Old, Weird Tech: How to Cast a 20-Ton Piece of Glass for a Telescope

"Imagine your eye 1,000,000 times as powerful as it is." This is the start of an odd 1939 book review in the New York Times that chronicled the making of a giant telescope for the Palomar Observatory in California. Odd because the it wouldn't be completed for another decade, but the 200-inch telescope, which would be the world's largest for almost fifty years, was already an icon.

Long before the moon walk, the Hale telescope at Palomar was the big scientific challenge of the day. Americans followed its construction process in the press. Lowell Thomas, the Walter Cronkite of his day, called it "the greatest item of interest to the civilized world in twenty-five years, not excluding the World War."

There's at least one possible explanation for all this fuss in a book called Imagining Space, by Roger D. Launius and Howard McCurdy. They note that the "golden age" of exploring the Earth ended when an airplane flew over the South Pole in 1926. After that, "space travel promised to continue that tradition, with its commensurate discovery of new lands, exotic species, and revolutionary science."

The sticking point technically for this operation was the telescope's mirror. Following a process first developed by Sir Isaac Newton, the reflector telescope uses a wide mirror to collect light and focus it, displaying an impossibly far away image.

The giant mirror blank for the Palomar scope was dubbed "The Giant Eye." After General Electric sunk $600,000 into failed attempts to make the mirror from fused quartz that could then be coated with metal, the telescope's mastermind, astronomer George Ellery Hale brought the problem to the upstate New York glass firm, Corning. Corning was best known for a classic set of dinnerware. But in the nineteen-teens, they'd introduced extra-strong Pyrex glass.

Images of the casting are like golden outtakes from a Chaplin movie. A half dozen of the firm's top glass pourers were brought in from its factories. They worked together to manipulate a giant ladle, which held 750 pounds of molten glass. The first casting failed, but all the men volunteered for a second attempt. The lead ladler, Charlie Wilson, held a heat shield in his teeth between himself and the glass furnace. He was due for dental surgery, but delayed it until casting was complete.

The disk had to be gradually cooled over a year and then transported to California transported. The giant glass disk, all 20 tons of it, was moved across the country by rail, a route that had to be exhaustively planned. It travelled at 25 miles an hour, and the operation went off without a hitch except for a single bridge which left just "a King James Bible width" of clearance. People lined up along the tracks to watch it go by.

When it arrived in Pasadena in 1936, the Pasadena Star News wrote that "There has been no such excitement since Ambler's feed mill burned." Score one for human ingenuity and American engineering! Cal Tech specialists ground away five tons of glass to shape it for use.

When the Hale Telescope was finally finished, a thousand people attended the dedication. Time and Life ran reports. Collier's Magazine ran an exclusive with the first images seen through the telescope.

Corning kept the botched mirror from the first pour attempt. They set up a small structure in the center of town, with a rounded top like the observatory. People could come to see it there until it was moved in the Corning Museum of Glass in 1951. For the museum's 60th anniversary this year the curators created a special exhibition of photos and ephemera from the making of the mirror. It opened earlier this month.

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Emma Jacobs is an independent journalist based in Philadelphia. She's a frequent contributor to NPR and other national publications and an urban enthusiast.

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