Blowing Distortion Out of Palomar’s Eye (Jun, 1950)

Preventing distortion in modern telescopes is a bit more complicated.

Blowing Distortion Out of Palomar’s Eye

ORDINARY electric fans—a dozen of them—plus an “overcoat” of insulating foil are helping the Big Eye of the Palomar Observatory to see clearer and farther into the vastness of the universe.

Arranged in a circle in the 20-ton metal cell that holds the 200-inch mirror, the fans cut down distortion caused by uneven temperature changes in the glass. A sudden five-degree change in temperature may distort the mirror surface a few hundred -thousandths of an inch while the glass adjusts itself. To the astronomer, even this minute distortion is serious when he is photographing celestial bodies millions of miles away. The fans reduce this distortion an estimated 50 percent by circulating air over the back of the mirror, causing the giant glass disk to contract or expand evenly. A coating of insulating foil around the rim of the mirror also helps maintain an even temperature over the whole mirror.

Although the Hale telescope at Palomar is already the most perfect large astronomical telescope ever built, astronomers are continuously working toward perfection. Recent repolishing of high spots around the mirror’s outer edge and the addition of the fans and insulating foil are part of these continuing efforts.

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