In the world of astronomy, the Puerto Rican Arecibo Observatory is a giant. For over 50 years, Arecibo has been the world’s largest single-aperture telescope. With a 305 m (1000 ft) diameter full dish, few others can come close in scale. However, by the end of this year, China will have completed construction on the Five hundred meter Aperture Spherical Telescope (FAST). With a 500 m (1640 ft) diameter, FAST will become the largest telescope in the world and offer the opportunity to look farther into the universe than ever before.
(Image: Arecibo Observatory, Puerto Rico)
But what exactly is it? FAST is a single-aperture radio telescope, which means it uses a single dish to reflect radio signals. Because there are many artificial, non-galactical sources of these signals, FAST is built partially underground and uses a wall to block unwanted signals. Natural sources of radio waves from above are thus unimpeded, and a reflector directs them to a common point.
One of the most significant challenges for a telescope of this size is the necessary support structure. For instance, lenses this big cannot be produced as they would deform under their own weight, so curved reflectors are used to direct the incoming light to a single location for data collection. To aid with achieving the correct shape, the reflector for FAST was built in a karst, which is essentially a giant pit of eroded limestone. A system of 4600 triangular panels connected to cables forms the reflecting surface, which is shaped into a paraboloid. Since FAST collects radio signals, it uses aluminum, which reflects radio waves, but not the wavelengths of visible light. That shape is preferred because all parallel light that enters a paraboloid is directed to a single focus, making data collection from a paraboloid reflector simpler. This massive telescope can also be pointed by moving the panels that make up the reflector, allowing for a pointing precision of 1 thousandth of a degree. Since the paraboloid must be maintained, the entire reflector is not evenly illuminated. In spite of that, FAST’s immense size allows for an effective dish size of 300 m, which is 50% larger than Arecibo’s.
To understand the magnitude of FAST’s capabilities, Arecibo Observatory, the current world leader in radio astronomy, offers an excellent point for comparison. With a collecting area of 196,000 square meters, FAST is more than three times more sensitive than Arecibo. Since a radio telescope’s image resolution depends on its size and the wavelength used, FAST can produce superior images over a larger range of wavelengths, and can also be directed much more effectively.
One of the downsides of large telescopes is that the entire reflector cannot be placed on a mount and oriented in any direction. The largest fully steerable telescope, for instance, the Green Bank Telescope, has a diameter of only 100 m. Telescopes on the scale of FAST and Arecibo cannot be so easier redirected. As a result, Arecibo uses a fixed reflector which points at its zenith, the point directly above it. Arecibo “aims” by moving the signal receiver, thereby only collecting light from certain parts of its reflector. This process introduces error and limits Arecibo’s visible range to within 20° of its zenith. But since FAST can reshape its reflector, it can view everything within 40° of its zenith, a substantial increase over Arecibo. FAST’s ability to reshape its reflector makes it superior in function to Arecibo in another way. Arecibo uses a spherical reflector for its ease of construction. However, its hemisphere shape does not properly focus light, which introduces spherical aberration, a source of error. FAST uses a dynamic system of mobile panels that can form the optimal paraboloid needed for proper focus. These advancements make FAST the single most sensitive telescope in the world.
While construction of FAST will not finish until September of 2016, there is already substantial speculation over the possibilities of its utility. FAST will be able to take a closer look at new galaxies and their redshifts, potentially offering new insights into the nature of dark energy. Also, since FAST uses a full dish reflector, it collects an enormous amount of photons, enabling it to detect incredibly faint signals. The increased sensitivity of FAST offers the opportunity to detect signals–perhaps even artificial ones–from within 28 light years which includes 1400 stars. It would be the first opportunity to observe any signals from other potential civilizations.