Overview (The Solar System)
Optical astronomy was the first form of astronomical observation. It relied on the human eye as an instrument capable of detecting light and discerning color and location. Today the term optical astronomy refers to observations made using visible and near-visible wavelengths of light, even if not made directly with the human eye.
Gamma, X, ultraviolet, visible, infrared, and radio waves all are forms of electromagnetic radiation. They differ only in wavelength, frequency, Photon energy, and the instruments used to detect them. The Wavelength range to which the human eye is sensitive, called visible light, spans only a small part of the entire electromagnetic spectrum, from violet light, at about 400 nanometers (10-9, or one-billionth of a meter), to red light, at about 700 nanometers. This is the same wavelength range that the Sun emits most intensely, which is no coincidence; human eyes evolved to react to the dominant wavelength range of sunlight. Furthermore, the Earth’s atmosphere blocks much of the Electromagnetic spectrum from reaching the ground. One of the two major wavelength ranges that can penetrate the atmosphere and reach the ground produces the so-called optical window; the atmosphere is almost completely transparent between about 300 nanometers in the ultraviolet and 1,000 nanometers in the infrared, and partially transparent out to about 10,000 nanometers. (The other range is the “radio window,” between about...
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Applications (The Solar System)
Optical astronomy observations can be divided into several techniques: imagery, photometry, spectroscopy, and polarimetry. These, in turn, are applied to studying various sorts of objects.
Imagery provides “pictures” of objects, showing their locations, shapes, and other features. Some of the first measurements in optical astronomy were made with micrometers attached to telescope eyepieces so visual observers could measure accurate positions for constructing star charts. (Earlier charts were made using naked-eye pointing devices.) Photography and electronic imaging provide a means of recording images that can be studied and measured later and that reveal details that cannot be seen with the eye peering through a telescope.
Photometry, literally “light measure” or measurements of brightness, also was performed initially with the eye but was highly subjective; it was difficult to form a standard of brightness based on human perception. Then photographic and electronic systems became available. With photographic emulsions, brightness could be measured objectively, by the degree of exposure recorded. With solid-state photon counters and other electronic sensors, the instantaneous brightness of an object can be measured at any time, and quantitative comparisons can be made with measurements taken on different nights or among different objects. Brightness variations too quick or subtle for the eye to detect also became...
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Context (The Solar System)
After millennia of having nothing more to use than the unaided eye, optical astronomy got a real boost in the 1600’s when Galileo manufactured his own refracting telescopes and used them to study the sky. His discovery of mountains and valleys on Earth’s moon, spots on the Sun, moons orbiting Jupiter, and many stars too faint to be seen without a telescope (including those that constitute the hazy white band of light called the Milky Way) spurred further telescopic observations and more discoveries. Sir Isaac Newton designed the first reflecting telescope, although problems in fabricating and silvering the mirrors caused reflectors to lag behind refractors in size and capability for many years. (Today, all large optical telescopes that are built are reflectors.)
Even after the introduction of the telescope, for 250 years more optical astronomy relied on the human eye as the only detector and the brain and hand as the only interpreters and recorders of data. The advent of photography sparked a radical change in astronomy. The development of dry photographic plates in the late 1800’s allowed long-duration exposures to be made for imaging faint, extended, diffuse objects whose central bodies astronomers had been able to view only dimly by eye. Photography also recorded images objectively for later study by any who were interested. The advent of electronic detectors in the 1980’s made recording light even more efficient. Improved...
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Further Reading (The Solar System)
Chaisson, Eric, and Steve McMillan. Astronomy Today. 6th ed. New York: Addison-Wesley, 2008. A well-written college-level textbook for introductory astronomy courses. Has three chapters on light and telescopes.
Field, George. The Space Telescope. Chicago: Contemporary Books, 1989. Description of the origins of the Hubble Space Telescope and the problems of ground-based and space-based astronomy. Includes descriptions of how optics and detectors work.
Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages to the Stars and Galaxies. Belmont, Calif.: Brooks/Cole-Thomson Learning, 2006. A well-written, thorough college textbook for introductory astronomy courses. Has two chapters on light and telescopes.
Freedman, Roger A., and William J. Kaufmann III. Universe. 8th ed. New York: W. H. Freeman, 2008. Thorough and well-written college-level introductory astronomy textbook. Includes two chapters on light and telescopes.
Hirsh, Richard F. Glimpsing an Invisible Universe: The Emergence of X-Ray Astronomy. New York: Cambridge University Press, 1983. Early history of X-ray astronomy through the High-Energy Astronomical Observatories. Written for the informed reader.
Kitchin, C. R. Astrophysical Techniques. 4th ed. Philadelphia: Adam Hilger, 2004. Discusses various methods used to study astronomy, along with instruments. A good reference for astronomy...
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