Laser guide star

An artificial guide star or laser guide star (english Laser Guide Star LGS) is a generated by a laser beam spot of light in the upper atmosphere. It allows as " artificial star " the correction of the atmospheric turbulence using adaptive optics, even at points of heaven, where no corresponding bright natural guide star (Natural Guide Star NGS) is available.

Guide stars for adaptive optics

Adaptive optics systems for large astronomical telescopes need the light spot of natural or artificial guide star to measure the turbulence of the overlying layers of the atmosphere to the telescope. The light of this guiding star is captured by the telescope and the distortion of its wavefront with a wavefront sensor, such as a Shack- Hartmann sensor, measured. So that the information is obtained in order to correct the wavefront of the actual target object by an electronic control system and a deformable mirror. This results in sharper images of objects outside of the atmosphere ( astronomical objects, but also earth satellites ). With good correction, the image quality is no longer determined by seeing but by the diffraction of light at the telescope mirror.

Since the distortion is very quickly changeable by air turbulence, the guiding star must be bright, to provide enough photons for many wavefront measurements in a second. He must be close are also in the isoplanatic patch on the target object, otherwise the light from the guide star and the target object on the way to the telescope would go through different turbulent regions of the atmosphere so that the temperature measured at guiding distortion would not be applicable to the target object.

It turns out that is available for most regions of the sky, especially far away from the star- rich plane of the Milky Way, not enough bright natural guide star. For rare object classes that would be a severe restriction of adaptive optics.

Laser guide stars

Remedy laser guide stars, which with a small telescope optics at the main telescope, a bright laser beam is sent in the direction of the target object.

In Rayleigh Laser Guide Star, a laser in the visible or near ultraviolet is used, the light is back-scattered by Rayleigh scattering by molecules and aerosols in the lower 10-20 km of the atmosphere. This initially results in a pillar of light, by using pulsed laser and utilization of the light travel time but this can be limited to a spot of light in a layer of the atmosphere.

With sodium Laser Guide Star a pulsed or continuous laser beam is used at the wavelength of the sodium D - line ( 589.2 nm) instead. The laser beam is scattered back into the atmosphere about 90 km altitude of the sodium atoms of the sodium layer.

Also, since the laser beam used for the LGS is deflected by the underlying layers of the atmosphere, a normal rating is generally observed in addition to the LGS. With the help of this star but only the shift of the observed image must be corrected as a whole rather than the entire wavefront, it is sufficient for a relatively faint star.

History

The first successful project of a laser guide star was carried out for military purposes in the 1980s at the Starfire Optical Range in New Mexico and at first was subject to confidentiality. In the 1990s, the first civilian experiments began in astronomical telescopes. Since 2000, laser guide stars are also installed at the 8m class telescopes such as the Keck Observatory and the Very Large Telescope.