The first step of the image reconstruction process is to defined
the object space Ho. This space is characterized
by two key parameters: the extension of its field,
and its resolution scale
(see Fig. 14.2).
The procedure wipe_init is used to set these
parameters properly.
The frequency coverage to be synthesized
is defined with the aid of the procedure wipe_aper.
This tool provides an interactive way of fitting an ellipse over
the experimental frequency coverage generated by the
experimental frequency list
(see Fig. 14.4).
Once
has been defined, the procedure wipe_beam
is ready for computing the neat beam
, as well as the
dirty beam
. The latter plays a key role in the action
of the convolutor
, while the Fourier transform of
the former is involved in the definition of the
data vector
(cf. Eq. 14.9 and 14.10).
The last step in the image reconstruction process concerns
the neat map. It is implemented in the wipe_solve
command. Before the initialization of the reconstruction, the
dusty map
is computed, and an optional support
can be selected (this support plays the role of the clean box
of CLEAN).
As WIPE can be slow when reconstructing large images,
it can be initialized with a few CLEAN iterations
to quickly build a first object representation space E.
When switching to WIPE, the program starts by optimizing
the solution provided by CLEAN with the corresponding
support. Then, at each iteration of WIPE, the support
grows, and for a given and fixed object representation
space E, the normal equation 14.16 is solved by using
the conjugate gradient method, which also provides the
condition number
of AE.
When leaving WIPE, a final smoothing of the current
object support is performed, removing (through an appropriate
morphological analysis) the details of the reconstructed image
smaller than the resolution limit of the reconstruction process.
The final reconstructed image
is the
function minimizing the objective functional 14.13 on that support.
The control of the robustness of the reconstruction process is
performed through an additional step with the wipe_error
command. This procedure computes with a fine accuracy the final
condition number , as well as the eigenvalues
and the critical eigenmodes of
.
One of the aims of this last step is to check that the features
present in the reconstructed image are not artefacts.
This can be done by comparing these features with those of the critical
eigenmodes. When there exists a certain similitude (between these
features), it is then recommended to restart the process with a
lower resolution, so that the final solution be more stable and
reliable.