPI on the Gemini telescope, achieve a Strehl ratio of 90% at a wavelength of 2.2 micrometers, but only within a very small region of the sky at a time.
   A wider field of view can be obtained by using multiple deformable mirrors conjugated to several atmospheric heights and measuring the vertical structure of the turbulence, in a technique known as Multiconjugate Adaptive Optics.
  Another cheaper technique, lucky imaging, has had good results on smaller telescopes. This idea dates back to pre-war naked-eye observations of moments of good seeing, which were followed by observations of the planets on cine film after World War II. The technique relies on the fact that every so often the effects of the atmosphere will be negligible, and hence by recording large numbers of images in real-time, a 'lucky' excellent image can be picked out. This happens more often when the number of r0-size patches over the telescope pupil is not too large, and the technique consequently breaks down for very large telescopes. It can nonetheless outperform adaptive optics in some cases and is accessible to amateurs. It does require very much longer observation times than adaptive optics for imaging faint targets, and is limited in its maximum resolution.
caption: Astronomers can make use of an artificial star by shining a powerful laser to correct for the blurring caused by the atmosphere.[11]
exif: {"Image ImageDescription": "ESO\u2019s Paranal Observatory is one of the best places on Earth to take in the night sky. This panorama of ESO\u2019s Very Large Telescope (VLT) atop the platform of Chile\u2019s Cerro Paranal was taken by Babak Tafreshi, one of the ESO Photo Ambassadors. A mix of colours sweeps across the sky in this image. From the fiery red sunset up to the dusty purple band of the Milky Way. Although the VLT site is one of the best places on Earth for astronomy, the telescope must still overcome the natural distortions caused by our atmosphere. This is done through a process known as adaptive optics. Astronomers can make use of an artificial star by shining a powerful laser \u2014 seen here projected from the telescope on the far right \u2014 to correct for the blurring caused by the atmosphere. The VLT, as a flagship facility for European astronomy, has opened up a new golden age of discovery in astronomy by making use of the most state-of-the art technologies.", "Image Software": "Adobe Photoshop CS6 (Macintosh)", "Image Artist": "Babak Tafreshi", "Image Copyright": "Babak Tafreshi, btafreshi@twanight.org"}