The Schmidt Camera

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In 1930, a brilliant but eccentric Estonian optician, Bernhard Schmidt, had a conversation about telescopes with Walter Baade, an astronomer at Mount Wilson Observatory, home of the 100-inch Hooker reflector, then the largest telescope in the world. It was clear telescopes were just going to keep getting bigger. George Ellery Hale was already hard at work on a 200-inch giant. It was not all gravy, though. Bigger mirrors naturally meant longer focal lengths and resultant smaller fields of view. Astronomers needed some kind of a supplementary telescope or camera, a "scout," to survey large areas of sky and pick out interesting objects for the big scopes to view and photograph. The seed planted by this conversation led Schmidt to develop the camera design that bears his name.

Schmidt's camera was simple to explain but difficult to produce. He began with a sphere-shaped primary mirror since spherical mirrors are easy to make, even in large apertures. Although they are easy to produce, spherical mirrors have a serious problem that limits their use in telescopes: spherical aberration. This is a defect that is very similar to chromatic aberration in refractors. When light is reflected from a spherical mirror, not all the rays come to focus at the same point. Those at the edge come to a focus closer in than those reflected from the mirror's center. The end product is not colored halos, as in chromatic aberration, but even worse, images that are badly blurred. This is the exact same problem that afflicted the Hubble Space Telescope when it was first launched. Schmidt was well aware of the effects of this aberration and knew he had to do something to "correct" for it if he were to use a spherical mirror in his astro-camera.

His great idea, the thing for which he is most remembered, was a special lens, a corrector plate, that he placed at the opposite end of his camera's tube from the primary mirror. This thin glass lens, which in Schmidt's camera is somewhat smaller in diameter than the primary mirror, bends incoming rays of light very slightly, just enough so rays at the edge of the corrector are at a different focus than those passing through its center. This different focus distance is identical to that of the mirror's edge and center, but reversed. Rays from the corrector edge focus at a longer distance than those passing through its center. The corrector introduces negative spherical aberration. This lens's negative spherical aberration and the mirror's positive spherical aberration cancel out, and, theoretically, result in an image that is perfectly sharp.

As mentioned, the Schmidt camera design was easier to describe than make. The corrector was the problem. Generating the complex ("fourth-order") curve that would produce negative spherical aberration was very difficult. Finally, Schmidt devised a trick that made grinding the lens a little easier. He placed a glass lens blank in a special cylindrical jig with the blank forming one end of the cylinder. A precise amount of vacuum was then applied to the cylinder to pull the glass blank inward slightly. The optician ground and polished the exposed side of the lens blank into a sphere shape, and when the vacuum was released and the blank sprang back, it almost magically assumed the required shape. The problem was that applying the exact amount of vacuum required and maintaining this pressure was maddeningly difficult. This method did work, however, and allowed Schmidt to successfully complete working cameras.

The Schmidt, which uses both lenses and mirrors to produce images, was the first catadioptric instrument in wide use by astronomers. It was not a catadioptric telescope, however. Its focus point is at an inconvenient position halfway between the corrector and the primary mirror. That makes it difficult to position an eyepiece for viewing. Schmidt was not concerned. He did not imagine his instrument would be used visually; it was to be a giant camera that did not have and did not need a secondary mirror or an eyepiece. Instead, he placed a film plate holder at the focus position. Astronomers accessed this "focal plane" through a door on the side of the tube. Schmidt's camera was very successful in professional astronomy, and one of the instruments built shortly after his untimely death in 1935, the 48-inch Oschin Schmidt at Mount Palomar, continues to do cutting-edge research today.

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