It will be completed in 2016
Unlike Arecibo, which has a fixed spherical curvature, FAST will use an active surface that adjusts to create parabolas in different directions, with an effective dish size of 300 m. This means that it will not be confined to pointing directly upwards, but capable of covering the sky within 40° from the zenith, compared to Arecibo's 20° range. Its working frequency will be 70 MHz to 3.0 GHz, with a pointing precision of 4 arcseconds
They began to assemble the telescope's reflector, which is 500 meters in diameter and made up of 4,450 panels. Each panel is an equilateral triangle with sides 11 meters long.
Once complete, the single-aperture spherical telescope called FAST will be the world's largest, exceeding the one at Puerto Rico's Arecibo Observatory, which is 300 meters in diameter
The new radio telescope will have an area of about 196,000 square meters vs the Arecibo 70,000 square meters. Therefore the new radio telescope will by about 2.5 times more area than Arecibo.
The giant dish is being built on a naturally formed bowl-like valley. "There are three hills about 500 meters away from one another, creating a valley that is perfect to support the telescope," said Sun Caihong, FAST's chief engineer.
The karst formation in the local landscape is good for draining rainwater underground and protecting the reflector, Sun said.
The surrounding area has "radio silence" as there are no towns and cities within a 5-kilometer radius and only one county center within 25 kilometers.
The possible contributions to high precision astrometry
As the largest single dish radio telescope ever built, FAST would observe the radio sky with unprecedented sensitivity.
Astronomical masers associated with stars and active galactic nuclei provide useful probes to study the dynamics of them (e.g., Reid & Moran 1981; Lo 2005). In some cases, maser observations could yield distance measurement by trigonometric technique. The shells of OH 1612 MHz emission surrounding OH-IR sources offer such a possibility. The light-travel diameter of the shell could be inferred by measuring phase-lags in the OH spectrum, but the angular diameter is measured by radio interferometry. The distance is then obtained geometrically (Shepherd et al. 1992). Such IR-OH sources would then
provide an independent calibration to the period-luminosity of the RR Lyrae that belongs to the same system.
Pulsars are cosmic clocks that provide ultra stable periodic pulses. The stability of some of the millisecond pulsars are competitive to or even better than the most stable atomic clocks. A group of millisecond pulsars well-distributed on the sky may provide an independent timing standard that could complement the atomic standard. Long-term monitoring of the ToA of a group of millisecond pulsars would also help in detecting the stochastic gravitational wave background. The high sensitivity and large sky coverage make FAST a powerful tool for discovering more pulsars and measuring
the ToA of pulsars with much better precision.
Ground station for space mission
FAST might also work as a very powerful ground station for the future space missions. The large collecting area will enable the downlink data rate increase by orders of magnitude. In three-way communication mode, FAST would be able to provide more precise ranging and Doppler measurements. In the case of a flyby event, FAST would then allow a better mass determination of the flyby object
Due to its large collecting area and geographical location, FAST could greatly improve the capability of the current international VLBI network. FAST lies at the edge of most of the VLBI networks.