Laser Guide Stars | previous | next | feedback |
Definition: small bright spots in the sky, generated with laser beams for use in astronomy with adaptive optics imaging
The quality and size of modern astronomical telescopes has been enormously increased; telescopes with mirror diameters of several meters and very high surface quality are used in many observatories. The image resolution of the best and largest of those telescopes is already no more limited by the optics themselves, but by atmospheric distortions: the light from astronomical objects can travel over huge distances in space without significant distortions, but temperature and pressure variations associated with turbulences in the earth's atmosphere can lead to significant distortions, even at favorable locations on mountains with a rather clear sky.
A straightforward solution to this problem is to use space-based telescopes. However, these cannot be as large as terrestrial ones, and are very expensive to build, launch, operate, and maintain. Therefore, the alternative solution of atmospheric correction is more and more adopted, which makes it possible to strongly reduce the effect of atmospheric distortions for earth-based telescopes: the wavefront distortions caused by the atmosphere are compensated for with adaptive optics, based e.g. on deformable mirrors with many degrees of freedom. Such a system obviously requires exact information on the current atmospheric distortions. These can be measured by analyzing the wavefronts from a distance point-like object such as a star (called guide star), since without distortions this light would have essentially plane wavefronts.
For precise wavefront correction, the guide star has to be close (in terms of direction) to the object under investigation, and has to be sufficiently bright. Unfortunately, however, one does not always find a suitable natural guide star. In this situation, an artificial guide star (or laser beacon), temporarily created by shining an intense laser beam into the atmosphere, can replace a natural star. Some laser light is then coming back to the telescope and can be analyzed e.g. with a Hartmann-Shack wavefront sensor. An improved scheme may even use multiple laser guide stars.
The position of the artificial guide star may somewhat drift, but this can be corrected e.g. by comparing it with that of a natural star, which does not have to be particularly bright.
Types of Laser Guide Stars
Figure 1: The William Herschel Telescope at the Roque de Los Muchachos Observatory, La Palma, with a green laser beam as used for a Rayleigh laser guide star. Credit: Tibor Agocs.
The two dominant types of laser guide stars are the sodium beacon and the Rayleigh beacon. The principle of the sodium guide star is to tune the wavelength of the laser radiation to a resonance of sodium atoms at 589.2 nm. This causes sodium atoms, naturally occurring in the mesosphere at an altitude of around 90 km, to absorb laser light and subsequently to emit fluorescence at the same wavelength. This approach has the nice feature of obtaining fluorescence light essentially only from a narrow range of high altitudes. Its disadvantage is that the required powerful orange/yellow laser source with a small linewidth is not easy to construct and accordingly expensive. Available technological options for sodium beacons include
- a Raman laser based on a bulk crystal, pumped with a frequency-doubled Q-switched neodymium-based solid-state laser
- an 1178-nm Raman fiber laser (or Raman MOPA), pumped with an ytterbium-doped fiber laser, with subsequent frequency doubling e.g. in periodically poled KTP
- sources based on sum frequency mixing of two laser sources (cw or pulsed), e.g. at 1064 nm and 1319 nm, or at 1583 nm and 938 nm
- a pulsed dye laser
In contrast, a Rayleigh guide star is based on Rayleigh scattering in the lower atmosphere. As this is not based on a narrowband resonance, the chosen wavelength is not critical; only it should be short because Rayleigh scattering is most efficient at short wavelengths. A common choice is that of a green laser source, such as a frequency-doubled solid-state laser, but a copper vapor laser (→ gas lasers) or an excimer laser can also be used. Such laser sources can be less complex than those of sodium guide stars, and at the same time more powerful, but the lower altitude of the backscattered light compromises the quality of the wavefront correction.
In many cases, laser guide star sources emit nanosecond pulses, rather than continuously. The pulsed format somewhat simplifies the nonlinear frequency conversion in the laser source, and it makes possible time-gated detection.
Laser Guide Star Systems in Use or in Development
While a number of different laser sources for laser beacons have been demonstrated, only a few observatories appear to be using laser guide stars so far: the Lick Observatory of the University of California, the Palomar Observatory of Caltech, and the Keck Observatory in Hawaii, all using sodium beacons. However, several larger observatories are currently developing laser guide stars and adaptive optics systems of various types. Examples are the Very Large Telescope, Gemini North, the Multiple Mirror Observatory (MMTO) in Arizona, and the William Herschel Telescope of the Isaac Newton Group in La Palma, Canary Islands.
Bibliography
| [1] | Keck Observatory in Hawaii, http://www.keckobservatory.org/ |
| [2] | Lick Observatory of the University of California, http://mthamilton.ucolick.org/ |
| [3] | Palomar Observatory of Caltech, http://www.astro.caltech.edu/palomar/ |
| [4] | Isaac Newton Group of Telescopes on La Palma, http://www.ing.iac.es/ |
See also: laser applications, Raman lasers, solid-state lasers, fiber lasers, nonlinear frequency conversion
