A slice of moon cheese, anyone?

Appearing to cut through the Moon in this Picture of the Week is one of the laser guide stars on Unit Telescope 4 (UT4 or Yepun), one of the four 8.2-m telescopes of ESO’s Very Large Telescope (VLT). At any altitude, ground-based telescopes are affected by the atmospheric distortion of astronomical light. To counteract this, astronomers use adaptive optics in which a flexible mirror is deformed in real time to counteract this effect, yielding sharp images. 

The laser you see here, a powerful beam of light, excites sodium atoms 90 kilometres high in the atmosphere, consequently producing a glowing artificial star. Astronomers use this star to measure the blurring effect of the atmosphere and drive the deformable mirrors.

To avoid disturbing airplanes, these lasers rely on an automated aircraft avoidance system. This system continuously monitors the sky and instantly switches the lasers off whenever an aircraft’s trajectory is predicted to enter the region of the sky where the lasers are pointed. In any case, the lasers wouldn’t be able to slice the Moon even if it was made of cheese!