One of the most often used phase-recovery procedure, especially with VLA data,
is Self-calibration. Due to imhomogenities in the atmosphere a flat wavefront
will be distorted when it reaches the telescopes. As a result of this the
wavefront phase is perturbed. However, it is possible to recover the Fourier
phases of the source brightness distribution from the observations. To do so,
one makes use of the closure phase , which is the sum of the observed
phases
, around any closed triangle of interferometer baselines:
For antennas, there may be
independent baselines, but only
unknown phase errors. This means that most of the phase information can be
recovered when many telescopes take place in an observation.
The Self-calibration routine involves a doubly-nested iterative loop, where an initial model, e.g., a point source, is Fourier transformed to the visibility plane. The observed visibilities are then used by the Self-calibration procedure to update the model visibilities, and the result is then Fourier transformed back to the image plane. In the image plane the CLEAN algorithm is used to deconvolve the image and to derive a new "model" image. By successive iterations, one may solve for all the antenna gains, using the phase closure information.