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FAST: Scanning As we go to press, the world’s largest single-dish radio telescope has started “listening” into signals from further out in space than has ever been possible. The 1.5 billion yuan, Five-hundred-metre Aperture Spherical Radio Telescope (FAST) in Guizhou Province, China has an Australian connection: its receiver was designed and built by the CSIRO at their Marsfield laboratory in Sydney. by ROSS TESTER F ive hundred metres in diameter, the FAST Radio support not only the dish but the receiver platform – more Telescope dwarfs the old leader, the Arecibo Obser- on this shortly. First proposed in 1994, it was approved and funded vatory in Puerto Rico, by 200 metres, or 164% larger. It was built by the Chinese National Astronomical Obser- in 2007. Construction commenced in 2011 (much of the intervening period was taken up in finding a suitable site) vatory in a natural karst basin at Dawodang, Pintang County and it was completed in July this year. in south-western China. The dish, or reflector, consists of 4450 triangular panels Apart from the topography and geology of the area suiting the dish construction (only limited earthworks were made from perforated aluminium. They’re 11m on each required), it was chosen because there are no cities or even side and are connected together to form an inverted geomajor towns within 8km of the site, making it electrically desic dome. Originally budgeted for CN¥700 million (approx. $AU140 very “quiet”. This is essential for a radio telescope seeking the unbe- million), the final cost was more than double this at CN¥1.5 billion. lievably faint signals from the far reaches of space. Its acronym, “FAST” is not entirely correct. Firstly, the A small village directly at the FAST site was relocated to make room and almost 10,000 people who lived within a “F” (standing for 500m) – not all of the 500m diameter can be used (in fact, only 5km radius of the site were each about 300m can be used paid CN¥12000 (equivalent to Main specifications of FAST telescope at any one time) and the about $AU2500) to relocate. “S” (Spherical) – while To put this in persective, Item Specification the dish construction is CN¥12000 represents about a Spherical reflector Radius 300m, Aperture 500m spherical, the usable secyear’s income! Opening angle 110-120° tion is actually a parabola. Natural sink holes for drainIlluminated aperture Dillu =300m While the overall inage in the karst basin (and arguFocal ratio f/D=0.4665 verted dome is fixed in one ably the reason for the basin) Sky coverage zenith angle ±40° place, it can be (and must also influenced the location. It Frequency 70-3000MHz be) somewhat movable to is surrounded by elevated areas Multi-beam(L-band) 19, beam number of future FPA >100 be of any use (otherwise it – ridges and small mountains Slewing <10min would be limited to how – which also lent themselves Pointing accuracy 8” (200mm) much sky it could view!). nicely to the towers which 16  Silicon Chip siliconchip.com.au deepest space The parabolic dish is nearing completion with just a few triangular panels yet to be mounted on their support cabling. What appears to be the receiver is at this stage on the ground (middle of dish). The supporting structure is made from aluminium to keep the weight to a minimum, but flexible steel cables underneath the panels can push or pull on the panel joins, thus moving them into a parabolic dish and aiming it at the area of the sky of interest. Maximum deviation between the ideal and the parabola thus formed is less than 0.67m across the illuminated area. The receiver platform Suspended above the dish on six cables, connected to the towers around its edge, is a light-weight feed cabin, mounted on a Stewart Platform (a platform which itself has integrated hydraulic/servo position setting) which gives very fine positional adjustment. This is moved into position by servo mechanisms mounted on each of the six towers into the focus of the parabola. These not only provide the precision of the dish – eight arcseconds – it also compensates for disturbances such as wind motion and temperature variations. Design positional accuracy is less than ±10mm. By the way, an arcsecond (abbreviated arcsec or asec) is 1/1,296,000 of a full 360° turn – or one sixtieth of one sixtieth of one degree. That precision is absolutely required for meaningful reception. When looking for signals thousands of light years out in space, even that tiny error can mean it’s millions of kilometres off! Underneath the feed