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Radioastronomie Basses Fr ́equences
Ecole CNRS de Goutelas XXX (2007)
Edit ́e par P. Zarka et M. Tagger
The Square Kilometre Array and the SKA
Design Studies
S.A. Torchinsky
Observatoire de Paris, 5, Place Jules Janssen, 92190 Meudon,
France
Abstract. The Square Kilometre Array (SKA) is the future ra- dio astronomy instrument which will deliver an order of magnitude
improvement in sensitivity compared to current radio astronomy
facilities. By taking maximum advantage of the latest digital tech- nology, the SKA is particularly well suited to extremely large sur- veys, including a number of experiments targeted at understanding
Dark Energy. The SKA project is described, with emphasis on the
European contributions.
R ́esum ́e. Le grand radiot ́elescope du futur « SKA » est pr ́evu
d’ˆetre un r ́eseau d’un kilometre carr ́e (Square Kilometre Ar- ray). Ceci d ́elivrerai un am ́elioration d’un ordre de grandeur par
rapport aux radio t ́elescopes existants. En profitant pleinement
des avanc ́es en technologie num ́erique, le SKA sera parfaitement
adapt ́e pour faire des tr`es grands relev ́es, y compris des ́etudes par- ticuli`erement con ̧cues pour ́eclairer le myst`ere de l’ ́energie sombre.
Ici, on pr ́esente le projet SKA avec l’accent sur les contributions
europ ́eens.
Table des mati`eres
1. Introduction 2
2. Dark Energy Experiments 2
3. The Square Kilometre Array Design Studies 3
4. Acknowledgements 6
1
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2 Ecole CNRS de GoutelasRadioastronomie Basses Fr ́equences
1. Introduction
The Square Kilometre Array(SKA) will be an enormous radio astro- nomy facility with first-light of the completed array expected by 2020, and
initial operations beginning in 2015. The SKA will be a massive array of
many receiving elements, and it will thus have both exquisite sensitivity
and angular resolution, as well as an ultra wide field of view. The fully
sampled field-of-view, of the order of 200 square degrees, makes the SKA
effectively a 10-gigapixel ultra wide field spectroscopic radio camera.
Survey science will especially benefit from this instrument. Unders- tanding Large Scale Structure, and Dark Energy in particular, is a key
science driver of the SKA. This is one of a number of Key Science projects
which include the nature and origin of Cosmic Magnetism ; the detection
and evolution of the first luminous objects during the Epoch of Reionisa- tion ; testing Einstein’s theory of General Relativity in the extreme field
limit ; and tracing the processes which lead to life on Earth including
planet formation, extra-solar planets, organic molecules, and perhaps in- dications of extra terrestrial intelligence (see Carilli & Rawlings(2004)
for more details).
The SKA is a global collaboration, currently including 17 countries
worldwide. There are two shortlisted sites for the instrument, both in
radio quiet zones with very low population. These are the Karoo desert
in South Africa, and the Mileura desert region in Western Australia.
The final decision for the site will be made in 2010, with construction
beginning soon thereafter.
2. Dark Energy Experiments
The spectroscopic capabilities in an ultra wide field of view allow
the SKA to make a billion galaxy redshift survey out to z=2. An initial,
all-sky survey ca be completed within a year, and will be used to detect
Baryonic Acoustic Oscillations (BAO) in the galaxy distribution, leading
directly to a measure of the Dark Energy equation of state parameter.
The SKA has the great advantage of simultaneously mapping the galaxy
sky positions and redshift, by detecting the λ21 cm electron spin-flip
transition of neutral hydrogen. The simulaneous mapping and spectro- scopy eliminates the systematic errors which limit the optical surveys
using photometric redshift estimates in follow-up observations. A review
of current and planned BAO experiments, as well as the relevance to
Dark Energy has been done in this volume by Abdalla (Abdalla (2007),
and see also Blake et al.(2004)).
