Correlator Setup including Multiple Phase Centers

The VLBA DiFX software correlator, developed at Swinburne University (Deller et al. 2007) and adapted to the VLBA by NRAO staff (Brisken 2008), offers great flexibility in correlating observed data.  Details on the current version of the correlator can be found in Deller et al. 2011 and the DiFX Guide.

Spectral Channel Width

VLBA users may select the desired number of “spectral points” for their observations, keeping in mind the following:

• A maximum of 4096 spectral points per baseband channel is allowed for routine DiFX processing.
• A total of 132,096 points, summed over all baseband channels and polarization products, is all AIPS can handle.
• The minimum spectral channel width is 2 Hz. The number of spectral points in all baseband channels must be identical within each correlation pass, but different correlation passes may have different numbers of spectral points.
• DiFX also supports "spectral zooming", where users select a specific subset of spectral points to correlate.  The location and width of these "zoom bands" must be communicated directly to the VLBA operations staff before correlation.

For more details on spectral channel settings, see the VLBA Observational Status Summary (OSS).

Integration Period

Users can specify nearly any correlator integration period they wish. However, individual integrations are quantized in multiples of the number of spectral points requested divided by the data channel bandwidth.

The default integration period is 2 seconds.

Multiple Correlator Passes and Multiple Phase Centers

The DiFX correlator allows users to perform multiple correlation passes on the observation data. Users must specify the number of correlator passes they need when defining their Resources in the Proposal Submission Tool (PST).

The DiFX correlator also allows for multiple phase centers to be specified per pointing. The current limit for the standard 10-element VLBA is about 600 phase centers at 2 Gbps (512 MHz polarization-summed bandwidth) for dual polarization. Two correlator passes may be required for 600 phase centers at 4 Gbps. Full polarization reduces the number of phase centers per correlator pass by a factor of 2. With multiple correlator passes, any number of phase centers may be achieved. For more information on the multiple phase center capability, see the OSS.

Users must specify the number of phase centers per pointing when defining their Resources in the PST. For a given Resource, use the "Number of Fields" item in to specify the maximum number of phase centers required for any antenna pointing. The requested spectral resolution and integration time should correspond to the desired initial number of spectral points per data channel (required to minimize bandwidth smearing) and the desired integration between u-v shifts (to minimize time smearing). A resulting expanded output data rate that exceeds the current limit, as well as any required multiple passes, must be justified specifically in the proposal.

Pulsar Gating

DiFX supports 3 time-based selection modes to facilitate pulsar observations:

1. Binary Gating
2. Matched-filter Gating
3. Pulsar Binning

For more information on pulsar observing, see the OSS and Brisken & Deller (2010). Users who will require pulsar gating must select the “pulsar gate” option when defining their Resources, and describe their setup in the Technical Justification section of the proposal.