Schmidt's-Eye View

Such great telescopes as the 200-incher on Mt. Palomar see only tiny patches of sky. They need a more wide-eyed instrument to tell them where to look. Last week CalTech and the National Geographic Society announced a joint project to map the whole sky in search of interesting objects for big telescopes to study in detail. The society will supply the funds; CalTech, which runs Palomar Observatory, will supply the Schmidt telescope to do the mapping.

The Schmidt telescope was invented by Estonian-born Bernhard Schmidt (1879-1935). Scientist Schmidt spent years studying the failings of refracting (lens) telescopes and reflecting (mirror) telescopes. Finally he devised a sort of compromise. His telescope has a concave spherical mirror, which is much easier to make than the parabolic mirror of a reflecting telescope. In front of it, to bring the light to a focus without "spherical aberration," is a correcting plate so slightly curved that it looks like plain sheet glass. The Schmidt telescope's advantage: it can take pictures of large patches of sky and have them turn out as sharp on the edges as in the center.

Palomar's Schmidt (called the "Big S") has a 72-in. mirror and a 48-in. correcting plate. It takes 14 in.-by-14 in. pictures that cover a square of sky as wide as twelve moons placed edge to edge in a row. The 200-in. sees only half the diameter of a single moon. At that rate, it would take the 200-in. about 5,000 years to observe the whole sky.

The Schmidts mapping project of the entire sky is expected to take four years. Each little patch will be photographed twice: once with a red filter and once with no filter. On the red plates the hot blue stars will appear to lose some of their brightness; the cool red stars will not. By comparing the two plates, astronomers can get some idea of the temperature of the stars that appear on them.

The Schmidt reaches about 300 million light-years into space--less than one-third as far as the 200-in. -- but it is more efficient than any earlier instrument designed for survey of the sky. As the Schmidt's pictures become available, astronomers all over the world will study them eagerly.

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