FAST (Five-hundred-meter Aperture Spherical radio Telescope)
FAST is a Chinese radio telescope. It is the world's largest and most sensitive radio telescope and three times more sensitive than the Arecibo Observatory. FAST is managed by NAOC/CAS (National Astronomical Observatories/Chinese Academy of Sciences) and funded by NDRC (National Development and Reform Commission). The ultimate goal of FAST is to discover the laws of the development of the universe. 1)
Figure 1: In this photo,released by the Xinhua News Agency on 24 Sept. 2016, an aerial view shows the Five-hundred-meter Aperture Spherical Telescope (FAST) in the remote Pingtang county in southwest China's Guizhou province (image credit: Liu Xu, Xinhua News Agency)
Measuring 500 meters in diameter, the radio telescope is nestled in a natural basin within a stunning landscape of lush green karst formations in southern Guizhou province. It took five years and $180 million to complete and surpasses that of the 300-meter Arecibo Observatory in Puerto Rico, a dish used in research on stars that led to a Nobel Prize.
Installation of the 4,450-panel structure, nicknamed Tianyan, or the Eye of Heaven, started in 2011 and was completed in July 2016. The telescope requires a radio silence within a 5 km radius, resulting in the relocation of more than 8,000 people from their homes in eight villages to make way for the facility, state media said. Reports in August said the villagers would be compensated with cash or new homes from a budget of about $269 million from a poverty relief fund and bank loans.
The FAST telescope will spend the coming decades exploring space and assisting in the hunt for extraterrestrial life. And once it commences operations in September 2016, the Chinese expect it will remain the global leader in radio astronomy for the next ten or twenty years. FAST is capable of forming a parabolic mirror. That will allow researchers a greater degree of flexibility. 2) — On September 25, 2016, the FAST telescope began operating in southwestern China.
FAST uses a data system developed at ICRAR (International Center for Radio Astronomy) in Perth, Australia and at ESO (European Southern Observatory) to manage the huge amounts of data it generates. The software is called NGAS (Next Generation Archive System), and will help astronomers using the telescope to search for rotating neutron stars and look for signs of extra-terrestrial life. The NGAS data system will help to collect, transport and store about 3 PB (Petabytes, 3 x 1015) of information a year from the telescope. 3)
The idea of sitting a large spherical dish in a karst depression is rooted in Arecibo telescope. FAST is an Arecibo-type antenna with three outstanding aspects: the karst depression used as the site, which is large to host the 500-meter telescope and deep to allow a zenith angle of 40 degrees; the active main reflector correcting for spherical aberration on the ground to achieve a full polarization and a wide band without involving complex feed systems; and the light-weight feed cabin driven by cables and servomechanism plus a parallel robot as a secondary adjustable system to move with high precision. The feasibility studies for FAST have been carried out for 14 years, supported by Chinese and world astronomical communities. Funding for FAST has been approved by the National Development and Reform Commission in July of 2007 with a capital budget ~ 700 million RMB. The project time is 5.5 years from the commencement of work in March of 2011 and the first light is expected to be in 2016. 4)
An international review and advisory conference on science and technology of FAST was held in Beijing in March 2006. The review panel unanimously concluded that the FAST Project is feasible and recommended that the project moves forward to the next phase of detailed design and construction as soon as possible. Funding for project FAST has finally been approved by the National Development and Reform Commission (NDRC) in July of 2007. The approved budget is now 700 million RMB. At the end of 2008, the foundation has been laid. The construction period is 5.5 years from the commencement of work in March 2011. The first light is expected to be in 2016.
FAST is an Arecibo-type spherical telescope. Figure 2 illustrates the optical geometry of FAST and its three outstanding features: the large karst depressions found in south Guizhou province as the sites, the active main reflector of 500 m aperture which directly corrects for spherical aberration, and the light-weight feed cabin driven by cables and a servomechanism plus a parallel robot as a secondary adjustable system to carry the most precise parts of the receivers. Inside the cabin, multi-beam and multi-band receivers will be installed, covering a frequency range of 70 MHz - 3 GHz. The telescope will be equipped with a variety of instruments and terminals for different scientific purposes.
