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13.1 Fringe Finders
VLBI fringe phases are much more difficult to deal with than
fringe amplitudes. If the a priori correlator model assumed for
VLBI correlation is particularly poor, then the fringe phase can wind
so rapidly in both time (the fringe rate) and in frequency (the delay)
that no fringes will be found within the finite fringe rate and delay
windows examined during correlation. Reasons for a poor a
priori correlator model include source position and station location
errors, atmospheric (tropospheric and ionospheric) propagation
effects, and the behavior of the independent clocks at each station.
Users observing sources with poorly known positions should plan to
refine the positions first on another instrument
(Section 17.1). To allow accurate location of any
previously unknown antennas and to allow NRAO staff to conduct
periodic monitoring of clock drifts, each user must include at least
two ``fringe finder'' sources which are strong, compact, and have
accurately known positions. Typically, a fringe finder should be
observed for 5 minutes every 1-3 hours. Consult Markowitz & Wurnig
(1998) to select a fringe finder for observations between between
20 cm and 7 mm; your choice will depend on your wavelengths but
J0555+3948=DA193, J0927+3902=4C39.25, J1642+3948=3C345, and
J2253+1608=3C454.3 are generally reliable in the range 13 cm to
2 cm. In addition, at 90 and 50 cm we recommend either
J1331+3030=3C286 or J2253+1608=3C454.3. Fringe-finder positions,
used by default by NRAO program SCHED (Walker 2011) and the
VLBA correlator, are given in the standard source catalog available as
an ancillary file with SCHED.
Next: 13.2 The Pulse Cal
Up: 13 PHASE CALIBRATION &
Previous: 13 PHASE CALIBRATION &
Contents
Jon Romney
2012-01-05