Murchison Widefield Array
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Murchison Widefield Array Telescope |
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| 16 antenna segment | |
| Type | Radio telescope |
| Location | east of Murchison
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| height | 380 m |
| Geographic coordinates | 26 ° 42 '11.9 " S , 116 ° 40' 14.9" O |
| wavelength | 3.8 to 100 m |
| Aperture | synthetic
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| construction time | since December 2011 |
| Installation | July 9, 2013 |
| Specialty | RF digitizer |
The Murchison Widefield Array ( MWA ) is a radio telescope in Australia . It is characterized by a large number of very simple and inexpensive antennas that are distributed over a large area in the Australian outback. In contrast to many other radio telescopes, the MWA has no moving parts. The telescope can still receive radio signals from one direction. The directional reception is done electronically.
Technical specifications
Construction phase I
The telescope of construction phase I can receive signals in the range from 80 MHz to 300 MHz. Reception is also possible in the adjacent spectral ranges , but is no longer profitable due to the decreasing antenna sensitivity.
The telescope produces approx. 50 TB of raw data per day, which are processed via a computer network . The 2048 dipole antennas are distributed in 128 groups of 16 antennas each in a square arrangement over an area of 2000 m². Some of the tiles are distributed in a relatively densely populated area, called a compact array , while another is set up at significantly larger intervals as a so-called extended array .
The analog-digital conversion of all signals takes place in the receiving band, one reason for the large amount of data. The MWA is located in the Mid-West Radio Quiet Zone , a radio protection zone with a 70 km radius, within which all radio operations are strictly regulated, if not prohibited. The next larger city, Geraldton (Australia) with 27,000 inhabitants, is a good 300 km away, which leads to a corresponding attenuation of the interference signals that usually emanate from residential and commercial areas.
Construction phase II
The electrotechnical properties of construction phase IIA and IIB are the same as those of construction phase I. An additional 128 tiles with corresponding receivers were installed. 72 of them in two compact hexagonal arrays and 56 in an expansive, irregular distribution for long baseline interferometry. The new tiles of the extended array from construction phase IIB are spread over 5 km in east-west direction and over 5.5 km in north-south direction.
With construction phase II, the number of tiles as well as the receiver were doubled. This also roughly doubles the amount of data that arises.
Construction principle
The telescope consists of a large number of individual ultra-wideband antennas . These are distributed in a pseudeo random grid over a large area (1.5 km in diameter). The antennas are rigidly mounted so that a single antenna has no variable directivity. The antennas themselves each contain two dipoles with which two linear polarizations can be detected.
Beam shaper
The directional effect of the telescope is achieved by beam formers , devices that generate a preferred direction for the acquisition of radio signals by adjusting the transit time in the measuring lines of the individual antennas. A beam shaper is used for each antenna tile with 16 antennas each. The beam formers process the 16 signals from the antennas independently of each other for the two polarizations. To set the direction, the signals are routed through a selectable combination of 5 delay lines . By controlling the delay, beam shaping is achieved which has no noticeable frequency dependence, in contrast to what would be the case with phase control . The beamformers are located directly next to the assigned antenna tiles and send their amplified analog output signal separately according to polarization to the receiver nodes . The beamformer is dimensioned so that one antenna lobe (main receiving direction) is formed per tile . Training different clubs would have been conceivable, but was not considered to be economically sensible.
receiver
The two signals from a total of 8 tiles are transmitted to a receiver node via coaxial cable . There are therefore 16 recipient nodes. The signals that have already passed bandpasses in the antenna tiles are filtered again at the inputs of the receiver nodes to suppress aliasing and constant components . These and other functions for signal processing and interference suppression in an as analog signal conditioning referred assembly included. The signals obtained therefrom, which until then no mixing (multiplication in the time domain) were subjected subsequently pass into a digitizer (ATMEL AT84AD001B) connected 655.36 Msample / s samples with 8 bit resolution quantized . The digitized signals are immediately filtered and divided into 1.28 MHz wide frequency bands , of which up to 24 are selected and processed for further transmission. The data of all the tiles connected to the receiving node are transmitted via fiber optics, but this is reduced to a third of the entire frequency band. The data volume transmitted by the receiving nodes roughly adds up to a little less than 100 Gbit / s.
Correlators
The correlators work according to the 'FX principle', which means that the signals are first transformed into the frequency range with filtering and then a correlation takes place. The filtering works with a minimum bandwidth of 10 kHz , which in turn means a high computing effort. The large number of narrowband data streams of the respective antenna tiles are then correlated in order to obtain phase and thus direction information. This involves CMAC (complex multiply and accumulate) operations on the order of 10 12 per second. This demanding task is mastered by FPGA boards. The resulting amount of data is as extensive as at the entrance of the correlator and is passed on to servers with conventional technology for further processing. Here come GPU clusters are used, which have proven to be particularly effective in the past for similar calculations.
Development and future
A test device with 32 tiles was built and tested between 2007 and 2011. In 2010 a total of 4.6 million Australian dollars were made available to build the MWA. Construction began in 2011 and after a test phase, the telescope went into official observation operation on June 9, 2013.
The system was expanded to 256 tiles by 2017. However, these cannot be operated simultaneously with today's receiver equipment, so that a suitable configuration with a focus on different operating modes must be selected in each case. An extension of the receiver and the correlator is desired, but not yet planned.
Applications and skills
A separate main receiving lobe can be generated with each antenna tile, which means that basically as many different targets as antenna tiles can be observed at the same time. However, since the antenna area of a tile is comparatively small, this use is rather the exception and not suitable for typical observations of astronomical objects. Due to the large extension of the tiles over a field of 1.5 km in diameter, baselines for synthetic apertures of this size can be generated. Like the VLBI telescope, this allows observations with a particularly high signal-to-noise ratio and angular resolution only on a smaller scale .
Beam shaping by controlled delay lines enables the telescope to be aligned without moving parts, which has the advantage of being able to do without expensive, wear-prone mechanics , but has the disadvantage that the angular resolution of the beam alignment of a tile is fixed by the gradations of the delay lines.
The large number of antenna tiles and their distribution should prove to be particularly beneficial for interference avoidance and interference suppression. Interference from terrestrial and orbital sources can be isolated, located and removed from the useful signal .
Special observation objectives
Among other things, the Murchison Widefield Array will provide information about the reionization of the universe.
Steven Tingay, a scientist at Curtin University in Perth , suggested using the MWA to locate space debris in the Astronomical Journal . According to Tangay, the radio waves from commercial radio stations from the Perth area are reflected by the rubble in space, so that these signals can be received and conclusions can be drawn about the situation in the near-earth orbit (between 400 and 2000 kilometers).
Web links
Individual evidence
- ^ The Murchison Widefield Array: Design Overview, Instrumentation and Methods for Astrophysics (astro-ph.IM)
- ↑ $ 4.6M awarded towards the Murchison Widefield Array ( Memento of the original from May 25, 2013 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. ICRAR press release
- ↑ The Murchison Widefield Array, Colin J. Lonsdale, American Astronomical Society, AAS Meeting # 211, # 11.01 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. ; Bulletin of the American Astronomical Society, Vol. 39, p.744
- ↑ Doodles against junk . In: Frankfurter Allgemeine Sonntagszeitung of December 8, 2013, page 61.
