device used to form magnified images of distant objects.
The telescope is undoubtedly the most important investigative tool in astronomy. It provides a means of collecting and analyzing radiation from celestial objects, even those in the far reaches of the universe.
Galileo revolutionized astronomy when he applied the telescope to the study of extraterrestrial bodies in the early 17th century. Until then, magnification instruments had never been used for this purpose. Since Galileo’s pioneering work, increasingly more powerful optical telescopes have been developed, as has a wide array of instruments capable of detecting and measuring invisible forms of radiation, such as radio, X-ray, and gamma-ray telescopes. Observational capability has been further enhanced by the invention of various kinds of auxiliary instruments (e.g., the camera, spectrograph, and charge-coupled device) and by the use of electronic computers, rockets, and spacecraft in conjunction with telescope systems. These developments have contributed dramatically to advances in scientific knowledge about the solar system, the Milky Way Galaxy, and the universe as a whole.
Today, the telescope is used to explore every region of the electromagnetic spectrum from the shortest wavelengths (gamma rays) to the longest (radio waves; see Figure 1
). The wavelengths of the spectrum are measured in three different units: angstroms (Å), micrometres (μ), and metres (m). Each of these units is customarily used for specific wavelength ranges, as shown in the figure. For example, the wavelengths for gamma rays and X rays are given in angstroms, those for infrared rays in micrometres, and those for intermediate radio waves in metres. (Centimetres are often used for short radio waves [microwaves] and kilometres for long radio waves.)
Astronomical observations were restricted to visible wavelengths until the 1930s, when Karl Jansky and Grote Reber of the United States opened the radio “window.” Since the 1960s the use of Earth-orbiting telescope systems has enabled astronomers to make observations in all other spectral regions as well.
The-electromagnetic-spectrumFigure 1: The electromagnetic spectrum.
Focal-length-of-a-lensFigure 2: Focal length of a lens.
Refracting-telescopeFigure 3: Refracting telescope.
The-72-inch-reflecting-telescope-at-Birr-Castle-County-OffalyThe 72-inch reflecting telescope at Birr Castle, County Offaly, Leinster, Ire., was the largest in …[Credits : Geray Sweeney/Tourism Ireland]
Concave-mirrorFigure 4: Concave mirror.
The-Cassegrain-reflectorFigure 5: The Cassegrain reflector.
The-Schmidt-telescopeFigure 6: The Schmidt telescope.
Cutaway-of-the-Hubble-Space-Telescope-revealing-the-Optical-TelescopeCutaway of the Hubble Space Telescope, revealing the Optical Telescope Assembly (OTA), the heart of …[Credits : Courtesy of the Hughes Aircraft Company]
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