VLBA to Begin Using the DiFX Software Correlator for Trimester 2010-A

Jon Romney, Claire Chandler, Walter Brisken, & Adam Deller

Figure 1

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Figure 2

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Figure 3

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NRAO’s implementation of the DiFX software correlator is the first element of the VLBA Sensitivity Upgrade project to be completed and, fortunately, it is one that can be put into operation, with scientific benefits to VLBA users, before the other elements are in place. DiFX was developed at Swinburne University in Melbourne, Australia, (Deller et al. 2007, PASP, 119, 318), and adapted to the VLBA operational environment by NRAO staff.

Ongoing point-by-point comparisons between DiFX and the original VLBA hardware correlator have yielded excellent results thus far. Amplitudes agree at better than (0.4 ± 1.7) percent, and phases to (0.1 ± 1.0) degrees, across a wide range of observing modes. The figures present a few test results, with the original correlator’s output shown in yellow and DiFX in green. Overlaid points are red, in each case. Figs. 1 and 2 show amplitude and phase spectra for a subset of baselines, at high and low resolution, while Fig. 3 displays the variation with time of the amplitude and phase of the central spectral point in the spectra of Fig.2.

We therefore expect that all VLBA observations proposed at the 2009 October 1 deadline will be correlated using DiFX. In the near future, we also expect to announce opportunities for rapid response “exploratory” proposals to use DiFX later in Trimester 2009-C, and for requesting DiFX correlation of projects already granted observing time but not yet correlated.

An updated version of the VLBA Observational Status Summary, available concurrently with this announcement, provides current information on NRAO’s implementation of DiFX. The principal new capabilities and other advantages DiFX offers at this time are summarized below.

• Spectral resolution as high as 4096 points per baseband channel, for any polarization configuration. Compared to the VLBA’s original, hardware-based correlator, this is an increase by factors of 4 and 32, for parallel- and cross-hand polar processing. There is also no overall limitation across multiple baseband channels.
• Elimination of the current resolution limits in extreme narrow-band observations, which are imposed by the restricted sample decimation and fixed hardware cycle of the original correlator.
• Significantly shorter integration periods, in principle as short as twice the reciprocal frequency resolution.
• A substantially increased correlator output data rate limit of 10 Mbyte per second of observing time – a ten-fold increase (at speedup factor 1) over the most recent maximum rate available on the original VLBA hardware correlator.
• More sophisticated pulsar processing than the simple binary gate implemented in the original VLBA correlator.
• Correlation of any mix of VLBA, Mark4 and Mark5B formatted data.

The maxima specified above are flexible, and can be waived on the basis of a sufficiently compelling scientific justification.

Please consult the updated VLBA Observational Status Summary for detailed information. Most aspects of DiFX are discussed in Section 7, while the new pulsar gating options are presented in Section 16 on pulsar observations.