Optical Astronomy

Until the 20th century, all observational astronomers studied the visible light that astronomical objects emit. Such astronomers are called optical astronomers, because they observe the same part of the electromagnetic spectrum that the human eye sees. Optical astronomers use telescopes and imaging equipment to study light from objects. Professional astronomers today hardly ever actually look through telescopes. Instead, a telescope sends an object’s light to a photographic plate or to an electronic light-sensitive computer chip called a charge-coupled device, or CCD. CCDs are about 50 times more sensitive than film, so today's astronomers can record in a minute an image that would have taken about an hour to record on film.



Telescopes may use either lenses or mirrors to gather visible light, permitting direct observation or photographic recording of distant objects. Those that use lenses are called refracting telescopes, since they use the property of refraction, or bending, of light (see Optics: Reflection and Refraction). The largest refracting telescope is the 40-in (1-m) telescope at the Yerkes Observatory in Williams Bay, Wisconsin, founded in the late 19th century. Lenses bend different colors of light by different amounts, so different colors focus slightly differently. Images produced by large lenses can be tinged with color, often limiting the observations to those made through filters. Filters limit the image to one color of light, so the lens bends all of the light in the image the same amount and makes the image more accurate than an image that includes all colors of light. Also, because light must pass through lenses, lenses can only be supported at the very edges. Large, heavy lenses are so thick that all the large telescopes in current use are made with other techniques.


The Hubble Space Telescope (HST), a reflecting telescope that orbits Earth, has returned the clearest images of any optical telescope. The main mirror of the HST is only 94 in (2.4 m) across, far smaller than that of the largest ground-based reflecting telescopes. Turbulence in the atmosphere makes observing objects as clearly as the HST can see impossible for ground-based telescopes. HST images of visible light are about five times finer than any produced by ground-based telescopes. Giant telescopes on Earth, however, collect much more light than the HST can. Examples of such giant telescopes include the twin 32-ft (10-m) Keck telescopes in Hawaii and the four 26-ft (8-m) telescopes in the Very Large Telescope array in the Atacama Desert in northern Chile (the nearest city is Antofagasta, Chile). Often astronomers use space- and ground-based telescopes in conjunction. See also Space Telescope.

Astronomers usually share telescopes. Many institutions with large telescopes accept applications from any astronomer who wishes to use the instruments, though others have limited sets of eligible applicants. The institution then divides the available time among successful applicants and assigns each astronomer an observing period. Astronomers can collect data from telescopes remotely. Data from Earth-based telescopes can be sent electronically over computer networks. Data from space-based telescopes reach Earth through radio waves collected by antennas on the ground.