The DiFX software correlator DiFX is an FX style correlator written in C++ Designed to run on commodity clusters Optimised Intel vector libraries are used for good performance Makes hard-to-reach areas of correlator parameter space accessible Now used by LBA, VLBA and (soon) AuScope, MPIfR (Bonn)

DiFX at the VLBA Acceptance testing is all but complete - correlator switchover by end of year (call for proposals already made in October) Investment has been modest: 10 node cluster costing ~$25,000 enables throughput of ~500 Mbps/antenna for 10 stations (roughly double the hardware correlator)

VLBA DiFX verification Visibility phase vs time: Colors are a little hard to see: blue is HWC, purple is SWC, cyan is where they overlap perfectly

New VLBA functionality: now Much increased spectral and temporal resolution: in principle almost infinite, in practice at least 16x previous is offered Much more flexible pulsar processing; previously only an on/off gate available Ability to process data in previously inaccessible formats (like the higher datarate Mark5B/B+)

Future features: Multiple PCs At any given instant, the phase centre (PC) of correlation can be changed by rotating the visibilities by a phase value equal to the sky frequency x  delay (between desired phase centre and current phase centre) When done in-correlator, very high spectral and temporal resolution means almost no smearing primary beam uv-shifted “pencil” fields

Future features: Multiple PCs At low frequencies (and high sensitivity) there will be many sources per primary beam Thus a dramatic improvement in survey speed! Applications in in-beam calibrator searches, star-formation regions, galaxy evolution studies and more