Figure 2: Left: FAST optical geometry, right: FAST 3-D model (image credit: NAOC/CAS)
Surveying Site: A practical way to build a large spherical telescope is to make extensive use of existing depressions which are usually found in karst regions. Site surveying in Guizhou province started in 1994, including geo-morphological features and the distribution of the karst depressions, climate, engineering environment, social environment, and radio interference. At least 400 candidate depressions were investigated with remote sensing, and the Geographic Information System. The expense of earthwork largely depends on the geometrical profile of a depression.
The Dawodang depression in south Guizhou has been selected as the telescope site (Figure 3). Total earthwork is estimated to be ~ 1,000,000 m3 according to the comparison of the reflector model with the digital terrain model images of the depression. The relatively low latitude (~26°N) of the site enables the observation of more southern galactic objects. The mild climate of the subtropical zone with a few days of frost and snow without ice build-up enable survival of low cost structures. There is no inundation of karst depressions because of their good drainage, but a tunnel is still budgeted in order to ensure telescope safety. No devastating earthquake has ever been recorded in history. The remoteness and sparse population guarantee a clean RFI environment and the safety of future FAST instruments. An agreement on a temporary radio quiet zone around the site has already been signed by the Chinese Academy of Sciences (CAS) and Guizhou provincial government.
Figure 3: Depression Dawodang, East: 107º21' North: 25º48', Altitude: ~1000 m. Right: image by quick bird with resolution of 0.6 m, the dimension of the circle is ~1000 m (image credit: NAOC/CAS)
Requirement from FAST sciences: FAST sciences put stringent limits on surface deformation of the main active reflector. The position of all the nodes need to be controlled precisely, which depends not only on the structure of the cable net, the back frame and the reflector elements, but also on status of thousands of down-tied cables and actuators. The tracking of FAST is realized by adjusting the reflector in real-time. So the speed of the deformation, which is related to the speed of actuators, should be high enough (about 15° per hour in the extreme case) to track a target in the sky. The shape and size of the panels are carefully chosen to minimize the required deformation from a sphere to form a paraboloid and to reduce intrinsic polarization.
Fundamental Questions for a Large Single Dish Radio Telescope: The origin of the observable universe, the origin of our world with the Sun and the Earth, and the origin of intelligent life are overarching questions of natural sciences. FAST, with its unparalleled collecting area, state of art receiver systems, and the digital backend of which the technology development largely follows the Moore‘s law, has a unique window for contribution through precise measurements of matter and energy in the low frequency radio bands.
At radio frequencies, a large single dish telescope is capable of observing the main component of cosmic gas, atomic hydrogen (HI), from the local universe to moderate redshifts. The gaseous galaxies can be either bright or totally dark in optical bands depending on their history of star formation. Therefore, a complete census of gaseous universe through blind surveys provides information of cosmology and galaxy evolution independent of those based on star light. One exciting development of research in cosmology is the apparent success of ΛCDM ("Double Dark" Standard Cosmological Model) simulations based on models in producing large-scale structures of dark matters. This is accomplished without knowing the actual content of neither dark matter nor dark energy. The critical test of such models and associated cosmology is to compare predicted structures to observable matters. One current mystery is the so called "missing satellite problem", i.e., the lack of detection of low mass halos predicted by dark matter simulations. Given the uncertainties in our knowledge of star formation (discussed later) and the very rudimentary treatment of star formation in simulations, the stellar content of these halos is essentially unknown. Therefore, by providing a census of HI complete to relatively low mass limit along with other large radio telescopes, FAST can significantly improve our knowledge of the origin of the universe.
Pulsars: The high sensitivity and larger sky coverage compared with Arecibo make FAST a powerful tool for detecting pulsars at large distances, such as millisecond pulsars, binary pulsars, double pulsars, extragalactic pulsars, etc. It is estimated that FAST equipped with multi-beam receivers would detect thousands of pulsars in the Milky Way Galaxy in less than a year of observing time. In such a new large scale survey, moreover, extremely interesting and unknown exotic objects may yet wait for discovery by the sensitive FAST as the telescope is put into operation. Among these discoveries, the most exciting one should be a pulsar-black hole binary, which will provide precise information of black hole. Besides this, FAST may also find sub-millisecond pulsars and pulsars that have a mass deviating from the 1.4 solar mass. This will give insight to the equation of state at supra-nuclear density, and further the strong interaction. In this way, a pulsar is a unique laboratory for studying two of four kinds of fundamental forces: gravitation and strong interaction.
