Steerable
Paraboloid In 1937, Grote Reber built the first steerable paraboloid antenna as an amateur project at Wheaton,
Ill. This antenna became the prototype of most modern large radio telescopes. During and immediately after World War II the large radar instruments built for military purposes were used as radio telescopes. In the 1950s the main thrust of radio
telescope construction was to build ever larger steerable paraboloids, a process that culminated in the 91.4-m (300-ft) paraboloid at Green Bank, W.Va., built in 1963, and the 100-m (328-ft) telescope near Bonn, West Germany, constructed in the late 1960s. The development of more precise antennas allowed operation at higher radio frequencies, which is important to the study of interstellar molecules. The most powerful for the study of such molecules is the 11-m (36-ft) radio telescope operated by the National Radio Astronomy Observatory at Kitt Peak, Ariz. An important variation of the paraboloidal antenna is the use of a fixed spherical antenna, as in the world's largest radio telescope, 305 m (1,000 ft) in diameter, constructed in 1963 near Arecibo, Puerto Rico. Another such antenna is the Soviet RATAN-600Ñan acronym for Radio Astronomy Telescope of the Academy of Sciences (Nauk)Ñin the northwestern Caucasus; it has about 1/4 the reflecting area of the Arecibo antenna. The operation of the paraboloid radio telescope is identical in concept to that of large optical telescopes. A reflector consisting of a paraboloid is oriented so that its axis is pointed at the place in the sky whose radio emission is to be measured. An excellent reflector surface at radio wavelengths need be smooth only to an accuracy of about wavelength or better, or typically about 1 millimeter. Thus ordinary sheet metal is usually used. The paraboloid reflects all rays coming to it from the place of interest to the focus of the paraboloid. At that point a radio antenna is located to capture the focused radio waves and convert them into an electrical signal of the same frequency as the incoming waves. This antenna is often a microwave horn or a simple dipole antenna. In the Arecibo telescope the reflector is spherical and focuses the radio emission to a long line, some 29 m (96 ft) long, rather than to a point. An ensemble of antennas must be placed all along this line to capture the focused radiation. The electrical signal is carried from the antenna through a waveguide or wires to a high-sensitivity radio receiver. This receiver has electronic or waveguide filters that allow only those radio frequencies to pass that the astronomer wishes to observe. The signals, which may be at a very low power level such as 10 to the minus 20th watt, are then amplified in a special
amplifier. The amplifiers used are low noise, meaning that they themselves add a minimum of radio noise to the weak incoming signals. The most commonly used amplifiers are the parametric amplifier, often cooled to a very low temperature with liquid air to give low noise performance, or the maser, a special form of amplifier using atomic processes to give exceptionally low noise performance. These must usually be cooled to a temperature of 4¡ C above absolute zero or lower, usually by immersing them in liquid helium, a procedure that leads to practical difficulties and high expense. Following amplification, a circuit detects the signal, meaning that it establishes the average power of the rapidly oscillating signal. This average radio power, which often changes many times in a second and may even change in less than a millionth of a second in some pulsar radiation, is then recorded. At present the most common recording method is an electronic digital voltmeter that provides a digital value of the radio emission; a computer is then used to record these digital values on magnetic tape. Later, the scientist involved will take these raw values and subject them to further mathematical manipulations in order to take into account suchthings as the amplifier gain and telescope size to lead to a value that is relevant to the physics taking place in the source observed. A common application of radio telescopes is in the observation of radio spectral lines of such things as atomic hydrogen, carbon monoxide, or formaldehyde. To do this expeditiously a radio receiver is preferred that receives a large number of adjacent frequency bands, or channels, simultaneously. This reception can be achieved by incorporating the appropriate number of selective filters in the radio receiver and providing each one with its own detector and data recorder; the latter may be a single computer time-shared by all the channels. This procedure is technically demanding and inflexible, however, because it calls for an entire new set of filters whenever channel bandwidth must be changed. One way to overcome these difficulties is to use an electronic device called an autocorrelator to analyze the radio signals. This device utilizes digital electronic circuitry to calculate the function describing the waveform of the amplified signal developed in the receiver. A simple mathematical treatment of this function in a computer, called a Fourier transformation, can recover the spectrum of the radio emission.