cabin is the nine-channel receiver, with the 1.23GHz-1.53GHz band around the hydrogen line siliconchip.com.au The hydrogen line Radio astronomers are very interested in one particular frequency, 1420.405751786MHz. This is the so-called “hydrogen line” (or H I line) and refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. Hydrogen is the lightest element and is believed to be one of the most widely spread elements in the universe. The microwaves of the hydrogen line come from the atomic transition of an electron between the two hyperfine levels of the hydrogen 1s ground state that have an energy difference of 5.87433µeV. Electromagnetic energy of this frequency passes very easily through Earth’s atmosphere and is one of the more promising pieces of evidence of extra-terrestrial “life” It’s also one of the most favoured frequencies used by SETI in their search for the elusive radio signals of space which may be an indication of inter-stellar communication. It was during such a search in October 1977 that a signal, believed to come from the Saggitarius constellation, was received by SETI radioastronomers from Ohio State University (USA) that was of such significance that it earned the sobriquet of the “WOW!” signal (See https://en.wikipedia.org/wiki/Wow! signal). It has never been detected since. With the significant increase in sensitivity of the FAST Radio Telescope, researchers are hoping that similar discoveries might become easier and/or more common. October 2016  17 Stages in the construction of the FAST Radio Astronony observatory in Guizhou Province, China. The site was chosen because it is a natural karst basin (karst being the dissolution of soluble rocks). (see panel P17) using a 19-beam receiver designed and built by Australia’s CSIRO as part of the Australian-China Consortium for Astrophysical Research (ACAMAR). Nineteen beams means that signals from different areas of space can be received at the same time. The working frequency range is 70MHz – 3GHz and FAST is capable of pointing anywhere within ±40º of its zenith. However, vignetting (reduction in sensitivity towards the edges) reduces the effective aperture to about 30º. What’s it looking for? Like virtually all radio telescopes, FAST is looking for a number of phenomena in the far reaches of space . . . except it is doing so with considerably increased (and unprecedented) sensitivity. Primarily, its targets include: Masers – a naturally occuring source of stimulated spectral line emission associated with stars and active galactic nuclei. These can sometimes allow distance measurement by trigonometry (not to be confused with terrestrial masers, the microwave equivalent of a laser). Pulsars – the rotating remnant of a collapsed star. The interesting thing about these is that they can form cosmic “clocks” providing ultra-stable periodic pulses (some of these are even better than the most stable atomic clocks on Earth!). Pulsars may provide detection Taken during construction from ground level looking up, this shows the supports for the movable dome panels on their matrix of triangular wire cabling. The receiver hardware is also shown, suspended from the six towers around the dome. Inset top right are some of the dish’s 4450 aluminium panels. 18  Silicon Chip siliconchip.com.au Comparison between Arecibo and FAST Arecibo Observatory Location: Puerto Rico Built: 1963 (upgraded 1977) Diameter: 305m Dish: fixed Postscript: Arecibo observatory was damaged by a 6.4 magnitude earthquake on Jauary 13, 2014 but is now back in full operation. of gravitational waves (see SILICON CHIP, April 2016). FAST is sensitive enough to look beyond our galaxy and possibly detect the first radio pulsar in another galaxy. Exoplanets – planets orbiting other stars. Some of these have at least the possibility of supporting life, so FAST may well detect radio emissions from extra-terrestrial intelligence. Hydrogen clouds – due to their sensitivity, FAST’s receiv- FAST Radio Telescope Location: SW China Built: 2011-2016 Diameter: 500m Dish: variable ers will allow examination of neutral hydrogen clouds in the Milky Way. New galaxies – similarly, FAST may discover tens of thousands of new galaxies, up to six billion light years away (a distance covering about half the age of the universe). A VLBI element? Due to its own large collecting area and geographical location, FAST may be used to complement the existing international very-long-baseline interferometry (VLBI) network (see SILICON CHIP, May 2005). FAST would increase the baseline detection sensitivity by an order