Main active reflector: As a huge scientific device, the supporting structure of the radio telescope FAST demands special requirements beyond those of conventional structures. The most prominent one is that the supporting structure should enable the surface formation of a paraboloid from a sphere in real time through active control. Fortunately, the peak deviation of the paraboloid of revolution from the spherical surface is only about 0.67m across the illuminated aperture of about 300 m. An adaptive cable-net design has been proposed for two main reasons: first, the small difference required by the deformation mentioned above should be easily achieved within the elastic limit of ordinary cable wires; second, the cable-net structure should accommodate with the complex topography of karst terrain easily, which will avoid heavy civil engineering between actuators and the ground.
Figure 4: Concept of the adaptive cable-net structure, the supporting structure for the FAST reflector (image credit: NAOC/CAS)
Leading the international VLBI (Very Long Baseline Interferometry) network: Angular resolution in astronomy is defined as the angular separation of two astronomical objects whose images can just be resolved. The angular resolution is θ=λ/D, which is the reciprocal of the aperture in units of wavelength. For a radio telescope, the wavelengths used are a million times larger than those in the optical band. If we want to obtain a resolution equivalent to the optical, we would have to make a "big dish" hundreds of kilometers in diameter, or even as large as the earth, and the surface deviations of the panels need to be kept to 1 millimeter, or even less. The necessary technology does not exist. Radio astronomers have found another way to improve the resolution without enlarging the antenna aperture — the radio interferometer. This has finally developed into the present very long baseline interferometry (VLBI). Two antennas joined in VLBI can sit on different continents, giving an angular resolution of θ=λ/B. The baseline B can be as long as the diameter of the Earth, and even longer if we send antennas into space. The resolution of a modern global VLBI network is finer than a marcsec (milliarcsecond), and is three orders higher than that at other bands. The main VLBI networks in the world are the EVN (European VLBI Network), VLBA (Very Long Baseline Array, USA) and APT (Automated Patrol Telescope, University of New South Wales, Australia). The main antenna apertures are 20-40 m, the diameter of the largest antenna is 100 m. When the 500 meter aperture telescope joins in the VLBI networks, it will naturally become the dominant dish due to its huge collecting area and a favorable location at the edge of all the existing networks. By then, China will be the leading force in international VLBI cooperation.
Detecting interstellar communication signals : SETI (Searching for Extra-Terrestrial Intelligence) is usually considered to be a high-risk task. However, if it succeeds, it will overshadow all other scientific achievements of mankind. Therefore, exploration by the scientific community, and support for SETI from governmental and non-governmental organizations in developed countries have never stopped.
The only available way for communicating with civilizations on distant planets is to search for extra-terrestrial "artificial" electromagnetic signals. The non-thermal Galactic background emission, quantum noise and cosmic microwave radiation are three noise sources which exist everywhere. Engineers in extra-terrestrial civilized societies also face a similar radio noise spectrum, and they might use the same microwave window as us.
The SETI experts believe that humans should concentrate the search in a frequency range from 1-3GHz, especially between the 21 cm neutral hydrogen HI line and the 18 cm OH line. The combination of H and OH forms water, H2O, so the narrow frequency band is also called the "water hole". Since water is the basic element for life on Earth, the extra-terrestrial "water population" would probably naturally search for us through the water hole.
The Phoenix project is one of the most ambitious SETI plans. It began in 1994, and searched for microwave signals from about 1000 nearby solar-like stars, using the biggest antenna in the world. In 2006, the ATA (Allen Telescope Array), supported by American private enterprise, began partial operation. The array will consist of 350 dishes, covering an area of 300 x 200 m2. It is specially designed for SETI science, to detect interstellar communication.
Calculations reveal that if we use an omnidirectional antenna with a transmitter power of 1000 MW (for comparison, the EIRP of a typical television station is about 1 MW, and the radiated power of the most powerful transmitter on Earth is about 10 million MW), then:
The Parkes 65 m telescope in Australia could detect the signal to 4.5 light years, and it would reach only one star — α Centauri. The Arecibo 305 m telescope detection distance is 18 light years, and it could reach 12 stars. FAST could search out to 28 light years, and would be able to reach 1400 stars. If we increase the transmitter's radiated power to 1000,000 MW, Parkes could reach 5000 targets, while FAST would be able to reach a million stars.
