The first hydrogen bomb, developed at Lawrence Livermore National Laboratory, was many times brighter than the sun. Now LLNL has introduced a new phenomenon: a virtual star.
Researchers at the laboratory, which is administered by the University of California, have collaborated with scientists at the Keck Observatory in Hawaii to produce a “virtual” guide star that will help produce more accurate images of the night sky. The guide star, a large laser, began operation at the Keck Observatory on Dec. 23.
The stars twinkle at night because of atmospheric turbulence. The varying density and consistency of the atmosphere causes the light from stars to distort. When the Keck telescope produces an image of a star it counters for these atmospheric effects by changing the shape of its mirrors to correct the wayward path of incoming rays of light. The system is called adaptive optics.
The trick for the telescope is knowing where to correct the image and by how much. Traditionally, this has been done by sampling light from the brightest stars, where the effects of atmospheric distortion are most obvious. A camera takes images of the light, which are fed into a computer. The computer is in control of 349 actuators that adjust the mirror at a rate of 670 times per second, producing a clear image.
The problem with this approach is that only 1 percent of the sky contains the kind of bright stars needed for adaptive optics. Astronomers are rarely fortunate enough to be focusing on one of these small patches of sky. The rest of the time they are left in the dark.
Researchers at the Keck Observatory and LLNL have attached a large laser to the telescope. The laser stimulates sodium atoms in the upper atmosphere, causing them to emit light. The result is a virtual star, bright enough for use in adaptive optics and which can be positioned anywhere in the sky.
The first trials with the new system have produced images four times clearer than those coming from the Hubble telescope, which does not suffer from atmospheric distortion.