Laser guide star

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ESO tested the new Wendelstein laser guide star unit by shooting a powerful laser beam into the atmosphere.[2]

Laser guide stars are an artificial star image created for use in astronomical adaptive optics imaging.

Adaptive optics (AO) systems require a wavefront reference source in order to correct atmospheric distortion of light (called astronomical seeing). Sufficiently bright stars are not available in all parts of the sky, which greatly limits the usefulness of natural guide star adaptive optics. Instead, one can create an artificial guide star by shining a laser into the atmosphere. This star can be positioned anywhere the telescope desires to point, opening up much greater amounts of the sky to adaptive optics. Because the laser beam is deflected by astronomical seeing on the way up, the returning laser light does not move around in the sky as astronomical sources do. In order to keep astronomical images steady, a natural star nearby in the sky must be monitored in order that the motion of the laser guide star can be subtracted using a tip–tilt mirror. However, this star can be much fainter than is required for natural guide star adaptive optics, because it is only used to measure tip and tilt and all higher order distortions are measured with the laser guide star. This means that many more stars are suitable and a correspondingly larger fraction of the sky is accessible.

Types

There are two main types of laser guide star system, known as sodium and Rayleigh beacon guide stars.

Sodium beacons are created by using a laser specially tuned to 589.2 nanometers to energize a layer of sodium atoms which are naturally present in the mesosphere at an altitude of around 90 kilometers. The sodium atoms then re-emit the laser light, producing a glowing artificial star. The same atomic transition of sodium is used to create bright yellow street lights in many cities. Rayleigh beacons rely on the scattering of light by the molecules which make up the lower atmosphere.

In contrast to sodium beacons, Rayleigh beacons are a much simpler and less costly technology, but do not provide as good a wavefront reference as the artificial beacon is generated much lower in the atmosphere. The lasers are often pulsed, with measurement of the atmosphere being time-gated (taking place a few microseconds after the pulse has been launched so that scattered light at ground level is ignored and only light which has traveled for several microseconds high up into the atmosphere and back is actually detected).

One of the launch telescopes for the VLT Four Laser Guide Star Facility.[1]

Progress

Laser guide star adaptive optics is still a very young field, with much effort currently invested in technology development. As of 2006, only two laser guide star AO systems were regularly used for science observations and have contributed to published results in peer-reviewed scientific literature: those at the Lick and Palomar Observatories in California, and the Keck Observatory in Hawaii. However, laser guide star systems were under development at most major telescopes, with the William Herschel Telescope, Very Large Telescope and Gemini North having tested lasers on the sky but not yet achieved regular operations. Other observatories developing laser AO systems as of 2006 include the Large Binocular Telescope and Gran Telescopio Canarias. The laser guide star system at the Very Large Telescope started regular science operations in June 2007.[3]

Dye Lasers played, and continue to play, a significant role in the development of laser guide stars, but are cumbersome to operate and use toxic chemicals.[4][5][6][7][8][9] New laser systems based on tunable diode lasers with subsequent narrow-band Raman fiber amplification and frequency-conversion are therefore being developed as alternatives.[10][11] Important output features in the tunable lasers used in this application are: diffraction limited beam divergence and narrow-linewidth emission.[12]

Gallery

  • The Earth’s rotation during the 30-minute exposure — and the movement of the laser as it compensated for this — is why the beam appears to fan out.

  • First Light of the ESO's VLT Laser Guide Star.

  • Keck Observatory projecting a laser into the night sky to create a sodium beacon guide star.

References

  1. ↑ "VLT's New Laser Launchers Arrive at ESO". ESO Announcement. Retrieved 22 February 2012. 
  2. ↑ "Laser Meets Lightning". ESO Picture of the Week. European Southern Observatory. 
  3. ↑ Markus Kasper; Stefan Stroebele, Richard Davies, Domenico Bonaccini Calia (13 June 2007). "Free from the Atmosphere – Laser Guide Star System on ESO's VLT Starts Regular Science Operations". ESO for the public. ESO. Retrieved 2 June 2011. 
  4. ↑ Everett, Patrick N. (1989). "300-Watt dye laser for field experimental site". Proceedings of the International Conference on Lasers '88: 404–9. Bibcode:1989lase.conf..404E. OCLC 20243203. OSTI 5416850. 
  5. ↑ Primmerman, Charles A.; Murphy, Daniel V.; Page, Daniel A.; Zollars, Byron G.; Barclay, Herbert T. (1991). "Compensation of atmospheric optical distortion using a synthetic beacon". Nature 353 (6340): 141–3. Bibcode:1991Natur.353..141P. doi:10.1038/353141a0. 
  6. ↑ Bass, Isaac L.; Bonanno, Regina E.; Hackel, Richard P.; Hammond, Peter R. (1992). "High-average-power dye laser at Lawrence Livermore National Laboratory". Applied Optics 31 (33): 6993–7006. Bibcode:1992ApOpt..31.6993B. doi:10.1364/AO.31.006993. PMID 20802559. 
  7. ↑ Duarte, Francisco J. (2001). "Multiple-Return-Pass Beam Divergence and the Linewidth Equation". Applied Optics 40 (18): 3038–41. Bibcode:2001ApOpt..40.3038D. doi:10.1364/AO.40.003038. PMID 18357323. 
  8. ↑ Pique, Jean-Paul; Farinotti, Sébastien (2003). "Efficient modeless laser for a mesospheric sodium laser guide star". Journal of the Optical Society of America B 20 (10): 2093–101. Bibcode:2003OSAJB..20.2093P. doi:10.1364/JOSAB.20.002093. 
  9. ↑ Wizinowich, Peter L.; Le Mignant, David; Bouchez, Antonin H.; Campbell, Randy D.; Chin, Jason C. Y.; Contos, Adam R.; Van Dam, Marcos A.; Hartman, Scott K. et al. (2006). "The W. M. Keck Observatory Laser Guide Star Adaptive Optics System: Overview". Publications of the Astronomical Society of the Pacific 118 (840): 297–309. Bibcode:2006PASP..118..297W. doi:10.1086/499290. 
  10. ↑ W. Clements, High-power guide star lasers are ready for next-generation AO astronomy, Laser Focus World 46 (6), 27-33 (2010).
  11. ↑ "Laser Guide Star / Astronomy". Toptica website. Toptica. Retrieved 2 June 2011. 
  12. ↑ Duarte, Francisco J. (2003). "Chapter 3". Tunable Laser Optics ([Online-Ausg.] ed.). Amsterdam: Elsevier Academic Press. p. 272. ISBN 0-12-222696-8. 

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