Update on the Square Kilometre Array Project

Russ Taylor and Sean Dougherty

Over the past several years an international initiative has been underway to develop the technology required to construct a giant radio telescope array with collecting area of one million square metres -- 100 times the collecting area of the Very Large Array. The Square Kilometre Array will have spatial resolution of millarcseconds, a field of view of a square degree, and the ability to simultaneously image large portions of the Universe in both solid angle and redshift. With such an increase in capability over current instrumentation, the cold universe will be revealed - from before the dawn of galaxies up to the present day - and provide an instrument for addressing some of the most compelling questions in modern astrophysics. It will be possible to trace the origins of structure, measure the conditions of the pre-galactic universe through the era of reionization, and trace the interstellar medium of galaxies and the intergalactic medium through the subsequent cosmic evolution of galaxies and large-scale structure. Modern technological advances make the realization of such a telescope a possibility in the post-2010 era.

The International SKA Organization

Like ALMA, the SKA will be one-of-a-kind world observatory costing close to $1 Billion. Thus, the SKA project has been devised from the outset as a cooperative international initiative. The international SKA consortium currently consists of representatives from fourteen countries in North America, Europe, and the Asia-Pacific region. These countries are signatories to the Memorandum of Understanding to cooperate in research and development toward a design for the SKA, and to develop an international organization to promote and construct the SKA. The consortium has established an International SKA Steering Committee (ISSC) with representatives from all the signatories to the agreement. Currently, the ISSC has 18 members. Canada has two members on the ISSC, Peter Dewdney and Russ Taylor. Russ Taylor also serves as the founding Executive Secretary of the ISSC. International interest in the SKA continues to grow. In the past year, three more countries have become official observers on the ISSC (Japan, South Africa, Russia), and within the coming year it is expected that the ISSC will be expanded to allow these countries to formally join the consortium. The ISSC has recently appointed a founding Director of the International SKA project. Richard Schilizzi, formerly the Director of the Joint Institute for VLBI in Europe, took up this position on January 1, 2003.The ISSC has established three international committees

1. The International SKA Science Advisory Committee (ISAC)
2. The International SKA Engineering Management Team (EMT)
3. The International SKA Site Evaluation and Selection Committee (SESC)

These committee have in turn established several working groups with representation from the international partners. Detailed information on the SKA project, including the international structure, the MOU, activities, timelines, memoranda etc. can be found on the International SKA web site at www.skatelescope.org.

The International SKA Development Timeline

The current internationally agreed SKA development timeline is depicted in the figure below. The process for the SKA facility definition (technology selection) leads to a final selection in 2007 with the first "down-select" of the current technology concepts taking place in 2005. The goal of the consortium is to have a complete, detail design proposal in place toward the end of the decade. The first opportunity for major capital funding is circa 2008 in Europe (as part of the European 8th framework) and in Australia. Capital funding opportunity in the US is expected after 2010 in response to the 2010 US Decadal review and the ramp down of ALMA capital. In the US, the Expanded VLA is seen as a technological and scientific step toward the SKA. Major SKA construction could therefore begin as early as 2010 with funding initially from Europe and Australia, and somewhat later from North America.

The figure above also illustrates the process for selection of the site for the SKA. Letters of interest for hosting the SKA central site have been received from Australia, US, Europe, China, Brazil, Argentina, South Africa. The site evaluation process is now beginning with an invitation for detailed white papers from each potential host due next May.

The SKA and Canada

Canada is one of the leading countries in the SKA project. We were active at the very earliest stages of the project, beginning almost 10 years ago when the concept was born within an URSI working group. We have a strong role in both the technological, scientific and organizational aspects of the project. In addition to our two members on the International Steering Committee, there are several Canadians formally involved in the SKA international structure. Russ Taylor served as the founding chair of the ISAC and currently serves as past-chair. Stephanie Cote served of the ISAC from 2000 to 2002, and has been replaced by Sean Dougherty who is the chair of the ISAC working group on the Life Cycle of Stars. Bruce Veidt serves on the EMT. Large International meetings of the project occur once each year, where Canada is well represented with typically between 6 and 10 attendees. The 2004 international SKA meeting will be hosted by Canada, at a location to be determined. A Canadian SKA Steering Committee has just been formed by the CASCA Radio Astronomy Committee. The membership of the SKA Steering Committee is shown in the table below.

The Canadian Large Adaptive Reflector

The Large Adaptive Reflector (LAR) is a concept for the SKA being developed in Canada, led by a group at NRC HIA and includes university and industry collaborators. The aim is to design a low-cost, large diameter radio wave reflector that will addresses the science goals of the SKA.The major design features or the LAR are:

1. A very wide frequency range - from ~100 MHz to 10?s of GHz, nominally 22 GHz, but potentially even higher.
2. A low cost reflector. With an f/D ~ 2.5 and a 200-300m diameter reflector the reflector surface essentially is flat. This permits spreading the weight of the reflector over a large area, similar to the roof of a building. This keeps the costs down compared to traditional reflector designs where the weight of the entire reflector is supported by the much smaller area of the "mount".
3. An aerostat-supported focus platform. Focal lengths of order a few hundred metres prevent using a conventional feed support design. The LAR feed is held in place by a tension-structure of six tethers, tensioned by the lift of a helium-filled aerostat - a stiff structure that effectively resists wind forces.
4. A large instantaneous field of view. The beam size (or field-of-view) of a reflector at a particular wavelength is determined by the diameter of the reflector. However, by using a focal-plane array feed that provides a large number of beams, a much larger field-of- view can be imaged, limited only by the size of the focal-plane array. The array feed of the LAR will be capable of imaging a field of view of 1 square degree at 1.4 GHz using 350 independent.

