Issue |
Astron. Astrophys. Suppl. Ser.
Volume 136, Number 1, April I 1999
|
|
---|---|---|
Page(s) | 211 - 216 | |
DOI | https://doi.org/10.1051/aas:1999208 | |
Published online | 15 April 1999 |
Hybrid mapping in VLBI and spurious symmetrization
1
Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
2
Joint Institute for VLBI in Europe, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
Send offprint request to: M. Massi
Received:
29
June
1998
Accepted:
9
December
1998
The standard procedure used in hybrid mapping of VLBI data is examined.
That this procedure can converge on wrong solutions has been pointed
in the past by many authors.
Walker (1996) indicated the bias in the
resultant data due to the use of a point source as starting model. Bååth (1989) suggested
the use of the original data set
in each iteration of self calibration,
and not the use of
the corrected one created by the previous iteration.
Linffield (1986) analyzed
the role of the coverage and lack of intermediate spacings.
However, the full procedure needed to avoid false features
is at present not clear and only experience with imaging helps
the user to avoid them. In this paper we
demonstrate that the problem is connected with the non-linear nature of self
calibration which leaves initial wrong assumptions frozen in the final
solution. We demonstrate how large closure phase values
linearize the process and we show a simple modification of the standard
procedure which helps to avoid creation of false features in the imaging.
Key words: methods: analytical / methods: data analysis / techniques: interferometric / instrumentation: interfereometers
© European Southern Observatory (ESO), 1999