Radio Telescopes

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Radio telescopes have several advantages. They let us "see" many celestial objects that emit powerful radio waves but little visible light. They let us "see" radio sources behind interstellar dust clouds in our Milky Way Galaxy that blot out visible stars (because radio waves pass through these clouds). Our atmosphere does not stop or scatter radio waves, so radio telescopes can be used in cloudy weather and during the daytime.

As with optical telescopes, more and clearer data are produced by ever-larger collectors. Aperture synthesis is a cost-effective way to get the performance of a single giant telescope from smaller ones. An interferometer combines beams of light from two or more telescopes to simulate one very large aperture whose resolving power is set by the separation of the smaller ones.

The Very Large Array (VLA) is the world's largest aperture synthesis facility. The VLA consists of 27 movable 25-m (82-foot) antennas located at a 2100-m (7000-foot) high National Radio Astronomy Observatory site in New Mexico, U.S. These can be used in different configurations to act as a fully steerable radio dish 36 km (22 miles) in diameter. Computers control the antennas, analyze and display observed data, and produce top quality, detailed images.

Very Long Baseline Interferometry (VLBI) gives the best resolution by stationing antennas continents apart. Each station has receiving, transmitting, data handling, and interstation communication equipment. Data recorded from coordinated observations of a specific radio source is correlated by computer to simulate one colossal dish.

The U.S. Deep Space Network (DSN) ►http://deepspace.jpl.nasa.gov/dsn^ has three 70-m (230-foot) radio telescopes set in California, U.S., Spain, and Australia. Stations are used for VLBI observations and constant contact with spacecraft as Earth rotates. The control center is at the NASA Jet Propulsion Laboratory in California.

The Very Long Baseline Array (VLBA) maps the most distant radio sources and finest details (Figure 2.18). It has 10 automated 25-m (82-foot)

Figure 2.18. The Very Long Baseline Array (VLBA) ►www.vlba.nrao.edu^ has 10 antennas at stations located across 5000 miles. The fine detail that the VLBA can "see" is like being able to stand in New York and read a newspaper in Los Angeles.

radio telescopes set across the U.S. from Hawaii to St. Croix, Virgin Islands, with an operations center in New Mexico. Astronomers monitor their research while VLBA operators remotely control antennas and check equipment over the Internet. Supercomputer processing of the recorded data from all 10 antennas subsequently can synthesize a single radio telescope 8000 km (5000 miles) in diameter.

Resolving power is maximized by using the VLBA with Earth-orbiting radio telescopes.

List at least three advantages of a radio telescope. (1)_

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