Lasers can help astronomers bring stars into focus. A laser, tuned to a sodium spectral line, shines backward through the telescope, and this laser light excites sodium atoms located in a thin layer of the atmosphere, producing in effect an artificial star. Astronomers can then see how the atmosphere distorts the laser star and then compensate to clarify their images. This relatively new technique means ground-based telescopes can be just as useful as those in Earth orbit. See First Supernova Discovered with Laser Guide Star Adaptive Optics and Rubber Mirrors and Artificial Stars for a fuller explanation.