FAST has three outstanding features: the unique karst depressions found in south Guizhou as the sites, the active main reflector of 500 m, which directly corrects for spherical aberration, and the low-mass focus cabin driven by cables and a servomechanism plus a parallel robot as a secondary adjustable system to carry the most precise parts of the receivers. Inside the cabin, multi-beam and multi-band receivers will be installed covering a frequency range of 70 MHz - 3 GHz. The telescope will be equipped with a variety of instruments and terminals for different scientific proposes. The main technical specifications of FAST are listed in Table 1. 5)
Table 1: FAST main technical specification
The FAST technical subsystems are (Figure 5):
• site exploration and earthwork
• observatory construction.
Active reflector system:
The FAST active reflector includes a main 500 meter aperture cable mesh composed of ~7000 strands of steel cables, reflecting elements, actuators, ground anchors, perimeter beam, wind-shield wall, noise-shield wall, etc. The reflecting element cable mesh is installed on the annular latticed perimeter beam. There are 2400 nodes in the network, by which 4600 reflecting panels are installed on the cable mesh to reflect the radio wave. Every node is connected with a down-tied driving cable and an actuator device, which is then connected with the ground anchor. A noise-shield wall is installed around the perimeter of the reflector, and outside is a wind-shield wall. All of these devices form a complete active reflector system. The construction of this system is aimed to build a 500 meter aperture active spherical reflector, which could realize to form a transient 300 meter parabolic dish under real time control.
The function of the reflector is to reflect the electromagnetic wave to the focus, so that the receiver can receive and record the signals. Also it can transform actively and hold the weight of the back frame, panels and wind load.
Feed Cabin Suspension System:
The feed cabin suspension system includes the following structure:
- The optical, mechanical and electronic integration first-order cable support system: 6 tower supports with height of about a hundred meters are built in the mountain around the depression. A kilometer-scale steel cable soft support system and the guide rope, cable reel are installed to realize the first-order spatial position adjustment of the feed cabin.
- The 10 m diameter feed cabin. A parallel robot is installed in the feed cabin for the second-order adjustment to realize a spatial position accuracy of 10 mm.
- The steering gear between first-order and second-order adjustment mechanisms to help adjust the attitude angle of the feed cabin.
- The power and signal channels between ground and the feed cabin.
- The safety and health monitor system: It includes lightning protection, cable stress force monitor, emergency prevention and dealing equipments.
Figure 6: Schematic view of the feed cabin suspension system (image credt: NAOC/CAS)
Measurement and Control System:
There is no rigid structure connecting the FAST reflector and focus. Therefore it requires high-accuracy measurements of their spatial coordinate in a common well-defined reference frame. Besides, all moving parts of the telescope require real-time measurement and reaction control during operations to meet the position accuracy requirement. Various modern measurement technology will be applied to the site exploration, earth work, reflector and feed cabin support, receiver and terminal systems, and the real-time detect and health monitor of the telescope during operation in future. Large dimension, high sampling rate, high accuracy measurement and control is the key points.
We plan to build a reference datum net comprised of 10 datum station with mm-accuracy. There are two most challenging tasks to be accomplished.
1) Real-time reaction control of the feed cabin: to read the 3D spatial positions of the feed cabin, four API laser trackers are used to measure 4 follow-up targets in the Stewart platform. Two API laser tracker systems are used to measure the position and attitude of the lower platform, and monitor the control results.
2) Surface scanning of the reflector: There are more than 2000 nodes, the joining points of the element panels on the reflector, and the number of nodes being illuminated is ~1000. As a plan, nine close-range instrumentations with accurate rotating platforms and digital cameras will be built to scan 1000 control nodes in the illuminated portion of the reflector during observations.
The project is also developing control technology to realize the spatial positioning of the receiver in the feed cabin.
