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Summary form only given. The international radio astronomy community are working toward the next generation telescope the square kilometre array (SKA). Australia's contribution is the Australian SKA Pathfinder which is being built by CSIRO in Western Australia. ASKAP is exploring the concept of placing a phased array feed at the focus of a 12 m parabolic reflector. This can be thought of a radio camera with the feeds of the phased array being the receptors and the dish the optics. However to optimize performance the receptor data must be calibrated and summed in a beamformer. The array generates beams that cover a large area of the sky. For ASKAP there are 192 inputs to the beamformer each sampled at 768 MS/s. For each of 36 antennas this is processed to generate up to 30 dual polarisation antenna beams which are decimated into 16384 frequency channels. This processing is implemented in an FPGA system that computes at greater than 1000 Tera arithmetic operations per second. Internal data flow in the system is 70 Tb/s. For ASKAP, FPGAs are the optimum choice in terms of total system cost which includes hardware and firmware development and total power consumption over the life of the instrument. Design of this system present a number of challenges including how to partition the system, efficient data flow and command and control.