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Wideband VLBI

The technique of very long baseline interferometry (VLBI) depends on faithfully recording the fluctuations of electric field caused by incoming radio waves from astronomical sources, as captured by the receiving system of an antenna. These recordings must be played back in precise time synchrony at a central location in order to combine signals from multiple antennas, and perform the interferometry, thereby achieving the characteristic and scientifically powerful milliarcsecond resolutions of the VLBI technique.

Since the early days of VLBI, recording systems have imposed limits on the performance of VLBI arrays. The reason is that the rate at which data can be recorded determines the timescale of the electric field fluctuations that can be measured. The more frequently one samples the voltage, the shorter the variations that can be tracked, but the higher the rate at which data is generated and must be recorded. Voltage fluctuations can be slowed down by restricting the frequency bandwidth, so limits in the recording system lead directly to narrow band filters in the VLBI system, deliberately discarding much of the information coming from the telescope. The recording system limits bandwidth, which in turn reduces the sensitivity of the VLBI array.

Haystack is at the forefront of a global effort to exploit modern electronics and storage system technologies to remove this VLBI bottleneck once and for all. By developing systems based on parallelized IO in arrays of commercial hard disk drives, and high-speed digital systems to efficiently process wide bandwidths into a form suitable for recording, engineers are rapidly expanding bandwidths, and rapidly decreasing costs.

Already, with recording rates of 1 Gbit/sec through the Mark 5 system, experiments combining the collecting area of the world's largest radio telescopes are achieving greater sensitivity than any in the history of radio astronomy, and further improvements will quickly follow. VLBI is in a transition from being an expensive but powerful technique with limited sensitivity, to being an inexpensive and yet more powerful technique, with sensitivity limited only by the constituent antennas of the array.

Ultimately, on-site recording will become obsolete as huge bandwidths are transmitted in real time over fiber-optic networks. This is referred to as e-VLBI, and it too is being pioneered by Haystack scientists and engineers.

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