If you were to leave the shutter open on your digital camera in a dark room and take a long exposure image, you would see something like the end of the movie playing above. And if you were an astronomer trying to take a picture of a celsestial object, those little streaks and spots that randomly appear would make you curse the very heavenly bodies you sought to photograph.

The streaky, spotty blemishes that show up randomly throughout the movie are referred to as cosmic rays. A cosmic ray can be an electron, a muon, a gamma ray, and perhaps even other exotic particles. These particles, like the photons of light we want to record in our images carry energy. And that energy can liberate electron hole pairs in a detector, generating the same signal as that caused by a visible photon.

Fortunately for the typical digital camera user, the light levels and exposure time of typical pictures are such that cosmic rays do not pose a problem. But for astronomers taking pictures of very dim objects in the night sky, they do. Special techniques are used, especially for space-based telescopes where the atmosphere does not filter out cosmic rays, to remove them from astronomical images. Luckily, because of their random nature it is unlikely that a cosmic ray will hit the same location in a detector in successive images, allowing one to filter them out in software by combining multiple images. Unlike lightning, however, cosmic rays can strike twice; you just have to expose long enough.

• Don Groom's Cosmic Rays


• Nasa's Cosmic Ray Site

• Robert Smith's Presentation


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