Optical astronomy includes all forms of astronomical
observations made in the wavelengths of visible light. This portion
of the electromagnetic spectrum passes most efficiently through Earth's atmosphere, so the sight organs of Earth organisms tend to be most sensitive to it. Large fractions of the energy output of stars fall within this band of the spectrum, as well. Humanity's earliest astronomical
observations were optical ones, made first by naked eye and later with
telescopes. The observations on which the heliocentric theory of the solar system was based were optical ones, and the later determination that the solar system exists within a galaxy of starsÑthe Milky Way (see Galaxy, The)Ñand that other such galaxies also exist all started with optical observations (see astronomy, history of). By the middle of the 20th century, however, limitations on instrumental engineering and various observational problems seemed to have closed out the prospect of further advances in optical astronomy. At the same time, new windows on the universe were being opened by the construction of detectors for other bands of the electromagnetic spectrum. Such fields of study now include gamma-ray astronomy, infrared astronomy, radar astronomy, radio astronomy, ultraviolet astronomy, and X-ray astronomy. Optical astronomy was far from finished, however. In fact, a virtual renaissance took place in the field during the later 20th century, as exhibited by engineering breakthroughs in building bigger and "smarter" Earth-based instruments and in creating powerful space-based instruments as well (see Space Telescope). Toward the end of the century optical astronomy had also returned to being an activity in which small-scale instruments and even amateur observations are making significant contributions. Basics of Optical Astronomy The light-gathering power of a telescope (see telescope, optical) is its ability to see faint objects as compared to the ability of the human eye. This power is directly proportional to the square of the diameter of the telescope's main mirror or lens, called the objective. The magnifying ability of a telescope, as distinct from its light-gathering power, is given by the ratio of the objective's focal length to the focal length of the combination of small lenses that forms a telescope's eyepiece. (Lenses are combined to correct for optical aberrations.) Usually a telescope has several eyepieces for varying magnification. A telescope's resolving power is defined as the minimum angular distance it can determine between two stars of moderate brightness under ideal viewing conditions. This minimum distance, measured in radians, is given by the ratio of the wavelength of the light to the diameter of the telescope's objective. Magnification must be sufficiently substantial to compress the emerging rays to the size of the pupil of the eye, but not so large that the apparent scale greatly exceeds the angular resolution. Astronomical telescopes are mounted on two perpendicular axes in order to give access to any point in the sky, and they are driven by electric motors so as to compensate for the Earth's rotation. Most telescopes use the equatorial system of mounting, in which one axis is parallel to the Earth's polar axis and the other axis is used for adjustments in declination. The development of computer-controlled drive along both axes, however, has made altazimuth mountingsÑmountings on vertical and horizontal axesÑdesirable, because they are simpler and more economical. Indeed, computerization is assuming a leading role in many modern telescope functions. For example, the New Technology Telescope at La Silla, Chile, is computerized to have an image analyzer examine the light from a faint star at least once each hour, providing a check on the curvature of the primary mirror and the positioning of the secondary.