(This aritcle originally appeared in the Q2 1987 STAR news letter)

WHERE IS THE EYEPIECE ON A RADIO TELESCOPE?

Radio Astronomy is the study of extraterrestrial radio signals reaching Earth. It is similar to optical astronomy in that both involve the study of electromagnetic energy. While optical astronomy deals with very short wavelengths of fractions of an inch, radio astronomy deals with wavelengths that may be 10 feet!

The major instrument in the science of radio astronomy is the radio telescope, composed of an ANTENNA, used to collect and feed radio energy to a RECEIVER. The most popular type of radio telescope is the DISH, similar to TV satellite receiver stations.  Others are the horn, used to discover the Hydrogen line in 1951 and the 2.7 K background radiation in 1963, and the YAGI, which is similar to TV antennas and was involved in the discovery of pulsars in 1967.

Other parts of the instrument include FILTERS to select the operating frequency or wavelength observed, RADIO FREQUENCY AMPLIFIERS for the weak sources observed, a DETECTOR used to convert radio frequency signals from alternating current to direct current so they can be analyzed, as well as output devices. These include SIGNAL DISPLAY METERS to measure signal strength, STRIP CHART RECORDERS to plot signal strength over time, and COMPUTER INTERFACES to digitize the detector output for recording to disk or tape for later analysis.

The field of view or beamwidth of the antenna is determined by a drift scan. Without tracking, a point source on the celestial equator will drift through the telescopes beam at approximately 1/4 degree per minute. If the source takes one hour to pass out of the beam, the antennas beamwidth would be about 15 degrees.

The antenna is very small compared to an optical telescope in terms of the operating wavelength, therefore compared to an optical telescope it’s resolution is lower. The radio telescope does not need polished optical quality surfaces at the long wavelengths at which it operates. At FM wavelengths, a chain link fence is shiny! There are several ways to obtain image quality comparable to an optical telescope. One way is to build larger radio telescopes, but economical and mechanical limitations soon prevail. Another is to align two antennas on an east-west line and combine their outputs. This INTERFEROMETER produces a series of squiggles called interference fringes when a drift scan is performed. The fringe frequency is proportional to the separation, or BASELINE, of the two antennas. Note that the envelope of the group of fringes is the same as the beam pattern of one of the antennas.

If the baseline is doubled, the fringe frequency and the number of fringes doubles but the envelope remains the same. The center fringe is always in the same place in the group regardless of baseline length, although its width will vary inversely proportional to the baseline length. If many drift scans with different baseline lengths are combined algebraically, all but the central fringe will cancel, yielding many times increased resolution in right ascension. With a fixed baseline length, the fringe frequency varies from zero for a polar source, to a maximum for a source on the celestial equator, therefore, by accurately measuring the fringe frequency, the declination of the source can be determined. Better than 1/100 arcsecond accuracy can be obtained by using baselines of thousands of miles.

A complex source can be considered as being composed of many point sources. The fringes are no longer clean squiggles like a point source, but each piece of the image can be extracted with the help of a computer and put into its proper location.

A better way of imaging a complex source is to use a strategically arranged cluster of antennas that track the source being studied. All of the data needed to generate an image can be collected within a few hours. The effective baseline length changes as the object nears the horizon, eliminating the need to relocate the antennas during a series of measurements. Radio telescopes capable of producing picture like images have existed for several years. Currently, the Very Large Array (VLA) in New Mexico, has the best resolution.

The advances in support equipment such as electronics for precise tracking with multiple telescopes, and computers for data processing, have allowed Radio Astronomy to achieve parity with Optical Astronomy.

To answer the title question, where is the eyepiece on a radio telescope? It’s the computer!

(Ed. note: Bob Thornburg wrote this article based on Jim Carroll’s notes of his May 8 talk.)