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...