Saturday, July 31, 2010

Preparing the way for the SKA international radio telescope

The design of and preparations for the €1,5-billion international Square Kilometre Array (SKA) project are being driven out of a suite of offices in the University of Manchester, in England. These house the SKA Programme Development Office (SPDO).
The SKA itself will be sited either in South Africa (in the Karoo, in the Northern Cape province, with outstations in some other African countries) or in Australia (with out- stations in New Zealand). The final decision on the siting of the instrument, which will be the biggest radio telescope ever built, is planned to be made in early 2012.
The University of Manchester, which is one of the world’s leading centres for radio astronomy, won the tender to host the SPDO. The university hosts the headquarters of both the world-renowned Jodrell Bank Centre for Astrophysics (JBCA) and of the UK’s Merlin long baseline interferometry (LBI) radio telescope network. All three agencies are housed in the same building.
The SKA project is currently managed by the SKA Science and Engineering Committee (SSEC), which is headed by a small executive committee. In addition, an Agencies SKA Group (ASG), composed of the national funding agencies which will pay for the SKA, concerns itself with the high-level policy issues of funding options, the site selection process and the short and long-term governance of the project.
“The SSEC is composed of leading scientists and engineers in the SKA collaboration who are responsible for the scientific and technical directions taken by the international project. The executive committee of the SSEC has teleconferences with the director of the SPDO every month,” explains SKA South Africa associate director: science and engineering, Professor Justin Jonas, who is one of South Africa’s representatives on the SSEC and a member of its executive committee. “The ASG liaises with the SSEC but is not in charge of the SSEC. In fact, neither the SSEC nor the ASG is a formally established legal entity. There is limited central SKA funding at the moment; development work is for the most part funded by institutions across the globe from local sources of money. This is expected to change within a year or two to a more centralised funding system.”

“Our office was formally set up on January 1, 2003. We now have 16 people and our job is to coordinate the international effort in developing and maintaining the science case and coordinating the systems design for the SKA and carrying out the characterisation of the two candidate sites for the SKA,” reports SPDO director Professor Richard Schilizzi. (Schilizzi was born in the UK, brought up in Australia, worked for many years in the Netherlands and recently returned to work and live in Britain.) “Our budget is currently about €700 000 a year, which comes from a common fund contributed by the SKA partner institutions and agencies around the world, plus from the European Union seventh research framework programme (or FP7) project, PrepSKA.” (PrepSKA is an acronym for the Preparatory Phase of the SKA).
“The process by which the final site decision is reached is still to be formally established. But the contours are becoming clear,” he adds. “It’ll involve a period in the latter half of this year and the first quarter of next year, in which the selection criteria are determined and the weights to be given to those criteria agreed on.” This will all have to be finished by the end of March 2011, because that is when the characterisation of the two sites (in South Africa and Australia) shortlisted for the SKA is set to be concluded.
This site characterisation – which includes determining the radio frequency interference (RFI) at both sites, using identical equipment – is being funded under FP7 PrepSKA. The RFI measuring systems are housed in a South African-designed container and include an Australian spectrometer, and have undergone acceptance trials in South Africa, with a combined team of SPDO, South African and Australian engineers.
A lot of other work is also going on. Under FP7 PrepSKA, there are seven work packages.
These are: Management, Systems Design, Site Characterisation, Governance, Procure-ment and Interactions with Industry, Funding Options and the Implementation Plan (including discussion of the nonscience benefits of the SKA).
The Governance, Procurement and Funding work packages are categorised as policy work packages and are led by funding agencies in individual European countries – Governance by the Netherlands, Procurement by Italy and Funding by the UK. For example, under the Governance work package there is a working group looking at potential legal entities to own and operate the SKA. There is currently a partnership agreement between the SKA programme members but this runs out next year, and could either be renewed or replaced.
“We’re certainly making good progress. We’re working to a schedule partly dictated by the FP7 funding from Brussels,” assures Schilizzi. “Just recently, we agreed on the baseline design for phase one. We’re about to start looking at the configuration of the telescope – how we would put down the SKA at either site. We plan to have a costed system design for the telescope by the end of 2012.”
Both South Africa and Australia are developing precursor telescopes for the SKA. Both will take the form of arrays of dish antennas. (It should be noted that the selection of the site for the SKA, and selection of the technologies to be used in the instrument, are completely separate issues.)
The South African project is designated MeerKAT and comprises up to 80 dishes and is expected to start doing science in 2013. Its prototype, the seven-dish MeerKAT Precursor Array (MPA – also known as KAT-7), in the Karoo, is now operational for engineering tests.
The Australian project is named Askap (Australian SKA Pathfinder) and will comprise 36 dishes. The first of these is now operational, and Askap is also expected to be finished and fully operational in 2013. Both these instruments will provide input for the final design of the SKA. However, the SKA will not merely be a very large MeerKAT or Askap.
