Optical telescopes. Optical telescopes collect and focus electromagnetic radiation in the range of 3000 to 10,000 angstroms reaching the surface of the earth (optical window). In optical astronomy, astronomers work with an image of the celestial light source. The image may be formed by refracting lenses or by reflecting mirrors. In either case, important properties of the image are size, brightness, and resolution of adjacent points. The properties depend in different ways on the aperture and focal length of the objective. Turbulence in the earth's lower atmosphere blurs and distorts a star's image, causing it to scintillate. Other atmospheric conditions hamper our view of the heavens: weather conditions, atmospheric pollution, air· glow, and the refraction of light passing through the atmosphere.
Reflecting and refracting telescopes. Telescopes that use lenses for the objective are known as refrac· ting telescopes. Telescopes that use mirrors are known as reflecting telescopes. Modern reflectors are arranged so that images can be formed at three differ· entfocal positions: the prime, the Cassegrain, and the coude foci. Each provides certain advantages depending on the brightness of the celestial object and the type of data to be obtained.
Large modern telescopes are all reflecting tele·scopes They are free from chromatic aberration. They may be supported by their edges and from the back unlike lenses. Furthermore, reflection occurs from the front surface, decreasing the quality requirements on the mirror material will allow astronomers to move from survey activities.
The universe is highly transparent to gamma rays because of their great penetrating power. One would therefore expect them to be capable of carrying the imprint of their origin from far-distant places because they pass so easily through interstellar matter. Thus the unique penetration of gamma rays reveals directional and temporal information about their origin in regions that are too dense for visible photons and even X rays to penetrate. Gamma rays serve as a probe to provide us with new insights into the structure of the cosmos.
Accessory instruments. Several types of radiation detectors, such as photographic emulsions and pho· toelectric devices, are attached to optical telescopes to analyze light in particular wavelengths. The most important properties of the radiation detector are its wavelength sensitivity, response over the wavelength interval, and the nature and range of response. Accessory instruments are used to analyze the wavelength composition of the radiation (i.e., type of spectrum) orthe amount of energy in the radiation (i.e., photometry).
Radio telescopes. Radio astronomy developed rapidly after the late 1940s. Devices to detect and locate precisely sources of radio radiation disclosed that celestial objects radiate energy in the radio portion of the electromagnetic spectrum as well as in the visible light portion. The collecting element of a radio telescope is a parabolic metal dish. The "image" is a plot of the brightness of radio radiation at particular wavelengths as the telescope scans the source of radio radiation.
Ultraviolet, X·ray, and gamma-ray radiation Telescopes, analyzing instruments, and radiation detectors are available to study cosmic radiation in the ultraviolet, X-ray, and gamma-ray portions of the electromagnetic spectrum. Because earth's atmosphere absorbs radiation in these wavelengths, such radiation studies must be carried outside the atmosphere b balloons, high flying aircraft, rockets, or satellites.
