|Square Kilometre Array|
The Square Kilometre Array (SKA) will be the largest radio astronomy facility ever constructed with more than 10 times the equivalent collecting area of currently available facilities. The SKA will primarily be a survey instrument with exquisite sensitivity and an extensive field of view providing an unprecedented mapping speed. This capability will enormously advance our understanding in fundamental physics including gravitation, the formation of the first stars, the origin of magnetic fields, and it will give us a new look at the Universe in the time domain with a survey of transient phenomena.
Surveys of the entire sky measuring neutral hydrogen in the middle and early Universe will give us a probe of the Baryonic Acoustic Oscillations at various epochs in the evolution of the Universe, thus constraining the equation of state of Dark Energy. The SKA is therefore likely to give the definitive answer on whether Dark Energy is a geometric effect of the theory of General Relativity (the Cosmological Constant) or whether Dark Energy is a negative pressure of the Universe associated with a currently unknown effect of physics, perhaps quantum gravity.
The SKA will probe the hydrogen distribution at very early times in the Universe, just before, and after the first stars began to shine. This period in the history of the Universe when the the first stars lit up and began to ionise the surrounding neutral medium is called the Epoch of Reionisation. The history of the Universe at that time of the formation of the first stars from the primordial hydrogen remains poorly understood. The SKA will make a tomographic survey of the Epoch of Reionisation, giving us vast observational data to feed back into our physical models.
The SKA will help answer fundamental questions in a number of other scientific areas. Large polarimetric surveys by the SKA will provide essential data for understanding the origin of Cosmic Magnetism. High time resolution and long term timing of pulsars in a correlated Pulsar Timing Array will likely detect gravitational waves on the order of nanohertz frequencies which are impossible to probe with any other experiment. Pulsar timing will also probe the limits of validity of Einstein’s General Theory of Relativity in extremely strong gravitational fields. Ultimately, with its vast improvement in observing parameter-space, including spectral, temporal, angular resolution, and wide field imaging, and perhaps most importantly, sensitivity, the SKA will discover new populations of exotic objects, and new phenomena which are not predictable, as so many discoveries in the past with new instruments have shown. For a summary of the principal questions addressed by SKA, see Torchinsky 2010. The SKA Key Science is described in detail in Carilli & Rawlings 2004.
The SKA will be a massive network of stations of antennas operating as a synthesis imaging instrument with each station being a contributing phased-array itself. The power of the instrument rests with its computational capability, and a large number of operating modes are envisioned which variously trade-off bandwith, sky coverage, spectral and temporal resolution, depending on the science goal of the particular observation undertaken.
The SKA project is currently in the design phase, and will begin construction of the first phase in 2016. As it is a network of antennas, the SKA can begin operations as soon as enough collecting area has been constructed to make it sufficiently sensitive for interesting science. SKA Phase 1 has been defined to be 10% of the full square kilometre of collecting area, and operations are expected to begin in 2020 (Garrett et al.). The SKA Phase 1 uses currently proven technology. At the lower frequencies, SKA will use a sparse distribution of simple dipole antennas, similar to LOFAR. At the higher frequencies, the technology for SKA Phase 1 will be wide band single pixel feeds at the focus of paraboloid dishes.
While the SKA Phase 1 will use mature technological solutions, there is a program of Advanced Technology currently underway which will develop completely novel methods for radio reception and signal processing. In particular, a revolution in radio receiving technology is underway with the development of densely packed phased arrays for use either directly on the sky, or in the focal area of large collecting mirrors. These technologies promise exceptional fields of view, while at the same time sampling the sky with high angular resolution.
Phased arrays which view the sky directly are called aperture plane phased arrays. The principle behind this technology is nearly as old as radio astronomy itself, and it’s simply the summation in phase of all the contributing antenna elements. The dense phased array brings this technology to the extreme, with individual antenna elements separated by distances less than a wavelength, and with integrated on-chip circuits doing the work of the coherent addition of elements. The result is a highly efficient, and flexible receiving system, which can be composed of thousands, or even millions of individual antenna elements.
Phased arrays which are placed in the focal area of a large collecting dish are called Phased Array Feeds (PAF). In this case, the phased array system is sampling the focal spot of the telescope, and it is possible with the selection of gain and phase parameters to have excellent control over the final antenna profile, reducing sidelobes, and deselecting unwanted signals. This control improves with the number of antenna elements which sample the focal spot, but the number of elements is limited by the physical size of the receiving structure, and in general cannot be less than half a wavelength distant from one another. For telescopes with a large focal length to diameter radio (f/D), such as the Nançay Decimetric Radio Telescope (NRT) there is the advantage of a very large f/D ratio, and so there is room for many elements.
This is an introduction to SKA written for a proposal for national funding submitted in January 2011.
S.A. Torchinsky, January 2011.