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When looking up at the night sky, the stars seem relatively close, but in real terms, there are light years between them. Stars can be used as a reference point by large ground-based telescopes to help maintain position when tracking of heavenly bodies. But sometimes, there are no nearby stars to act as a reference, so lasers are used to generate small bright spots in the sky. These laser guide stars as they are called can be used by astronomers with adaptive optics imaging to obtain pictures superior to those taken by the Hubble Space Telescope.
Temperature and pressure variations caused by the Earth’s dynamic atmosphere cause havoc with the image resolution of ground-based telescopes. Before it can reach the telescope’s lens, light must weave its way through layers of air which are subject to turbulence; this causes the light to become blurred and images to lack focus. This atmospheric distortion of light – also known as astronomical seeing – can be corrected using adaptive optics.
Adaptive optics make use of deformable mirrors and supercomputers and requires precise information on current atmospheric distortions, measured by analyzing wavefronts from a distant point-like object, such as a guide star.
A guide star has to be close enough to the object of interest and be sufficiently bright but finding a suitable natural star to act as a point of reference is not always possible, so an artificial one is created using lasers. The use of laser guide stars means that much more of the sky is now available for exploration using adaptive optics.
A laser is shone into the atmosphere, and the light that returns to the telescope can be analyzed to see how the light’s wave is bent as it travels through the Earth’s atmosphere – this is where the supercomputer comes in. It monitors the changes in the atmosphere and sends instructions to the deformable mirror to bend in such a way that the light straightens when it is reflected, thus producing a crisp image.
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There are two variations of laser guide stars; a sodium beacon, or Rayleigh beacon. A sodium beacon tunes the wavelength of the laser radiation to the resonance of sodium – 589.2nm. Sodium atoms in the mesosphere – around 90km above the Earth – absorbs the laser light and emit fluorescence at the same wavelength, therefore creating a glowing artificial star. This is advantageous as it allows astronomers to obtain fluorescent light from a narrow range at high altitudes but generating the orange/yellow laser source with a small linewidth that is required is difficult and costly. Raman lasers based on bulk crystals pumped with a frequency-doubled solid-state laser, a Raman fiber laser, or a pulsed dye laser are among the technological options available.
The alternative is a Rayleigh beacon which employs Rayleigh scattering in the lower atmosphere. A green laser source is usually used – such as a frequency-doubled solid-state laser – although copper vapor lasers and excimer lasers are also employed. There are less complex and more powerful than a sodium beacon, but the quality of the wavefront correction can be compromised at lower altitudes because of the backscattered light.
The Keck Observatory in Hawaii has employed adaptive optics since 1999: they state the Keck II telescope can measure and correct atmospheric turbulence using a deformable mirror that changes shape 2,000 times per second, resulting in a tenfold increase in image clarity than was previously possible. In 2004, the Observatory deployed the first laser guide star based on a sodium beacon and boasts it can now produce sharp images of celestial objects anywhere in the sky with greater crispness and detail than those provided by Hubble, which sits above the Earth’s atmosphere and is therefore not subject to distortions.
Artificial optics systems are also present at the Lick and Palomar Observatories in California with many more under development at most of the major ground-based telescopes. Indeed, in April 2016, the 4 Laser Guide Star Facility (4LGSF) was installed on the Very Large Telescope – owned by the European Space Agency and based in Chile. It utilizes four laser beams to produce four artificial stars by exciting sodium atoms in the mesosphere, therefore enabling better correction of widening the field of view corrected by adaptive optics.
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