Sometimes there just isn’t a bright star you can calibrate your sensitive telescopes by, in the right place at the right time. And unfortunately, even at the high altitude of the European Southern Observatory’s Paranal site in Chile, with its still air and impeccably clear skies, the atmosphere of Earth creates turbulence that distorts any observations made of objects in space. Without any correction, the Very Large Telescope array’s four 1.8-metre- (six-foot-) aperture instruments are totally unable to resolve images any better than an everyday 20-centimetre (eight-inch) telescope.
To counter this, a clearly visible star near the target is used to calibrate instruments – unfortunately only one in 100 are bright enough. So in the absence of a suitable star, the VLT makes its own. Yepun is one of the four units that make up the VLT and it’s equipped with the Laser Guide Star (LGS) – a powerful instrument that beams a yellow laser 90 kilometres (56 miles) into the atmosphere, where it hits a layer of sodium atoms thought to be left over from meteorites. By energising these atoms a reference point is created, an artificial star that can be used to prevent the VLT’s images from blurring.
Image Credit: ESO