The VLA
Published on Jul 27, 2007 at 10:34 am.
1 Comment.
Filed under astronomy, observatories, radio astronomy, wonders.
My next choice for one of the Seven Wonders of Astronomy is another radio observatory. This one is different, though, from Arecibo. Instead of a single large telescope, the Very Large Array, near Socorro, New Mexico, is a collection of 27 separate radio telescope antennae.
The Very Large Array (VLA) is operated by the National Radio Astronomy Observatory (NRAO). The Arecibo Observatory that I blogged about a few days ago is the largest single dish radio telescope in the world. And, there is good reason to believe that a larger dish will likely never be built. However, radio waves are much longer than optical light, so much larger instruments are needed to get the same resolution. Even Arecibo lacks the resolution of even modest optical telescopes. To achieve the resolution that astronomers demand would require a radio telescope miles across. That is clearly too big for a single dish. However, radio waves, being so large, are fairly forgiving to work with. This allows for the possibility of building an interferometer. An interferometer works by signals from widely separated antennae being combined electronically. If signals from a source travel different distances to a receiver, then they become out of phase. Because radio radiation acts as waves, if the signals are out of phase by half a wavelength (meaning that the signals go half a wavelength farther to one receiver than another) then they will destructively interfere. So, signals coming from slightly different directions towards an array of antennae will take different distances to go to different antennae. The signals can be electronically delayed so that signals from a particular source in the sky will add together, but signals from other locations will not. Thus, the array can determine the exact position for the source of the signal. This is the information needed to make high resolution images. Interferometers are used all the time in ordinary radio communications, so the concept is well understood. However, normally the antennae of an array are close enough together for wires to run from one to another. The VLA was envisioned to be an array stretching over several miles. This was too far to run cables, so each antenna would be self contained and would record radio signals. These signals would then be combined later using rather sophisticated computer software performing a Fourier transform. The result simulates what would have been received by a radio dish of an aperture equal to the diameter of the array. Nothing quite of this magnitude had been tried before.
Funding for the project was approved in 1972, and construction began the following year. By September 1975, the first antenna was in place, and the first signals were received and processed by the next year. The facility was formally completed and dedicated in 1980.
Each component of the VLA is itself a radio telescope with a dish of diameter 82 feet (25 meters). The components are arranged along the legs of a giant Y-shape. Amateur astronomers are well aware of the fact that sometimes a low power eyepiece is best for observing something, and sometimes a high power is best. The same can be said of photography: sometimes a wide angle lens is what you want, and sometimes a telephoto lens is better. The same is true of radio astronomy. The VLA, while not changing magnification, can actually act in an analogous fashion because the individual components can be moved around to different postions along each leg of the Y. In the most far flung positions, the array can have the resolution of a dish nearly 22 miles (36 kilometers) across. But, there are three other commonly used orientations of smaller, more compact arrangements of the components, which act like smaller dishes. The total surface area of dishes collecting radio signals when all telescopes are pointed at something is about the same as that of a single dish of diameter 422 feet (130 meters), still much less the sensitivity of the Arecibo telescope.Â
Moving the components is not something done lightly (or easily!). Each component is a 230 ton structure. The legs of the Y are along railroad tracks. A special vehicle moves along those tracks, slides under the component to be moved, and then lifts it up off of a stand that it had been sitting on. The vehicle then carries the structure to another stand and sets it down. The components are fairly tough, but they are still sophisticated scientific instruments, so great care is needed to move them.  A maintenance building is available for components to be cycled out of service for maintenance when needed. Of course, it is difficult to move them and it takes some time to rearrange all of the elements and get things situated to be able to use the array again. So, the array stays in a particular configuration for about three months or so before being out of commision for several days rearranging components.  Plans call for expanding the VLA capabilities, ultimately with the addition of extra dishes located across New Mexico, greatly improving resolution of the array.
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By using interferometry, the VLA can act like a much larger instrument than could be built as a single radio telescope. It has make remarkable discoveries, and is a premier instrument in making detailed radio images of celestial objects, such as the galaxy M81 seen below. For that reason, it clearly deserves a position on the list of the Seven Wonders of Astronomy.
-Astroprof
Images courtesy of NRAO/AUI
Astroprof’s Page » The European Southern Observatory on August 4, 2007 at 7:29 pm: 1
[…] One of the sites used by ESO is the La Silla Observatory, the site of 18 telescopes. La Silla is where the New Technology Telescope, one of the first telescopes with active optics to maintain the shape of its mirror. La Silla is located directly between Cerro Tololo and Las Campanas Observatory (operated by Carnegie Institution). ESO also operates the Paranal Observatory. Paranal has fewer telescopes, but it is home of the Very Large Telescope (VLT). The VLT is actually four separate 8.2 meter telescopes which can operate independently of one another. However, VLT is special in that the light of those four telescopes can be combined to form an optical interferometer (analogous to the radio interferometer, the VLA, that I had written about earlier). Four smaller telescopes can now also be tied into the VLT. […]