of
the primary antenna beam area. This restricted interferometer beam arises
in the correlation process, from smearing due to averaging in time (with,
typically, a 2-second period) and/or across bandwidth (``chromatic
aberration'' over, typically, 0.5 MHz spectral resolution), at positions
away from the correlation phase center. Thus, imaging of targets that
are widely spaced in the primary beam requires multiple processing passes
in typical correlator implementations. If the visibilities are maintained
at high time and frequency resolution, it is possible to perform a uv
shift after correlation, essentially repointing the correlated dataset to a
new phase center. However, this approach would require prohibitively large
visibility datasets.
DiFX implements multiple uv shifts inside the correlator, to generate
as many phase centers as are necessary, in a single correlation pass. The
output consists of one dataset of normal size for each phase center. This
mode consumes around three times the correlator resources of a normal
continuum correlation, due to the need for finer frequency resolution
before the uv shift, but the additional cost is only weakly dependent
on the number of phase centers. For reasonable spectral and temporal
resolution requirements (for example, adequate for smearing 
10% at the
50% contour of the VLBA primary beam), 200 phase centers require only 20%
more correlator time than 2 phase centers. Extremely high spectral and/or
temporal resolution (e.g. for shifts even closer to the edge of the primary
beam) carry a higher overhead per additional phase center. This mode thus
should be requested only for imaging of three or more sources within any
single antenna pointing. The output data rate must be justified if it
exceeds the current limit specified in Section 7.4.
Multiple phase-center correlation is requested in the NRAO Proposal Submission Tool by setting the ``Number of Fields'' item in the resource section to the maximum number of phase centers required for any antenna pointing specified in a given resource. The requested spectral resolution and integration time should correspond to the desired initial number of frequency channels per sub-band (required to minimize bandwidth smearing) and the desired integration between uv-shifts (to minimize time smearing). SCHED version 9.4 includes facilities to support specification of the actual phase center locations.
For more details on wide-field imaging techniques, see Bridle & Schwab (1999), and Garrett et al. (1999).