The latest baseline design for the SKA involves three compact arrays, or cores, each containing a separate array. One would be an array of dish antennas and the other two would be aperture arrays, consisting of many small ‘elemental’ antennas, arranged in groups of tens, hundreds or even thousands of elements. One of the aperture arrays would be optimised for long wavelengths (this being the sparse aperture array) and the other for shorter wavelengths (the dense aperture array). The aperture arrays are fixed in position but can be steered electronically to point at different parts of the sky and can even point in more than one direction simultaneously. (Previously, the concept had been a single core with planar arrays in the centre, surrounded by a broad ring of dishes.)
“Each of these core arrays will be about 5 km in diameter,” states SPDO international project engineer Professor Peter Dewdney, a Canadian. “This configuration was formally adopted only recently by the SSEC. Each core will house a different technology . . . because each technology covers a different range of wavelengths (and, so, frequencies). For the SKA, we’re talking about wavelengths of about 4 m to about 3 cm – it could be 2 cm. We call the antennas ‘receptors’ and no single receptor design can cover this entire range of wavelengths. But covering all these wavelengths will allow the SKA to examine a very wide range of celestial objects and phenomena.”
In the sparse aperture array, the antennas will take the form of two crossed dipoles. These – and other planar antennas – have very small electrical voltages induced in them by the incoming radio waves. The signals from the array elements are combined digitally to produce a usable signal. Recording, pro- cessing and analysing these signals provides information about the source of the radio waves. The sparse aperture array will cover the wavelengths ranging from 4 m to 1 m, and prototypes of these antennas are expected to start operation in 2012 or 2013.
The dish array will cover the wavelength range from 1 m to 3 cm or even 2 cm, if the dishes can be manufactured accurately enough. “We have avant garde dish designs – avant garde in several respects, from both the manufacturing side and the electronics side,” highlights Dewdney. “But the principle of all dish designs is the same – they’re basically radio mirrors which concentrate incoming radio waves at a point we call the focus of the dish.”
Regarding manufacturing, radio telescope dishes have traditionally been made from metal, but, for the SKA, composite dishes have been developed and are being tested. “Composite dishes are being pioneered simultaneously and separately by South Africa and Canada,” he highlights. “The scale and accuracy involved in the production of composite dishes are unprecedented.”
Canada is using carbon fibre to produce both the dish and its support structure, while
South Africa’s composite dishes (which do not use carbon fibre) use both metal and composite support structures. To date, Canada has built one 10-m-diameter composite dish, while South Africa has built eight – the single 15-m Experiment Demonstrator Model (XDM), at the Hartebeesthoek Radio Astronomy Observatory, in Gauteng province, west of Pretoria, and the MPA’s seven 12-m dishes.
But composite dishes do not reflect radio waves by themselves, so they must be fitted with a metallic radio reflecting surface in order to work. “The XDM dish uses a flame-sprayed aluminium surface. A layer of aluminium powder was sprayed on to the mould and the composite material and laid on top,” explains SKA South Africa (SKA SA) associate director Anita Loots. “This is quite labour intensive but it gives the dish a solid reflective surface, which means no avoidable restrictions on the wavelengths it can receive.”
The Canadian and the seven South African MPA dishes, however, have metallic meshes embedded in them to reflect radio waves. This approach is easier and cheaper than that used for the XDM. Unfortunately, the size and shape of the mesh limits the radio frequencies the dishes can receive. So it is essential to determine the optimum mesh design to achieve maximum dish performance. Thus, the first three South African MPA dishes each have a different design of mesh embedded in them, while the Canadian dish uses a fourth design.
“There is quite a lot of research and develop- ment (R&D) still to be done on the dishes,” says Loots. “Only when we have completed the test programme will a decision be made on which approach to use for MeerKAT. But ours (the MPA) is the first composite array in the world that actually works.”
The Canadian dish is likely to be used in the SKA Dish Verification Programme led by the US Technology Development Programme (TDP). The TDP antenna is expected to be a prototype for the SKA dishes and will be built and tested in the US.
“The composites industry is developing rapidly,” points out Dewdney. “New composites are developed all the time. Strength to weight ratios are improving, costs are coming down and the chemistry is improving.”
For Askap, the Australians plan to use 12-m-diameter metal dishes, saving on costs by having them manufactured in China. All the dish designs proposed for the SKA will be examined in the Dish Verification Programme, the outcome of which will determine which design is adopted for the SKA.
Whatever dish design is chosen, it is intended that they will be capable of accommodating phased array feeds. “Like the aperture arrays, phased array feeds will be able to observe in several directions at the same time, like a multipixel camera, whereas traditional dishes can look in only one direction at a time with the equivalent of a single pixel,” explains Dewdney. “Phased array feed technology is being pioneered in Australia, Canada, the Netherlands and the US. A group in the Netherlands has actually demonstrated astronomical capability with phased array feeds using a prototype on the Westerbork Synthesis Radio Telescope.”