A continuum survey complements the BAO spectroscopic survey,
and will provide a large database for a statistical weak-lensing survey,
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The Square Kilometre Array and the SKA Design Studies 3
with the goal of measuring Cosmic Shear (Zhang & Pen(2005), Zhang &
Pen(2006)). Over 10 billion galaxies are expected to be catalogued Blake
et al.(2004)), and large, clean, sample can be extracted. Galaxies with
intrinsically stretched morphologies, for example due to starbursting, can
be removed from the sample, while still retaining a very large sample,
distributed throughout the sky. By further dividing the sample into red- shift bins, the evolution of Cosmic Shear can be determined as a function
of redshift (Blake et al.(2004)). Statistics on gravitational microlensing
traces the intervening mass distribution, and the combination with red- shift evolution adds the ability to measure the angular diameter distance
contribution to the lens equation, effectively probing Dark Energy.
A programme of proper motion studies of galaxies will provide a
precise measurement of the local Hubble constant H0. With its high
sensitivity, and angular resolution using baselines of up to 3000 km, the
SKA will monitor proper motion of extragalactic maser sources over a
period of years, and eventually decades. A large sample of sources over a
moderate timescale of several years will already provide ∼ 1% precision
on H0. Such precision provides a robust constraint on the estimation of
the equation of state of Dark Energy (Blake et al.(2004)).
If it turns out that the accelerated expansion of the universe is due,
at least partly, to a negative vacuum energy, and not entirely a purely
geometrical factor in General Relativity (ie. the Cosmological Constant
with w ≡ 1), then one can consider measuring the sound speed of Dark
Energy (Torres-Rodr ́ıguez & Cress(2007)). Such an experiment is possible
with the SKA, and can measure differences from a w = 1 model to the
10% level.
3. The Square Kilometre Array Design Studies
The Square Kilometre Array Design Studies (SKADS) is a Euro- pean Community project funded by the EC Framework Programme 6. It
was proposed in 2004, and formally began on 1 July, 2005 with a total
duration of 4 years. SKADS brings together the science and technology
development in the partner institutes with the overall aim of producing
a designed and costed SKA which is well matched to the requirements
of the SKA scientific goals. SKADS is funded at the level of 38Me with
nearly 11Me provided by FP6, and the rest by the partner national
funding agencies.
Much of the scientific interest in the SKA within Europe is related to
science using very large surveys. On the technological side, this capability
is best provided by the concept of the Aperture Plane Phased Array
(Faulkner et al (2007)). Thus, SKADS focuses on the development of the
Aperture Plane Phased Array which exploits fast digital technology to
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4 Ecole CNRS de GoutelasRadioastronomie Basses Fr ́equences
Figure 1.: An artist’s conception of a SKA station. The Aperture-Plane
Phased-Array occupies a large area, with many small receiving elements
viewing the sky directly. This technology provides a very large field of
view, and digital technology permits are large number of synthesised beams
to fully sample the field of view with high spatial resolution. (image by
Xilostudios)
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The Square Kilometre Array and the SKA Design Studies 5
make a flexible, ultra-wide field, multitasking telescope that can do many
different astronomical observations all at the same time.
SKADS aims to prove the technical feasibility of the aperture array
concept by building and testing prototypes. Three demonstrators are
under construction : BEST, EMBRACE, and 2PAD.
The Basic Element for SKA Training (BEST) focuses on testing
components and algorithms. The validity of some concepts that are at
the heart of the SKA philosophy are being verified using a part of the
large Northern Cross Radiotelescope in Medicina, Italy. The technique of
multi-beaming must be proven, and this involves electronically creating
multiple pixels in the field-of-view of a single parabolic receiver. Adap- tive beam forming will also be studied which involves combining signals
from separate receivers with the proper phase delay in order to create a
given pointing direction. Finally, BEST is developing and testing algo- rithms for mitigating and possibly eliminating man-made radio frequency
interference.
