The world's most powerful telescope is under construction on Mount Graham, thirty miles northeast of Tucson. When completed in 2004, it will produce images ten times sharper and twenty-five times brighter than what can be achieved with the Hubble Space Telescope.

Until now I had assumed that we had reached the limit with ground-based telescopes. This was the major reason for putting the Hubble Telescope into orbit ten years ago. Locating the Hubble above the atmosphere has produced amazing images even though the mirror is only eight feet in diameter.

Recently I had the opportunity to visit the University of Arizona in Tucson where the construction of the telescope is being coordinated. Italy and Germany are partners in the construction and use of the telescope. We met with Dr. Peter Wehinger who is intimately involved with the its planning and construction. It is called the Large Binocular Telescope (LBT). He explained the reasons why it will be the most powerful in the world, surpassing the Hubble by a factor of ten. The limitations on ground-based telescopes have been the size of mirror that could be built and the distortions caused by turbulence in the atmosphere.

As the name indicates, the LBT will have two mirrors side by side on the same mounting. Light from the two mirrors will be combined to produce one image. Each mirror is 28 feet in diameter, but when combined they will have the resolving power equivalent to a 75-foot diameter mirror. Just for comparison, the mirror at the Mount Palomar Observatory is only about 16 feet in diameter.

The reason why the LBT can be built is that a new method of casting large mirrors was invented by Dr. Roger Angel, director of the Stewart Mirror Lab at the University of Arizona. He noticed that if he rotated a container with water, the surface became concave due to the centrifugal force of rotation. The surface was actually parabolic which is exactly the shape of a mirror used in a telescope. After much developmental work, the Lab built a furnace large enough to construct a mirror 28 feet in diameter, the largest mirror ever made. The furnace is built on a turntable that rotates at a speed that produces the parabolic shape required.

The mould for the mirror is constructed on the turntable. To decrease the weight of the mirror, a honeycomb structure is installed on the turntable. Chunks of glass are spread over the surface of the honeycomb. The furnace is then sealed and the heat turned on. Once the glass melts, the turntable begins to rotate at seven revolutions per minute. When all the glass has melted and filled in all the spaces, the heat is reduced gradually over a period of twelve weeks to prevent any stress in the mirror that might cause the glass to crack. The slow cooling also produces a much stronger molecular structure in the glass.

One of the two mirrors has already been cast and the back has been ground and polished. The size of the mirror is hard to visualize until you stand next to it. The mirror is 36 inches thick at the outside edge, and 28 feet in diameter. Soon, the mirror will be inverted and the parabolic surface will be produced by a specially constructed milling machine. The grinding and polishing will take approximately 12 to 18 months. This seems like a long time, but the Mount Palomar mirror took four years and it is half the size, or one-quarter of the surface area. The LBT mirror will take less time because the technicians begin with a parabolic surface which is very close to the desired shape. The Mount Palomar mirror was made from a flat disc which meant that a great deal of glass (about 25% of the original disc) had to be removed to form the parabolic surface. Dr. Wehinger estimates that it would take ten years to grind the 28-foot mirror if they started with a flat surface.

Atmospheric turbulence has always been a problem with earth-bound telescopes and, until recently, was thought to be the limiting factor in telescope construction and use. Now a new method of correcting for atmospheric turbulence has been developed. The LBT will be equipped with adaptive optics which correct for the distortion caused by the atmosphere.

As I was leaving the lab, I mentioned to Dr. Wehinger that I had been to Hawaii and dropped in to see the Keck Observatory Centre. While I was talking to one of the astronomers, he mentioned that they are about to identify four more stars that have a planet in orbit. They study stars to detect a wobble. From this they can deduce that the wobble is caused by a nearby planet. I commented that it is unfortunate that we will never see a planet outside our solar system because the light from the star could be a billion times brighter than the planet. He replied, "That is exactly what we intend to do with the LBT. The two mirrors can be adjusted so that the light from one is 180° out of phase with the other. When the light beams are combined, the result is a null effect; in other words, it is as if the star had been turned off. We therefore expect, for the first time, to be able to actually observe a planet in orbit around a star."

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