The 48-inch Schmidt telescope (Oschin telescope) at the Palomar Observatory is a standard Schmidt camera telescope using both lenses and mirrors to create a wide field of view for photographing large sections of the sky at one time.  Construction on the Schmidt telescope began in 1939 and was completed in 1948.
The telescope was manufactured in the Caltech machine shops and consists of a tube 20 feet long in a fork-type mounting which allows the telescope to sweep all parts of the sky from the north pole to as far south as declination minus 45 degrees. The combined weight of the fork and tube is more than 12 tons. This assembly moves on a 2-inch ball bearing in the polar axis. The tube, partly cylindrical and partly conical, is made of 5/16 inch welded steel plate. The telescope shutter consists of two rotating shells located inside the tube behind the correcting plate. This construction allows the correcting plate to be removed or auxiliaries to be mounted without removing the shutters. The mirror and its cells are mounted at the lower end of the tube and are kept at a constant distance from the focal surface, regardless of temperature fluctuations, by means of three floating metal alloy rods.
The telescope uses Selsyn indicators which take their signals from declination and right-ascension gears and transmit the position electrically to the control desk. Other electrical features include automatic limit switches which stop the telescope four degrees from the horizon, automatic control of the dome's rotation, and automatic regulation of the wind-screen height. The telescope is driven by a 1/25-horsepower synchronous motor.
Two sizes of photographic plates are used in the camera, 10 inches square and 14 inches square. The spherical mirror is 72 inches in diameter and has a radius of curvature of 241 inches. The corrector plate at the upper end of the tube is 49.75 inches in diameter. The 48-inch telescope, at an optical speed of f/2.5 covers an angular field of 7 degrees. The telescope is guided by two 10-inch refractors attached on each side of the tube.
The telescope was overhauled recently to prepare for the new Palomar sky survey project sponsored by the National Geographic Society. A new 48-inch-diameter corrector plate was made by Grubb-Parsons in England and installed in the telescope. This new plate produces better images over a wider range of wavelengths than the original. Other new equipment includes an automatic guider and an internal calibration source for placing technical information at the edge of each plate while the sky exposure is being made. There is also a photometer to monitor sky brightness and provide a zero point for the calibration. 
The dome housing the 48-inch Schmidt is 48 feet in diameter and 48 feet high and is located about a quarter mile east of the 200-inch Hale reflector. Darkrooms, offices, and a study are found on the ground floor while the second floor is occupied by the telescope.
Although not as well known as the 200-inch Hale reflector, the 48-inch Schmidt telescope (Oschin telescope) at the Palomar Observatory has performed invaluable scientific research and has prepared the way for many of the important discoveries made by the 200-inch. This instrument was first used in 1950, to carry out two surveys of the Northern Hemisphere, one through a red filter and one through a blue, so that a comparison of the two black and white prints would reveal how cool (red) or how hot (blue) a star was. The surveys involved taking 1758 plates of the northern sky and recorded stars never seen before. The Palomar sky survey is the standard reference atlas for deep sky observation and provides a base line with which to measure changes in deep sky observation targets in future surveys. It is used as a standard reference tool for all modern observatories doing deep sky observation. 
The successful construction and use of large reflecting telescopes in the 20th century increased the need to have accurate deep sky star maps that would provide suitable targets for observation. Although large telescopes, such as the 100-inch Hooker reflector at Mount Wilson, could peer deeply into the cosmos, they could only cover a small section of the sky at one time. The optics of the large parabolic mirrors in these reflectors permitted observation of objects only in a narrow viewing area. These telescopes were therefore unsuitable for mapping the sky to discover interesting targets for future observation. What was needed was a reflecting telescope that could see deeply into space, and also photograph large sections of the sky to produce accurate sky maps needed to guide the large reflectors.
A solution to this problem was reached by a Russian-German optician, Bernhard Voldomar Schmidt (1879-1935) who, in the 1920s, conceived of a compromise between reflectors and refractors, instruments that made use of both mirrors and lenses.  Schmidt's solution was to design a telescope that used a spherical mirror with a "corrector plate," a lens that could be placed at the center of curvature of the sphere and through which the light would pass. The corrector plate lens is thickest in the center, less thick at the edges and least thick between the edges and the center. Schmidt designed the plate to refract the light passing through it in such a way as to make up for the spherical aberration introduced by the mirror, without introducing visual errors such as chromatic aberrations commonly found in large reflectors.
Since the "Schmidt telescope" could take photographs over wide sections of the sky and gain large amounts of information at one time, its use spread rapidly in the 1930s, to increase the efficiency of the large reflectors. A Schmidt telescope could survey a wide field, producing single photographs containing the images of as many as 1,000,000 stars and 100,000 galaxies, and, if anything in that field looked suspicious or interesting, a reflector could then observe the object more closely. 
During the construction of the large 200-inch Hale reflector on Palomar Mountain the astronomers at Caltech realized the need to build a large Schmidt telescope to work with the 200-inch reflector. The result was the construction of the 48-inch Schmidt telescope which could photograph large sections of the sky in a short period of time.
During the 1950s the National Geographic Society and the Palomar Observatory collaborated on the original Palomar Sky Survey. This work took five years to complete and remains the deepest photographic record of the entire northern sky. When the project was begun the heavens were almost unexplored territory at the faint limiting magnitudes reached with the 48-inch Schmidt. The idea was to identify objects that could be studied in greater detail with the 200-inch reflector. 
The Palomar Sky Survey was an unqualified success by the time of its completion in 1957. Many discoveries were made with these photographs, including distant star clusters, dwarf spheroidal galaxies, dwarf galaxies in the local group, and clusters of galaxies. Astronomers have used the survey for morphological studies of galaxy classification and for work on peculiar galaxies. The survey has been heavily relied on to identify objects at wavelengths outside the visible spectrum, such as radio galaxies, X-ray sources, and quasars. It has also proven valuable for making finder charts to aid in deeper studies with other telescopes. 
The 48-inch Schmidt is now being used to complete a new survey of the sky that will supplement and update the original sky survey. When completed this survey, taken together with the original work, will form a data base of information about deep sky objects, that will enable astronomers to observe changes in the sky over a period of 30 years and to formulate new theories about the nature and extent of the universe.
Abell, George O. Exploration of the Universe. 4th ed., Philadelphia: Saunders College Publishing, 1982.
Asimov, Isaac. Eyes On The Universe. Boston: Houghton Mifflin Company, 1975.
Kirby-Smith, H.T. U.S. Observatories: A Directory and Travel Guide. New York: Van Nostrand Reinhold Company, 1976.
Learner, Richard. Astronomy Through the Telescope. New York: Van Nostrand Reinhold Company, 1981.
_______________. "The Legacy of the 200-inch," Sky & Telescope, April 1986, pp. 349-53.
Staff of the Palomar Observatory. Giants of Palomar. Hansen Planetarium, 1983.
Schombert, James. "Surveying the Northern Sky," Sky & Telescope, August 1987, pp. 128-31.
Wilson, Albert G. "The Big Schmidt," Scientific American, December 1950, pp. 35-40.
Woodbury, David O. The Glass Giant of Palomar. New York: Dodd, Mead & Company, 1954.
Wright, Helen. Palomar: The World's Largest Telescope. New York: The Macmillan Company, 1952.
(click on the above photographs for a more detailed view)