The Glass Giant
The story begins...
For the American astronomer George Ellery Hale, bigger was always better. In 1897, at the age of 29, he had become director of Chicago’s new Yerkes Observatory, whose 40-inch refracting telescope remains the largest instrument of its kind in the world. The lenses of refractors collected and focused light, but Hale found that, at sizes bigger than 40 inches, those lenses would distort under their own weight and absorb too much starlight. Hale dreamed of probing the mysteries of the universe by applying the principles of physics and chemistry to the stars. In order to realize this ambition, it would be necessary to build much larger telescopes.
Hale redirected his interest to reflecting telescopes, which permitted the use of mirrors to collect more light and yield brighter images than were possible with refractors. He started with a 60-inch mirror that was donated by his father. Later, with the backing of the steel magnate Andrew Carnegie, the 100-inch Hooker Telescope was erected at Mount Wilson, California. But Hale wanted an even bigger telescope. In 1928, he wrote an article titled “The Possibilities of Large Telescopes” for Harper’s magazine. While recounting the successes of the Yerkes and Mount Wilson telescopes, he worried about the restrictions they imposed. “Starlight is falling on every square mile of the earth’s surface,” he noted, “and the best we can do at present is to gather up and concentrate the rays that strike an area 100 inches in diameter.”
Having addressed concerns that large mirrors would result in poorer image quality, Hale concluded that “a 200-inch or even a 300-inch telescope could now be built and used to the great advantage of astronomy.” He even managed to secure a pledge of $6 million from the Rockefeller Foundation to construct a 200-inch telescope for the California Institute of Technology. The principal challenge was casting a satisfactory blank of glass. If temperature fluctuations caused the glass to expand and contract unequally, images of the stars would be badly distorted. What was required, Hale determined, was a type of glass with a low coefficient of expansion. The mirror of the Mount Wilson telescope was made of plate glass, but that material would not work for a 200-inch mirror.
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