Aiming a laser pointer into space

© ESO / Y. Beletsky, TNO/ESO / Fred Kamphues

Reliable guide-star lasers are revolutionizing astronomy. The “Guide Star Alliance” development team led by Wilhelm Kaenders, Wallace Clement and Domenico Bonaccini Calia made it onto the podium at the Berthold Leibinger Innovation Prize.

Stars don’t actually twinkle — but the atmosphere makes it seem like they do. All the refractive anomalies, temperature gradients and turbulence in the air distort our view, preventing astronomers from getting a clear look at outer space through their telescopes. Even when the sky seems perfectly clear, this interference can make the light from a star appear much brighter, obscuring any smaller astronomical bodies in its vicinity. Dim objects that the telescope normally has no difficulty in resolving simply disappear, and the surface features of planets merge into a blocky gray mush.

There are two ways to counter the effects of these disturbances: either by heading into space, like the Hubble telescope; or by locating a guide star with a known brightness. This latter option allows a computer to eliminate the distortion − the twinkling − of the guide star and derive the in- formation it needs to control a variable mirror, physically deforming it in real time to compensate for the atmospheric distortion. That removes the twinkle from the other stars in the telescope’s field of view, too.

Create your own guide star

But what if you want to observe a portion of the sky where there are no suitable guide stars? Then you can just create your own using a laser beam! The idea, which originated in the 1990s, is to align a laser parallel to the telescope’s axis. At a height of around 90 kilometers, the laser light stimulates sodium atoms to absorb and re-emit light, creating a clear, bright “star.” For this to work, the laser must have a wavelength of exactly 589.2 nanometers, and that can only be achieved using dye lasers. With the required output in the low two-digit watt range, dye lasers require levels of technology and maintenance that can only be described as intensive.

But now Wilhelm Kaenders, Wallace Clements, Domenico Bonaccini Calia and their colleagues in the Laser Guide Star Alliance have come up with a new laser guide star system for large telescopes. The laser delivers exactly the right power output yet requires no more effort than using a laser pointer — at least in terms of the overall scale of running a large astronomical observatory. The first system to be installed has been guiding the European Very Large Telescope array on Cerro Paranal in Chile since April 2016.

It marks the start of a new era of astronomy, providing the technology to allow terrestrial telescopes with large mirrors to finally reach their full resolution potential. Mankind will soon be able to gaze even deeper into space.  

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