The visibility function needs to be determined for many points
in the -plane as possible. If many telescopes are available, the visibility
can be measured simultaneously for many baselines.
Connected element interferometers are essentially of two kinds :
1) linked directly by means of cables or waveguides km .
2) radio linked km .
Even if few telescopes are available, better coverage can be obtained
making use of the rotation of the earth ("supersynthesis"). An east-west
interferometer of baseline length traces out an ellipse in the
where and are respectively the hour angle and the declination
of the field center, and is the observed wavelenght.
If several telescopes are aligned on an east-west line (Westerbork Synthesis
Telescope (NL)) several concentric ellipses are traced out simultaneously.
For the ellipses degenerate into straight lines. Therefore north-south
interferometer arms are needed to have good coverage also for equatorial
Due to the earth rotation, the phase difference between the signals at
two telescopes separated by a baseline , is rapidly changing in time:
where is called the fringe frequency and is the earth angular
For istance, if km and cm , Hz .
In order to be able to calculate the cross correlation of the signals, with
typical integration times of few seconds, this effect has to be compensated for.
The signal of one of the two telescopes is opportunely delayed so that, at the
correlator input, the phase difference between the signals propagating
in the direction , is zero. This operation ("tracking delay") needs an
accurate geometric and atmospheric model at the correlator.