The Electronic Multi-Beam Radio Astronomy Concept (EM- BRACE) is the demonstrator of the aperture-plane phased-array concept
which is the main focus of SKADS technology development. The EM- BRACE project is led by Parbhu Patel at ASTRON, and will have de- monstrators built at Westerbork in the Netherlands, and at Nan ̧cay in
France. EMBRACE will be a single-polarisation telescope covering a fre- quency range from 500-1500 MHz, with a ±45◦
scan angle. There will be
a total collecting area of 300 m2 at the Westerbork site, and a further
100 m2 at Nan ̧cay. The individual receiving structures in each 1 m2
tile
are Vivaldi antennas. Each tile has 64 elements placed in parallel rows,
and a dual-polarisation tile is being developed which uses a novel method
for mechanically interlocking the individual antenna elements.
The combination of signals from the 64 antenna elements is done in
an integrated analog circuit called a beamformer chip. This is where the
phase shift is applied in order to vary the scan range. Different imple- mentations of the beamformer chip are under development at ASTRON
and at Nan ̧cay.
The entire EMBRACE development maintains a focus on cost, as
well as on performance. EMBRACE components are designed with em- phasis on reproducability, and the ease of mass production. The SKA
will be an instrument composed of tens of thousands of antennas and
tiles, each with large numbers of the same components. Mass production
is essential for making an affordable SKA.
The ultimate capability of an aperture-plane phased-array is reali- sed with the development called 2PAD : the Dual Polarisation All Digital
aperture array tile. This concept exploits digital technology to the fullest
extent. The signal from the sky is digitised right after reception at the
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6 Ecole CNRS de GoutelasRadioastronomie Basses Fr ́equences
Vivaldi antenna element, and from then on, only digital electronics are
used. This concept promises unpredecented flexibility and performance
for a telescope, limited only by the computer power and speed of data
transfer. It remains to be demonstrated that low system noise level, power
consumption, and feasible data transfer rates can all be achieved at an
affordable cost, but the fully digital solution provides the maximum pos- sibility for simultaneous observing, post-analysis of transient signals, and
virtually instantaneous telescope repointing. The SKA based on 2PAD
technology would truly be a software telescope, limited only by computer
processing power.
The Science Simulations are at the heart of the effort in SKADS. Wi- thin this context, the science drivers of the SKA are modelled in detail
and they ultimately provide the technical specifications for the instru- ment. The core activity is the pure sky simulation which feeds into the
telescope and network simulators. These tasks, in turn, take input from
the technical development as well as the measured behaviour from pro- totype components. The result is a virtual observation which is analysed
to see if the science goals can be met. Requests for specific areas of im- provement are passed-on to the technical development, and this process
is iterated. In particular, the SKADS Design & Costing (Alexander et al.
(2007)) effort is the focus of the science and technical interactions.
4. Acknowledgements
SKADS is supported by the European Community Framework Pro- gramme 6, Square Kilometer Array Design Studies (SKADS), contract
no 011938. The author is honoured to acknowledge full support from
FP6.
R ́ef ́erences
Abdalla, F.B., “Baryonic acoustic oscillations : past, present and
future”, this volume
Alexander, P., Bolton, R.C., Faulkner, A.J., Torchinsky, S.A., et al,
“SKADS Benchmark Scenario - Design and Costing” SKA Memo
#93, http ://www.skatelescope.org
Blake, C. A., Abdalla, F. B., Bridle, S. L., & Rawlings, S. 2004, New
Astronomy Review, 48, 1063
Carilli, C. L., & Rawlings, S. 2004, New Astronomy Review, 48, 979
Faulkner, A.J., Jones, M.E., Alexander, P., Kant, G.W., Patel, P.D.,
Picard, P., Keller, R., Montebugnoli, S. “Design Considerations for
an SKA Quality, Cost Effective Aperture Array” 2007 Proceedings
of EMTS, Ottawa, Canada
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The Square Kilometre Array and the SKA Design Studies 7
Greenhill, L. J. 2004, New Astronomy Review, 48, 1079
SKA website : http ://www.skatelescope.org
SKADS website : http ://www.skads-eu.org
Torres-Rodr ́ıguez, A., & Cress, C. M. 2007, MNRAS, 376, 1831
Xilostudios Videoproduzioni http ://www.xilostudios.com
Zhang, P., & Pen, U.-L. 2005, Physical Review Letters, 95, 241302
Zhang, P., & Pen, U.-L. 2006, MNRAS, 367, 169