Software correlation, where a correlation algorithm written in a high-level language such as C++ is run on commodity computer hardware, has become increasingly attractive for small to medium sized and/or bandwidth constrained radio interferometers. In particular, many long baseline arrays (which typically have fewer than 20 elements and are restricted in observing bandwidth by costly recording hardware and media) have utilized software correlators for rapid, costeffective correlator upgrades to allow compatibility with new, wider bandwidth recording systems and improve correlator flexibility. The DiFX correlator, made publicly available in 2007, has been a popular choice in such upgrades and is now used for production correlation by a number of observatories and research groups worldwide. Here we describe the evolution in the capabilities of the DiFX correlator over the past three years, including a number of new capabilities, substantial performance improvements, and a large amount of supporting infrastructure to ease use of the code. New capabilities include the ability to correlate a large number of phase centers in a single correlation pass, the extraction of phase calibration tones, correlation of disparate but overlapping sub-bands, the production of rapidly sampled filterbank and kurtosis data at minimal cost, and many more.The latest version of the code is at least 15% faster than the original, and in certain situations many times this value. Finally, we also present detailed test results validating the correctness of the new code.

High angular resolution images of extragalactic radio sources are being made with the Highly Advanced Laboratory for Communications and Astronomy (HALCA) satellite and ground-based radio telescopes as part of the Very Long Baseline Interferometry (VLBI) Space Observatory Programme (VSOP). VSOP observations at 1.6 and 5 gigahertz of the milli–arc-second–scale structure of radio quasars enable the quasar core size and the corresponding brightness temperature to be determined, and they enable the motions of jet components that are close to the core to be studied. Here, VSOP images of the gamma-ray source 1156+295, the quasar 1548+056, the ultraluminous quasar 0014+813, and the superluminal quasar 0212+735 are presented and discussed.

We have used the NRAO Very Long Baseline Array to image the nucleus of NGC 1275 (3C 84) at frequencies of 15 and 43 GHz, with an angular resolution of about 0.6 and 0.15 mas, respectively. On scales of 0.05-5 pc (0.15-15 mas), the previously identified regions of radio emission are seen with unprecedented clarity: a bright and complex central core, a southern cocoon-like "expanding bubble," a faint, thin jet connecting the core to the bubble, and an inverted spectrum northern "counterfeature." The inner 0.5 pc of the core has bright knots of emission located along a line with multiple sharp bends, as if sprayed from a precessing nozzle with a full opening angle of ∼40Њ. These knots move at 0.05c, 0.08c, and 0.2c (‫30.0ע‬c), for components at increasing distances from the core, indicative of acceleration along the jet. However, there are rapid and progressive flux changes in the slow-moving knots, corresponding to a phase velocity of ∼ . 0.9c ‫ע‬ 0.1c

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