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 particulary 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 antenna location errors, atmospheric (tropospheric and ionospheric) propagation effects, and the behavior of the independent clocks at each antenna. Users observing sources with poorly known positions should plan to refine the positions first on another instrument such as the VLA. To allow accurate location of any previously unknown antennas and to allow NRAO staff to conduct periodic monitoring of clock drifts, each user should include at least one, and preferably two, ``fringe finder'' sources which are strong, compact, and have accurately known positions.

The sources listed in Table 5 can be used as fringe finders between 20 cm and 7 mm, except as noted. In addition, at 90 and 50 cm we recommend either

J1331+3030=1328+307=3C286=J1331+30 or
J2253+1608=2251+158=3C454.3=J2253+16.

Fringe-finder positions, used by default by the NRAO program SCHED (Walker 1997) and the VLBA correlator, are given in the standard source catalog available as an ancillary file with SCHED. Fringe-finder names are as they appear in the standard source catalog.