1947: After 13 years of grinding and polishing, the Palomar Observatory mirror is completed at Caltech.

It was, at the time, the largest telescope mirror ever made in the United States, measuring 200 inches in diameter. Following its completion, the disk was mounted in Palomar’s Hale Telescope and first used in January 1949 to take pictures of the Milky Way. Edwin Hubble was the first astronomer to make images using the new scope.

The mirror began as a 20-ton piece of molten Pyrex, a new glass blend, at the Corning Glass Works in upstate New York. Pyrex expands and contracts far less than regular glass, making it less prone to distortion, a problem that plagued the 100-inch mirror already in operation at Palomar.

After being heated to 2,700 degrees Fahrenheit, the Pyrex was poured into a ceramic mold. It was carefully cooled at an average rate of one or two degrees per day for 11 months, then allowed to reach room temperature. After that it was shipped west to Caltech in Pasadena, where the glass was painstakingly ground to perfection in a process lasting more than a decade.

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The era of giant telescopic lenses began in the 1700s, when astronomers recognized that the bigger the lens (or reflecting mirror), the better the image. In 1774, English astronomer William Herschel mounted several 9-inch mirrors in a 10-foot-long telescope and recorded, with satisfaction, that he had spent the first night looking at “Saturn’s rings and two belts in great perfection.”

Herschel followed that up with a 48-inch behemoth, requiring a telescope so large that it could no longer be manually operated. This led to the building of the world’s first observatory, a 60-foot high, wood-framed structure that looks nothing like a modern observatory.

The Palomar Observatory opened in the 1930s after astronomer George Hale (for whom the telescope is named) determined that the Mount Wilson Observatory was no longer an ideal site because of the encroaching lights of a growing Los Angeles. The new site he chose was atop Mount Palomar, 100 miles southeast of Pasadena.

The Hale Telescope is one of seven operational scopes at Palomar. It was the largest optical telescope in the world until completion of Hawaii’s 10-meter Keck I telescope in 1993.

Source: Caltech

Photo: The 200-inch mirror blank for the Hale Telescope had only a rough, flat front surface in 1936, before it was shipped from Corning to Caltech for grinding and polishing. (Courtesy Palomar Observatory/Caltech)

This article first appeared on Wired.com Oct. 3, 2008.