The feeds and receivers are planned to be built through international cooperation, which cover a frequency range from 70 MHz to 3 GHz. The present 9 feeds and receivers in plan are displayed in Table 2. 7)
Table 2: Specification of the receivers
The proposed construction mainly includes:
(1) Feeds and low-noise receivers: to develop feeds and polarizer at 9 bands, LNAs, band selection filters, radio frequency circuit, frequency mixers and IF circuits.
(2) Refrigeration machine: Helium GM refrigeration machines and vacuum Dewar are used to refrigerate the head amplifier and other key instruments, especially for receiver at bands higher than 560 MHz, which requires a temperature of 10-20 K to keep low noise.
(3) Optical fiber to transform IF data: to develop a wide-band optical fiber transformation. IF data is converted to light signal, and then transform in the optical fiber over a distance of about 3 km, and convert back to IF data to the digital terminals.
(4) Digital data-processing terminals: to construct multi-band terminals for cosmic neutral hydrogen, pulsar de-dispersion, molecular lines observations, VLBI data-record terminal, SETI terminal and digital record and process terminal based on computer clusters.
(5) Receiver monitor and diagnosis system: to monitor the receiver in real-time, and test the remote trouble diagnosis to shorten the feed cabin maintenance time in harbor.
(6) The time frequency standard: this will be provided by GPS and high stationary hydrogen clock to fulfill the requirements of pulsar, spectral lines and VLBI observations, etc.
Mission status and some results
• April 30, 2018: China's FAST, still under commissioning, discovered a radio millisecond pulsar (MSP) coincident with the unassociated gamma-ray source 3FGL J0318.1+0252 in the Fermi Large Area Telescope (LAT) point-source list. This is another milestone of FAST. Fast has discovered more than 20 new pulsars so far. This first MSP discovery was made by FAST on Feb. 27 and later confirmed by the Fermi-LAT team in reprocessing of Fermi data on April 18th. 8) 9)
- The newly discovered pulsar, now named PSR J0318+0253, is confirmed to be isolated through timing of gamma-ray pulsations. This discovery is the first result from the FAST-Fermi LAT collaboration outlined in a MoU signed between the FAST team and Fermi-LAT team.
- "This discovery demonstrated the great potential of FAST in pulsar searching, highlighting the vitality of the large aperture radio telescope in the new era," said Kejia Lee, scientist at the Kavli Institute of Astronomy and Astrophysics, Peking University.
- Radio follow-up of Fermi-LAT unassociated sources is an effective way for finding new pulsars. Previous radio observations, including three epochs with Arecibo in June 2013, failed to detect the MSP. In an one-hour tracking observation with the FAST ultra-wide band receiver, the radio pulses toward 3FGL J0318.1+0252 were detected with a spin period of 5.19 milliseconds, an estimated distance of about 4 thousand light-years, and as potentially one of the faintest radio MSPs.
- A millisecond pulsar is a special kind of neutron stars that rotate hundreds of times per second. It is not only expected to play an important role in understanding the evolution of neutron stars and the equation of state of condense matter, but also can be used to detect low-frequency gravitational waves.
- The pulsar timing array (PTA) attempts to detect low-frequency gravitational waves from merging supermassive black holes using the long-term timing of a set of stable millisecond pulsars. Pulsar search is the basis of gravitational wave detection through PTAs.
- "The international radio-astronomy community is excited about the amazing FAST telescope, already showing its power in these discoveries. FAST will soon discover a large number of millisecond pulsars and I am looking forward to seeing FAST's contribution to gravitational wave detection," said George Hobbs, scientist of the Commonwealth Scientific and Industrial Research Organization (CSIRO) of Australia and member of the Gravitational Wave International Committee (GWIC).
- FAST will be under commissioning until it reaches the designed specifications and becomes a Chinese national facility.
