New kinds of telescopes allow today's astronomers to "look" farther into space and "see" more fascinating sights than at any time in the past.
Most radio telescopes use a curved "dish" antenna, which corresponds to the main mirror in an optical reflector, to collect and focus radio waves from space. This antenna must be very big to collect long radio waves and produce clear images (Figure 2.17).
You cannot see, hear, or photograph these radio waves directly. Instead, they are redirected to a tuned radio receiver that amplifies, detects, and records their electronic image. Computers may display radio images digitized, as a contour map that shows the strength of the radio source (Figure 6.19 b) or as a radiograph (Figure 6.18), which is a false color picture that shows how the radio source in space would "look" to a person with "radio vision."
Radio astronomy began in 1931 when U.S. engineer Karl G. Jansky (1905-1950) discovered radio waves coming from the Milky Way. Since then radio waves have been received from diverse sources including our Sun, planets, cold interstellar gas, pulsars, distant galaxies, and quasars.
The largest single radio antenna ever built is a 305-m (1000-foot) dish with an 8-hectare (20-acre) reflecting area. It is fixed in a valley at the Arecibo Observatory, Puerto Rico. ►www.naic.edu^
The Robert C. Byrd Green Bank Telescope (GBT), at the National Radio Astronomy Observatory (NRAO) in West Virginia, U.S. ► www.nrao.edu ^ is the world's most powerful, accurate, and sensitive fully steerable single antenna. Its innovative 100 x 110-m (330-foot) dish is specially shaped to direct radio waves to the side, where a receiver collects the signals without blocking the dish.
Identify the antenna and prime focus of the radio telescope shown in Figure 2.17. (a)_; (b)_
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