St. Lawrence River, Canada, 1974.Photograph by Sam Abell, National Geographic Creative
ESO's Paranal Observatory projecting a laser into the night sky to create a sodium beacon guide star. Laser guide stars are an artificial star image created for use in astronomical adaptive optics imaging. The, by now classical approach, is to use a narrow-line laser emitting at a sodium resonance line wavelength to create a yellow artificial “star” in the ~ 95 km altitude sodium cloud around the Earth. When working with an Adaptive Optics system, this beacon provides a bright reference source to correct atmospheric turbulence in real time in fields devoid of bright enough natural stars; note however that a moderately bright natural star is still needed to correct global image motion in the field (see a short tutorial here)
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).
Credit: ESO/Gianluca Lombardi (glphoto.it)
Intermediate radio frequency amplifier