Oct. 3, 1947: Birth of Palomar's 'Giant Eye'

[Steve]

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 and cooled at an average rate of one or two degrees per day until it reached room temperature 11 months later. 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. 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...

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