Laser guide stars look undeniably badass, deathrays slicing through the celestial sphere. More importantly, astronomers use these artificial stars to compensate for atmospheric blurring in real-time, allowing ground-based telescopes to take crystal-clear images of the sky.
Laser guide star from within the Very Large Telescope. Image credit: ESO/Yuri Beletsky
The atmosphere is not a steady, static thing. It moves constantly, jiggling, deflecting, refracting, and reflecting any light passing through it. This turbulent mixing is part of why stars twinkle, which, while very pretty, is downright frustrating for astronomers when their pretty stars are reduced to a blurry mess.
Part of the problem can be reduced by limiting how much atmosphere light has to travel through before reaching the telescope. Observatories on the top of high mountains where the air is thin and astronomers are panting to take a full breath are popular. The endangered Lick Observatory kicked off the trend of mountaintop observatories, looming above San Jose at 1,283 m. It was followed by Lowell Observatory, Mauna Kea Observatory, the Very Large Telescope, and oh-so-many others.
Multiple laser guide stars tracking with the Earth's rotation in a timelapse. Image credit: W.M. Keck Observatory
But even telescopes so high that workers need to occasionally take a break and huff some oxygen to get their breath back are still subject to the whims of atmospheric wiggle.Then one day, that frustration led to innovation. If only we knew exactly how the sky was causing starts to scintillate, we could compensate, working backwards from the speckle to a beautiful, clear pinpoint of a star.
That's the concept behind adaptive optics: measure the shape of the wavefront, then deform a mirror to compensate for distortion. All it requires is a known, bright source of light, a distortable mirror, and a sufficiently capable computing system.
Lasers are a very bright light source. Image credit: ESO/T. Kasper (AVSO)
Gemini South GeMS laser guide star during system verification observations. Image credit: Gemini Observatory/AURA
The application of adaptive optics showed up in fiction before reality: Poul Anderson adapting the concept to the hard science-fiction classic Tau Zero in 1970, while scientists didn't develop real adaptive optics until spurred by the Cold War and improvements in computing power. Now it's everywhere, helping optical telescopes like the Keck Observatory and the Very Large Telescope compete with the Hubble Space Telescope for clarity of image.
Laser guide star on Yepun, part of the Very Large Telescope. Image credit: ESO/Dave Jones
While adaptive optics can use natural guide stars, that only works if a sufficiently bright star is located right near whatever the target-of-interest is. Instead of being limited by convenience, astronomers developed laser guide stars, artificial stars of perfect, known characteristics to drive their adaptive optics systems.
A 360-degree panorama of the Keck Telescope at Mauna Kea in Hawaii. Image credit: W.M. Keck Observatory
Laser guide stars come in two varieties: sodium guide stars, and Rayleigh beacon guide stars.
This is where I admit the title was a bit of a lie: neither form of laser is hot enough to "sear" the sky, nor singe, spark, or otherwise cause a noticeable ruckus.
A 5-point constellation of laser guide stars at the Gemini Observatory. Image credit: Gemini Observatory/AURA
A sodium guide star uses a yellow laser (wavelength of 589.2 nanometers) to excite a layer of sodium atoms in the mesosphere (80 to 90 kilometers altitude). The excited sodium atoms re-emit the light, much like yellow sodium street lamps.
Rayleigh beacon guide stars use an ultraviolet laser that propagates to the stratosphere (15 to 25 kilometers altitude), using the backscatter to trigger the adaptive optics.
Rayleigh beacon guide star at the Steward Observatory. Image credit: Steward Observatory
Rayleigh beacon guide stars are cheaper and simpler, but because they produce backscatter in the lower atmosphere, really only provide correction for those last few kilometers. Sodium guide stars are more expensive and complex, but because they provide a higher-altitude guide star, produce a better reference wavefront for how starlight scintillates as it makes the journey through our atmosphere.
Timelapse of the laser guide star on Yepun, the fourth Unit Telescope of the Very Large Telescope. Image credit: ESO/Dave Jones
To avoid getting tangled up in ground scatter, laser guide stars usually pulse on and off, with measurements time-gated so only data collected in the microseconds after a pulse are used in observations.
Laser guide star at the Very Large Telescope at the Paranal Observatory in the Atacama Desert of Chile. Image credit: ESO/H.H.Heyer