Last Update: February 15, 2000
Difwrap is a program written in Perl to provide a Graphical User
Interface (GUI) to Difmap. The software currently provides a means
to estimate errors in source models generated by the modelfit
function in Difmap. Difwrap now allows for some limited configuration
of Difmap through a GUI.
Model Error Analysis
This function allows for the determination of errors in source models obtained
using the modelfit command in Difmap. It is based on the
process first described by Tzioumis et al. (AJ 98, p36, 1989)
The parameter of interest was stepped by small increments near
the best-fitted value. At each step, this parameter was kept fixed, while
all the other parameters were allowed to relax during model fitting. The
resultant model was compared with the data. When the visual fit
to the data became unacceptable, the limits of the acceptable range of
values had been determined. Even though this method is subjective, it is
thought to give a conservative estimate of the model-parameter range allowed
by the data.
This method has the advantage that it allows for the inter-dependency of
model components to be taken into account during model fitting. For example,
if the (u,v) coverage is limited, there can be a strong inter-dependency
between the size and flux density of the model component.
The main problem with this method is many trial models may be needed
before a good understanding of the errors can be obtained. Carrying out
this task by hand is tedious and time-consuming as it requires the generation
of many model files, followed by a visual comparison of every model with
the data. The model error analysis function of Difwrap removes much
of this drudgery. It works in the following way:
The model components are presented to
the user who decides which parameter (flux density, position or component
size/shape) requires an error estimate.
The user then decides what range of parameter values require trialing and
how many points to sample within that range.
Lastly, the user selects which error fitting method is appropriate. There
are three to choose from:
Difwrap will then create a test model for each grid point, carry
out the requested method and store the final model and goodness of fit
No Modelfit or Selfcal. For each parameter value that is trialed,
the goodness of fit of the trial model is measured without running modelfit
to allow the other parameters to relax. This method is less conservative
than that described by Tzioumis et al. as it does not provide an
error estimate that accounts for the inter-dependedness of model components.
If you believe your model component parameters are reasonable independent
of each other, you might decide to use this over the other methods as it
is significantly faster. Because it it fast, this method is also useful
to provide an estimate of the minimum errors before starting on one of
the other (slower) methods.
Modelfit only. For each parameter value that is trialed, the goodness
of fit of the trial model is measured after executing the modelfit
task with the number of specified iterations. This is the method described
by Tzioumis et al.
Modelfit and Selfcal. For each parameter value that is trialed,
the goodness of fit of the trial model is measured after repeatedly executing
the modelfit task with the number of specified iterations followed
by phase self-cal until there is no improvement in the goodness of fit.
This method provides an even more conservative estimate of errors than
When this is complete, the user can select how
the goodness of fit values are to be presented.
After inspection, the estimated error values
are presented and the user may carry out another error analysis if required.
Difwrap uses the Perl module Expect to communicate with
a Difmap session. The Tk and PGPLOT modules are
used to provide the GUI. Difwrap requires the following versions
(or later) of these modules:
Difwrap has been tested on Sparc Ultras running Solaris 2.5.1
and 2.6 with Perl version 5.004_04.
Documentation for Difwrap is available in HTML and postscript formats:
Difwrap may be obtained from web site at code.google.com.
Installation instructions are provided in the
accompanying README file.
- Several bug fixes. It is now possible to save images of rms/chi-squared distributions.
New configuration window to allow configuration of Radplot, Vplot and Projplot
flags. Also some configuration of map parameters is avialable.
In model error analysis, it is now possible to show the residual and restored
maps. (Thanks to Dan Homan).
the main menu is now smaller an appears in the lower left corner of the
better management of windows
exit and quit commands are disabled during the "Interactive session" thus
forcing an exit through the GUI.
Difwrap forces Difmap to open a /xs device on startup. This avoids a problem
experienced by users who do not make a /xs device open when they start
various small bug fixes
If you notice any bugs or have any suggestions, please send them to firstname.lastname@example.org.
Jim Lovell email@example.com