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13.3 Fringe Fitting
After correlation and application of the pulse calibration, the
phases on a VLBA target source still can exhibit high residual fringe
rates and delays. Before imaging, these residuals should be removed
to permit data averaging in time and, for a continuum source, in
frequency. The process of finding these residuals is referred to as
fringe fitting. Before fringe fitting, it is recommended to edit the
data based on the a priori edit information provided for VLBA
stations. Such editing data are delivered in the FG table
(Section 11). The old baseline-based fringe search
methods have been replaced by more powerful global fringe search
techniques (Cotton 1995a; Diamond 1995). Global fringe fitting is
simply a generalization of the phase self-calibration technique
(Section 13.5), as during a global fringe fit the
difference between model phases and measured phases are minimized by
solving for the station-based instrumental phase, its time slope (the
fringe rate), and its frequency slope (the delay).
Global fringe fitting in AIPS is done with the program FRING or
associated procedures. If the VLBA target source is a spectral line
source (Section 16) or is too weak to fringe fit on
itself, then residual fringe rates and delays can be found on an
adjacent strong continuum source and applied to the VLBA target source
(Section 13.6).
Next: 13.4 Editing
Up: 13 PHASE CALIBRATION &
Previous: 13.2 The Pulse Cal
Contents
Jon Romney
2012-01-05