Modern telescopes are marvels of engineering. Space telescopes like Hubble give us magnificent views that are simply unobtainable from Earth. Larger ground-based optical telescopes are so sensitive that they are built on mountaintops in remote locations to reduce sources of extraneous light and get above the densest part of the chaotic, vision-disrupting atmosphere. But optical telescopes are currently limited to around 10 m in diameter. The biggest, most impressive telescopes ever created are the radio telescopes. These monstrous machines are impressive sites to behold.
Starting small, in a sense, there are telescope arrays such as the Allen Telescope Array (ATA). This array was built for the SETI (Search for Extraterrestrial Intelligence) Institute to scan the sky for signals from alien intelligences. The ATA is made up of a group of 42 6.1-metre telescopes spread out over 300m. 6.1 metres is not very large as telescopes go, especially for radio telescopes. However, the 42 telescopes can work in concert to make much better results than any single dish can obtain. Essentially, the signals from each telescope can be combined to make a much bigger telescope, albeit one with a lot of holes in the dish.
The biggest advantage of the arrays such as the ATA is economic; it is much cheaper to assemble many small telescopes than one massive one. Furthermore, these projects can be expanded; ATA has plans to grow to 350 telescopes, though these plans are constantly constrained by lack of funding. Unfortunately, a telescope array has less sensitivity than a single dish would, so it can’t pick out as many features. However, the array has a similar angular resolution to a filled dish and so can pick out features of the same size. This array also has another advantage, especially when you are trying to scan the entire sky for radio signals; by changing how the signals from each telescope are combined, researchers can make several virtual telescopes at once, each looking in a slightly different direction.
The radio telescope arrays sound impressive when you think about the size of the virtual dish they produce, especially in the case of the continent-spanning Very Long Base Array. But that impressive image loses out to the sight of a massive full-dish array. The largest and most famous of these massive radio telescopes is Arecibo, located in Puerto Rico. Arecibo has a diameter of 305 metres, making it the same size as the ATA, but with much higher sensitivity. Such a large dish is understandably un-movable. Instead, the 800 tonne radio receive that collects radio waves from the dish is suspended overhead by cables, moving back and forth to collect transmissions from different parts of the sky.
China is working on a new radio telescope, the FAST (Five-hundred-metre Aperture TeleScope). As the name suggests, this project will dwarf Arecibo with its 500 metre made of 4450 triangular panels. This is a mammoth undertaking, and will allow for radio astronomy like never before. The Chinese government has even gone as far as to evacuate civilians in a 5 km radius around the site to, according to a Communist party official, “create a sound electromagnetic environment.” Among its goals is studying the beginning of the universe, and searching for extra-terrestrial intelligence from beyond our galaxy. FAST is currently scheduled for completion in September of this year; amazing new discoveries are sure to follow shortly.
Radio astronomy allows for many unique measurements and observations. As China’s investment in FAST shows, the discoveries are far from being complete. While optical telescopes continue to improve, it is almost certain that machines like FAST and Arecibo will not be surpassed for years to come.
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