Laser Guide Stars

Laser guide stars are artificial reference beacons created by shining a laser into the upper atmosphere, providing the bright point source adaptive optics needs where no suitable natural star is available.

Definition

A laser guide star is an artificial point of light produced high in the atmosphere by a laser, used as a wavefront reference for adaptive optics in directions where natural guide stars are too faint or absent.

Scope

This topic covers sodium-layer guide stars that excite atoms about ninety kilometres up and lower-altitude Rayleigh beacons, the cone effect or focal anisoplanatism that limits how well a single beacon samples the turbulence, the need for a natural star to sense overall tilt, and the operational issues of laser safety and aircraft and satellite avoidance.

Core questions

• Why are natural guide stars often insufficient?
• How is an artificial star created in the atmosphere?
• What is the cone effect and why does it limit performance?
• Why is a natural star still needed alongside a laser beacon?

Key theories

Sodium and Rayleigh beacons

A laser tuned to a sodium transition makes atoms in a layer about ninety kilometres up glow, while a Rayleigh beacon uses light scattered by lower air molecules, each producing an artificial reference star.

Focal anisoplanatism, the cone effect

Because the beacon is at finite altitude, the cone of light it samples misses some turbulence that a distant star's parallel beam would traverse, limiting correction and motivating multiple beacons.

Tilt indeterminacy

Light returning from a laser beacon retraces its own outgoing path, so it cannot measure overall image tilt, which must still be obtained from a faint natural star.

Clinical relevance

Laser guide stars greatly enlarge the fraction of the sky over which adaptive optics can work, since suitable natural guide stars are rare, and they are essential to wide-field and extremely large telescope adaptive-optics systems that map the turbulence in three dimensions.

History

The laser guide star concept was proposed in the 1980s, with early demonstrations partly arising from declassified defence research. Sodium beacons became operational on major telescopes around the turn of the century, and multiple-beacon systems now support wide-field adaptive optics.

Key figures

• Laird Thompson
• Renaud Foy
• Antoine Labeyrie

Seminal works

• hardy1998
• roddier1999

Frequently asked questions

Why create an artificial star instead of just using a real one?

Adaptive optics needs a bright reference very close to the target on the sky, but suitable natural stars exist in only a small fraction of directions. A laser can place an artificial beacon almost anywhere, vastly increasing the sky coverage over which sharp adaptive-optics imaging is possible.

Why does a laser guide star system still need a natural star?

Light from a laser beacon travels up and back along the same path, so any overall tilt it would measure is cancelled out. A faint natural star is therefore still required to sense the overall image motion, while the laser beacon handles the finer, higher-order distortions.