107

Very Large Array, Socorro, NM

34° 4′ 43.98″ N, 107° 37′ 5.49″ W

A Virtual Antenna

In western New Mexico, in the middle of an empty plain, sit 27 radio telescopes that work together to study distant galaxies, stars, quasars, and pulsars by examining their radio transmissions. The Very Large Array of dishes are mounted on railway tracks arranged in a Y shape with 21-kilometer-long branches (Figure 107-1). By mathematically combining data from all 27 radio telescopes, the array acts as if it were a single dish 36 kilometers across. (Building a 36-kilometer radio telescope would have been financially unfeasible; the Very Large Array, by contrast, cost a relatively affordable $79 million.)

Figure 107-1. The Very Large Array; courtesy of David Bales (www.davidbales.com)

The dishes in the Very Large Array are moved using a specially built transporter (which is usually part of any tour). They cycle through four major configurations, called A, B, C, and D, every 16 months. The A configuration has the dishes spread as widely apart as possible—this gives the maximum possible magnification. The D configuration has the dishes only 600 meters apart and is used to study an individual radio source in detail. The B and C configurations lie between the extremes of A and D.

Such large virtual dishes are needed because the ability of a radio telescope to distinguish details—the resolution—depends on the wavelength of the signal being listened to divided by the size of the dish. The larger the dish, the smaller the resolution possible. Because radio waves have a much longer wavelength than light waves, radio telescopes need to be much bigger. A 1-meter optical telescope has the same resolution as a radio telescope many kilometers wide.

Each dish in the Very Large Array is similar in construction to a home satellite dish—it’s a parabolic reflector (see Chapter 48), but instead of having a radio receiver at the focal point of the parabola, there’s a second reflector that sends the received radio signal into the middle of the dish where the actual radio receivers are located (see Figure 107-2).

Figure 107-2. Parabolic dish with a central reflector

Data from the dishes is combined to get a radio picture of the sky. As the Earth rotates, more radio pictures are taken at different angles, building a clearer picture of the radio sources being observed.

When you arrive at the Very Large Array, the first place to go is the visitor center, where a short video introduces radio astronomy and the technique used to reconstruct a radio image from multiple dishes (interferometry). A fun experiment for children involves a pair of dishes facing each other—whisper into one dish, and the whisper is clearly heard in the other. The Very Large Array welcomes photographers, but remember to turn off your cell phone—the antennas are very sensitive and even a small phone can interfere with them.

Practical Information

Visiting information is at http://www.vla.nrao.edu/. The Very Large Array runs tours twice a year that coincide with the Trinity Test Site tours (see Chapter 106). It’s about a two-hour drive between the two sites.

Interferometry

Real telescopes (optical or radio) suffer from a limit in their resolution caused by diffraction. A telescope observing a distant star is equivalent to a point of light passing through a small hole, and it generates a diffraction pattern called an Airy disc (Figure 107-3).

Figure 107-3. An Airy disc

The minimum resolution for the telescope is governed by the overlapping of Airy discs from the different points of light being observed. Each point will create an Airy disc—to be able to distinguish the two points, the maximum (the bright portion in the center of one Airy disc) cannot be closer than the first minimum (the innermost dark band) of the second disc.

The larger the telescope, the better the resolution. But making very large telescopes—of the size necessary to view distant stars—is either very expensive or simply impossible.

However, by using a pair of telescopes it’s possible to observe a light source (a star, for example) and superimpose the two images to get a fringe pattern of dark and light bands corresponding to the interference between the same light waves as received at two different positions. A similar thing happens in the well-known double slit experiment (see Figure 107-4), where light from a lamp passes through a pair of slits and creates a fringe pattern.

Figure 107-4. Double slit experiment

The fringe pattern can be used to reconstruct the pattern of light—for example, the star being observed—with a resolution related to the distance between the two slits (or telescopes). The spacing of the dark and light bands (and how they differ) can be used to determine the structure of the light being observed.

Radio astronomy uses the same technique—interferometry—but looking at radio waves instead of light. A pair of antennas listens to the same point in the sky, and an interference pattern is made from the observations. Because the radio waves arrive at the two antennas at slightly different times, very accurate atomic clocks are used to synchronize the signals received, by delaying one signal to match the other, and build the interference pattern (see Figure 107-5).

Figure 107-5. A pair of antennas

In an array of radio telescopes, each pair of telescopes contributes information about the radio signals received along the straight line joining the two telescopes. To make high-quality pictures of the sky, it’s necessary to either move the antennas to build up a picture from many lines, or use many pairs of telescopes. Since the Earth also moves, it’s possible to make additional observations just by observing the same radio sources as the Earth rotates.

The fringe patterns are used to determine the brightness (from a radio signal perspective) of the star being observed, and brightness measurements from each pair of telescopes are combined using a Fourier transform to build up a picture of the sky.

The Very Large Array has 27 telescopes, giving 351 possible pairs. And as the Earth moves, the telescopes can track an object, providing many more observed lines to build up a complete picture.