of magnitude. Ground station for space missions – FAST might also be called into play for future long-distance space missions. The large collecting area would enable the downlink data rate to increase by orders of magnitude over other dishes. SETI – The Search for Extra-Terrestrial Intelligence – is a world-wide search program using unused time by computer users trying to find evidence of, well, ET! Some of the radio-telescopes which have occasional down-time feed data into SETI and it is to be hoped that FAST may be one of those. Comparison between FAST and Arecibo A close-up look at the dome housing the telescope receiver. Minute radio signals are reflected off the parabolic dome into this receiver at its focus. siliconchip.com.au The basic design of FAST is very similar to the Arecibo Observatory radio telescope in Puerto Rico. Both are fixed primary reflectors installed in natural hollows, made of perforated aluminum panels with a movable receiver suspended above. There are, however, three significant differences in addition to the size. First, Arecibo’s dish is fixed in a spherical shape. Although it is also suspended from steel cables with supports underneath for fine-tuning the shape, they are manually operated and adjusted only for maintenance. It has two additional reflectors suspended above to correct for the resultant spherical aberration. Second, Arecibo’s receiver platform is fixed in place. To October 2016  19 Early in the construction, this photo shows the infrastructure partially completed – but more importantly, the cosmos FAST will be searching. support the greater weight of the additional reflectors, the primary support cables are static, with the only motorized portion being three hold-down winches which compensate for thermal expansion. The antennas are mounted on a rotating arm below the platform. This smaller range of motion limits it to viewing objects within 19.7° of the zenith. Third, the FAST dish is significantly deeper, contributing to a wider field of view. Although 64% larger in diameter, FAST’s radius of curvature is 300m, barely larger than Arecibo’s 270m, so it forms a 113° arc (vs. 70° for Arecibo.) While Arecibo’s full aperture of 305m can be used when observing objects at the zenith, the effective aperture for more typical inclined observations is 221m. Acknowledgement: most photographs in this feature courtesy SC CSIRO and/or Chinese National Astronomical Observatory The Arecibo Message To mark the recomissioning of the Arecibo radio telescope in November 1974, a digital message was transmitted into space which was designed to (hopefully!) show anyone who received it a little about who sent it and where they (we!) came from. Dr Frank Drake, then of Cornell University and colleagues wrote a three-minute message consisting of 1679 binary digits (approximately 210 bytes) and was transmitted with a power of 1MW, on a frequency of 2380MHz. To mark the difference between “0” and “1”, the frequency was shifted up by 10Hz. 1679 has its own significance: it’s a semiprime number (ie, the product of two prime numbers – 73 and 23 – arranged retangularly as 73 rows by 23 columns). The message, was aimed at a cluster of stars some 25,000 light years away – so if it is received and decoded, any answer will not be detected for some 50,000 years (about 500,000,000,000,000,000km round trip, give or take!). What does it mean? There were seven parts to the message, shown in the colour graphic at right for clarity (the actual message was in mono). The top lines (white) show the numerals 1 to 10. The second set (purple) show the atomic numbers of hydrogen, carbon, nitrogen, oxygen and phosphorous. These elements make up deoxyribonucleic acid (DNA). 20  Silicon Chip The third set (green) show the formulas for the sugars and bases in the nucleotides of DNA. The next, white and blue, show the number of nucleotides in DNA amd a graphic of the double helix structure. Following this in red is, obviously, a man (red) including his average dimension (blue/white) and the human population of Earth (white). The yellow row is a graphic of our solar system, unfortunately not to scale because that was impossible to do – but the size of the nine planets is somewhat relative. The third planet from the left is deliberately offset to mark the planet from which the signal was sent. Finally, there is a graphic (purple) of the Arecibo radio telescope and the dimension of the transsmitting antenna dish (blue and white). Incidentally, there hasn’t yet been any reply to the Arecibo message! siliconchip.com.au