Famous Astrophotography Cameras

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Technology:	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 an Estonian 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 centre of curvature of the sphere and through which the light would pass. The corrector plate lens is thickest in the centre, less thick at the edges and least thick between the edges and the centre. 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,

	History:	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 original 10” x 14” Photographic Glass Plates have now been replaced by a 112 CCD array which provides a 4 x 4 degree wide field. 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. In 1987 the 48” Palomar Schmidt Telescope was renamed the Samuel Oschin Telescope, after he and his wife had made a substantial donation to the Palomar Observatory.

	Fame:	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 Oschin Telescope was responsible for the discovery of 90377 Sedna on 14th November 2003 and Eris, the "10th Planet" on 1st May 2005 from images taken 21st October 2003. The peculiar Type Ia supernova SN 2002cx was discovered with the Oschin telescope on the 12th May 2002.

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