These design features are aimed at addressing the full range of SKA science goals, in particular deep spectral and broad-band imaging over large areas of sky. Such capabilities are required in order to study the evolution of neutral gas, at large scales for determining the structure of the universe over large angular scales, and on smaller scales for studying the structure and evolution of galaxies, from the epoch of re-ionization to the present day.The LAR is an ideal instrument for addressing this science for three reasons. Firstly, it is a large fully-filled aperture, which has a higher brightness temperature sensitivity than an array of smaller apertures spread out over a similar area (unfilled aperture), making the LAR an excellent concept for observation of the lowest brightness temperature emission at high redshifts. Second, with its large instantaneous field-of-view, highly efficient surveys of large areas of sky are possible. Lastly, its very broad band capability, from around 100 MHz up to 10's of GHz it is possible to study high redshifted hydrogen at redshifts z ~ 7 (~100 MHz) as well as CO J=1-0 at redshifts in excess of 4.2 (<20GHz), and opens up the area of thermal emission science (> 5 GHz).There is a lot of effort underway to demonstrate the technologies required for the LAR concept. This work can be broken down into four main areas:

2. Reflector design. A concept for the reflector design has been developed at DRAO in conjunction with a group led by Prof. Sigi Steimer at UBC as part of a student MSc thesis. The design is based around light-weight steel panels that can be supported by a simple support structure, very similar to the trusses used in building construction (check out the roof at your local supermarket to get the idea!). This makes the whole reflector structure very light, and hence easy to support on a series of actuated legs. Implementation of this design is being investigated at AMEC Dynamic Systems of Port Coquitlam. A test section of reflector will be available for control and RF evaluation later this Summer.
3. Feed design. The prime focus feed of the LAR will consist of a large array of "Vivaldi" antennas. Vivaldis have the very attractive property of very large bandwidth performance - certainly 2:1, and potentially as high as 5:1. The RF performance of such an array is being investigated at DRAO, in conjunction with Ed Reid, a PhD student at U of Alberta, under the supervision of Dr. Dave Routledge. This work is also studying how to best couple the feed to the receivers.
4. Feed platform. Several concepts for the structure that supports and "points" the feed plate have been developed at DRAO. The complete feed package will need to be lightweight and designs using exotic materials such as carbon fibre are underway. Control of this platform is also being investigated jointly at DRAO and McGill.

The LAR as the technology solution for the SKA

Over the past year considerable effort by the international consortium has gone into a critical technology review to a) identify fundamental technology roadblocks for the technologies that are under study and b) match the capabilities of the technologies against the scientific goals of the SKA. This review took for the form of "white papers" prepared by each of the groups developing SKA technologies (Third box in the timeline figure above). The white papers were discussed within a large forum at the latest international SKA project meeting in The Netherlands, and were also critically reviewed by the International SKA Science Advisory Committee and the Engineering Management Team. The white papers and the reports of the ISAC and the EMT are posted on the SKA International web site at www.skatelescope.org/ska_memos.shtml.No fundamental engineering issues for the LAR were identified by the EMT. In terms of scientific capability, the LAR remains the only concept that can provide the full range of frequency coverage in the current SKA specifications. The ISAC has identified and agreed on 18 Level 1 science drivers for the SKA and prepared a compliance matrix measuring the requirements for these science drivers against the SKA technology concepts. The matrix is a dynamic document. The latest version can be found at www-astro.physics.ox.ac.uk/~sr/ska/ska_matrix.html. The Canadian and US concepts meet the largest set of science drivers. The Chinese and European concepts have a fundamental high frequency limit that rules out a large range of key science. The Indian concept is the similar to the US concept, using very inexpensive antennas constructed using the GMRT design. Currently, staff at DRAO are carrying out imaging simulations of an array of LARs to address some critical questions concerning the imaging capability of a 60-element array of LAR's required for the SKA.The Canadian LAR concept remains a strong contender for the SKA provided we can continue to develop the technology. Several countries have plans for ?demonstrator? antennas. With so many new technologies involved in the LAR concept a proto-type will be required to demonstrate the feasibility of the LAR concept for the SKA design. In Canada, the goal for such a demonstrator is a 300-m reflector working up to 1.6 GHz and with a feed system that will be capable of instantaneously imaging 0.5 square degrees. Such a telescope will be the larges steerable radio telescope in the world and will show case all the fundamental technologies required for the LAR. Most importantly, it will be able to tackle many of the cutting-edge experiments related to low surface brightness HI targeted for the SKA. One goal of the current development work is to arrive at a detailed costing and design for an LAR prototype.