Dense aperture arrays are more complicated to develop than sparse arrays because the electrical voltages induced in their antennas interact with each other because they are closely packed together. Characterisation of this behaviour and its calibration will determine the performance of the dense array.
“Alternative technologies for the dense aperture arrays are being developed in the UK and the Netherlands. While work on sparse aperture arrays is pretty much global, dense aperture development work is con- centrated in Europe,” he states. (For infor- mation on the British development work, see Engineering News, July 9, 2010.)
For all the different types of antennas, there are two primary parameters that have to be met: the best possible efficiency in trans- ferring the incoming radio waves to electrical signals, and low instrumental noise (inter- ference). Digital systems, as will be used in the SKA, generate significant amounts of radio frequency interference. It is of paramount importance that measures are taken to contain this interference and not allow it to feed back into the antennas and dwarf the signals which the telescope seeks to detect.
It is expected that R&D on the dense aperture array and the phased array feeds for dish antennas will advance sufficiently to allow a decision to be made on their use during 2016.
MeerKAT, Askap and (hopefully) the TDP are not the only SKA precursors/pathfinders, nor are all SKA precursors/pathfinders concerned mainly with antenna and signal processing technology. In the UK, a major upgrade of the country’s Merlin (Multi-Element Radio Linked Interferometer Network) LBI network will also contribute to development of key SKA technology.
Interferometry involves using two or more radio telescope dishes (or other antennas) to look at the same object in the sky. The signals received by each dish are fed into a com- puter and, because the dishes are not in exactly the same place (even if they they are only a few tens of metres apart), the distance travelled by the signals to each is not identical and combining them creates an interference pattern that can be analysed by computer to provide high-resolution images of celestial objects.
The distance between the dishes is called the baseline. With long baseline interferometry, this baseline can be hundreds of kilometres long. With very long baseline interferometry (VLBI), the baseline can reach thousands of kilometres long. VLBI can involve radio tele-scopes in different continents. It can even involve radio telescopes on earth and in space.
Merlin is a permanent LBI network involving seven radio telescopes at six locations across England. The Merlin radio telescopes are also frequent participants in European VLBI network operations, where the baselines stretch across Europe.
Merlin’s heart is the Jodrell Bank observatory, in Cheshire, which has two of the network’s radio telescopes – the 53 year old, 76-m-diameter, 3 500-t mass Lovell (also known as the Mark 1A) instrument, which is the world’s third-largest fully steerable radio telescope, and the elliptical Mk 2 dish, which measures about 32 m along its main axis. The Jodrell Bank observatory is also the location of the Merlin correlator system. (A third, small, radio telescope at Jodrell Bank is not part of Merlin). The other five instruments in Merlin are at (from west to east) Knockin, Darnhall, Pickmere, Defford and Cambridge.
“The quality of the results in LBI is very highly dependent on the phase coherency of the individual elements in the network – every telescope has to observe the same source with the same timing and frequency phase reference,” explains JBCA digital systems engineer Chris Shenton. (He is also the digital systems engineer for UK PrepSKA). “To achieve this, we distribute a centralised reference frequency from Jodrell Bank, which is generated by a maser.” ‘Maser’ is an acronym for Microwave Amplification by Stimulated Emission of Radiation, and a maser produces a narrow beam of monochromatic coherent radiation.
Originally, in Merlin, this reference frequency was distributed to the other radio telescopes, and analogue signals from each telescope were transmitted back to Jodrell Bank through a UK-wide network of microwave links. This microwave network is now being replaced by a fibre-optic network in an upgrade called e-Merlin.
“With the Local Oscillator distributed over the fibre-optic network, we can achieve a very precise wide area timing distribution, with picosecond (0,000 000 000 001 of a second) resolution,” reports Shenton. “This is done using a centralised timing reference and local timing facilities. The central reference is used to periodically recalibrate the local timing references. e-Merlin will show one possible solution to the problem of how to carry out wide area timing and frequency phase distribution in the SKA.” This will be essential for the SKA, with its thousands of antennas and its tens of outstations.
So far, four of the Merlin radio telescopes have been linked by fibre optic cables, two of which are now also capable of receiving their timing via fibre. “We’ve already been running SKA-related experiments,” he highlights. “With the Darnhall radio telescope, which is some ten miles (17 km) south-east of Jodrell Bank, we successfully replaced the microwave link mechanism for transferring the reference phase with the fibre-optical link and demonstrated that the quality of the new link is both far superior and much more robust than the microwave link.”
Another experiment saw the successful testing last year of a small-scale direct digital receiver, which directly converted mid- frequency radio waves into digital signals. Currently, at many radio telescopes, high- frequency signals received are converted into a lower intermediate frequency (IF) before they undergo digital conversion. “We want to eliminate IF and have direct conversion to digital, because using IF has cost and complexity implications,” says Shenton, “We’re trying to simplify the system."
Edited by: Creamer Media Reporter


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