Radio astronomy transformed: Aperture arrays - Past, present & future

Abstract

I review the early development of Aperture Arrays and their role in radio astronomy. The demise of this technology at the end of the 1960's, and the reasons for the rise of parabolic dishes is also considered. For almost 40 years, Aperture Arrays languished as historical curiosities while large single dishes or arrays of dishes dominated the radio astronomy agenda. The parallels with the Antikythera mechanism (see these proceedings) are briefly presented. Aperture Arrays re-entered the world of radio astronomy as the idea to build a huge radio telescope with a collecting area of one square kilometre (the Square Kilometre Array, SKA) were debated within ASTRON and the rest of the radio community around the world. Huge advances in signal processing, digital electronics, highspeed networking and high-performance computing systems had transformed Aperture Arrays in terms of their capability, flexibility and reliability. In the mid-1990s, ASTRON started to develop and experiment with the first high frequency aperture array tiles for radio astronomy. Operating at frequencies of up to 1-2 GHz for studies of neutral hydrogen, tiled systems such as AAD, OSMA, THEA & EMBRACE were built and tested. In the slipstream of these efforts, Phased Array Feeds (PAFs) for radio astronomy were invented and LOFAR itself emerged as a next generation telescope and a major pathfinder for the SKA. Meanwhile, the same advantages that aperture arrays offered to radio astronomy (large multiple field-of-views, rapid electronic steering, reliability, durability, flexibility, cost and performance) had already made dishes obsolete in many different civilian and military applications. The first commissioning results from LOFAR and other Aperture Arrays (MWA, LWA and PAPER) currently demonstrate that this kind of technology can transform radio astronomy over 2 decades of the radio spectrum, and at frequencies up to at least 1.5 GHz. This "reinvention of radio astronomy" has important implications for the design and form of the full SKA. Building a SKA that is simply the "VLA on steroids" is simply not good enough - we have the ability to do much, much better. Like the Antikythera mechanism itself, we must amaze future generations of astronomers - they and the current generation deserve nothing less.