Figure 7: Radio follow-up of Fermi-LAT unassociated sources is an effective way for finding new pulsars. Previous radio observations, including three epochs with Arecibo in June 2013, failed to detect the MSP. In an one-hour tracking observation with the FAST ultra-wide band receiver, the radio pulses toward 3FGL J0318.1+0252 were detected with a spin period of 5.19 milliseconds, an estimated distance of about 4 thousand light-years, and as potentially one of the faintest radio MSPs (image credit: NAOC/CAS)
• March 15, 2018: China's FAST (Five-hundred-meter Aperture Spherical Radio Telescope), the world's largest single-dish radio telescope, has discovered 11 new pulsars since its September 2016 inauguration, according to China's National Astronomical Observatories. 10)
• On September 24, 2016, China officially put into operation the world's largest single-aperture telescope. Scientists said they welcome their foreign counterparts for space research after first debugging the facility to ensure its best optimal performance. 11)
- Chinese President Xi Jinping sent a congratulatory letter to scientists, engineers and builders as the world's largest radio telescope was officially put into use in southwest China's Guizhou province on Sunday. 12)
- A launch ceremony was held in Pingtang County, Guizhou, for the Five-hundred-meter Aperture Spherical Telescope (FAST).
- Its launch is significant for China to achieve major breakthroughs in frontier scientific fields and to expedite innovation-driven growth, Xi said, adding astronomy is crucial to propelling scientific progress and innovation.
• On July 3, 2016, with the command given by Yan Jun, the FAST project general manager, National Astronomical Observatory director, the last reflection surface element was slowly lifting and moving to air and finally fell to the designated location on cable net. 13)
- The active reflectors are an important part of the FAST telescope, a total of 4450 blocks of the reflective panel unit, including 4273 basic types and 177 special types. Reflector unit length is 10.4 to 12.4 meters, each unit has a mass of 427 to 482.5 kg, the thickness is about 1.3 mm.
- On August 2, 2015, the FAST reflector unit lifting project started construction. A block of reflector units on the ground after assembling, measurement, inspection and strict steps form qualified units, through the tower crane, transport vehicles, cable crane and so on a series of complex high process will be every piece of units shipped to the designated location to install. To overcome the large scale and high precision assembling construction difficulties and large span, position higher hoisting construction problems, after 11 months of effort, an area of nearly 30 football fields of the reflecting surface by a block reflector units gradually laying was completed.
- Reflective surface engineering is also the last FAST equipment engineering, and its successful completion marks the successful completion of the main project of FAST.
- The reflector unit design, manufacturing and assembling tasks were done by the China Electronics Technology Group Corporation No. 54 of the consortium with Zhejiang southeast space frame Co., Ltd, while the reflector unit lifting tasks were undertaken by the Wuchang Heavy Engineering Co., Ltd..
Figure 8: On July 3, 2016, all 4450 reflector panels have been installed on FAST (image credit: NAOC/CAS)
Figure 9: Installing the last reflector panel (image credit: NAOC/CAS)
1) "China begins operating world's largest radio telescope," Gillian Wong, Sept. 25, 2016, URL: http://phys.org/news/2016-09-china
2) Matt Williams, "Now, Witness The Power Of This Fully Operational Radio Telescope!," Universe Today, July 7, 2016, URL: http://www.universetoday.com/129769/now-witness-
3) "Australian Technology Installed on Largest Single-Dish Radio Telescope,"Space Daily, Sept. 29, 2016, URL: http://www.spacedaily.com/reports/Australian_
4) Rendong Nan, Di Li, Chengjin Jin, Qiming Wang, Lichum Zhu, Wenbai Zhu, Haiyan Zhang, Youling Yue, Lei Qian, "The five-hundred -meter Aperture Spherical Radio Telescope (FAST) Project," International Journal of Modern Physics D, URL: https://arxiv.org/ftp/arxiv/papers/1105/1105.3794.pdf
8) "FAST's first discovery of a millisecond pulsar," Space Daily, 30 April 2018, URL: http://www.spacedaily.com/reports/FASTs_
9) "China's New FAST Radio Telescope -World's Largest- Makes Breakthrough Discovery," The Daily Galaxy, 29 April 2018, URL: http://www.dailygalaxy.com/my_weblog/2018/04/chinas-
10) "China's big silver bowl captures signals from 11 more pulsars," Asia Times, 15 March 2018, URL: http://www.atimes.com/article/chinas
11) Hou Liqiang and Yang Jun, "World's largest single-aperture telescope put into operation," China Daily, 25 September 2016, URL: http://www.chinadaily.com.cn/china
12) "Xi commends launch of world's largest radio telescope in China," China Daily, 25 Sept. 2016, URL: http://www.chinadaily.com.cn
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).