Running Difwrap

Choosing ``Interactive session'' will allow you to interact with Difmap as normal. To return to Difwrap, type <Control>-W (i.e. hold down the [Control] key and press the W key).

Choosing ``Model error estimates'' will start up the error analysis GUI as described in Section 2.2 while choosing ``Exit DifWrap'' will exit the program.

How to use the error analysis GUI

1.

After starting Difwrap, choose ``Interactive session'' from the main menu and carry obtain the best fit model to your data as normal.

2.

Type <Control>-W to return to the main menu and select ``Model error estimates''. You will be presented with window (Figure 2.2) which you can use to choose the model component of interest, the range of values to test and the error fitting method.

     

Figure 2.2: Model error analysis setup window.

 

3.

Model Components. Near the top of the window is a list of your model components. Those parameters with a green background you have selected as variable while those with a red background are non-variable. You can alter the variability state by clicking on the square buttons in the columns marked by a ``v''. Select the model component of interest by clicking on a button in the left-hand column. Select the parameter of interest (either flux density, Position or Size) from the buttons above the list of model components. In figure 2.3, the size of component number 1 has been selected.

     

Figure 2.3: Model component selection.

 

4.

Gridding. For each value that describes a parameter, the range and number of values you wish to test must be selected. This is done through the Gridding section of the window. The number of grid points to be sampled for each range of values is selected together with the maximum offset of each parameter. This can be done by moving the horizontal sliders to the desired position or by typing in the desired values manually in the boxes in the Val column. For example, in Figure 2.4 5 grid points have been requested in each dimension^2.1 and the range of values  mas in major axis,  in axial ratio and  in major axis position angle from the best fit will be trialed.

     

Figure 2.4: Gridding selection.

 

5.

Error Fitting Method. Lastly, the error fitting method (as described above) must be selected together with the number of modelfit iterations. For example, in Figure 2.5 the ``Modelfit only'' method has been chosen with 10 iterations in modelfit.^2.2

     

Figure 2.5: Error fitting method selection.

 

6.

Once this setup page has been configured, the analysis is started by pressing the Start Error Analysis button.

7.

A Status window will appear providing an indication of the progress of the modelfitting (Figure 2.6).

     

Figure 2.6: Error fitting method selection.

 

8.

When Difwrap has finished sending commands to Difmap, a new window will appear from which you can choose how the results are to be presented (Figure 2.7). The goodness of fit (either rms or Chi-squared) of each trial model can be plotted as a function of the trial values. The user can select a grid point and Difwrap will ask Difmap to display the corresponding model with the data using vplot, radplot or projplot. Select the Difmap command you would like executed from the ``inspect data using'' box and how the goodness of fit results are to be shown from the ``plotting'' box.

     

Figure 2.7: Plotting and Analysis window.

 

9.

Figure 2.8 shows Chi-squared as a function of Flux density for a 9-point grid analysis to estimate the error in flux density of a model component. Clicking on a point with the left mouse button will result in the corresponding model to be displayed with the data. That point will then be surrounded by an orange circle to indicate that the point has been inspected. If the user decides that the model corresponding to that point is a good representation of the data, clicking the middle mouse button over it will change its colour from red to green, indicating acceptance. In this example, the central three grid points have been deemed acceptable while the others are unacceptable. Clicking the right mouse button in the window returns the user to the plotting window (Figure 2.7) and the estimated flux density errors are displayed in the message window (Figure 2.9). Difwrap estimates the errors by taking the most extreme accepted points. The contents of this window can be edited and saved or printed out using the ``Save as'' and ``Print'' commands respectively under the ``File'' menu.

     

Figure 2.8: A plot of modelfitting results.

       

Figure 2.9: The messages window showing results of a flux density error analysis.

  

10.

Further plotting may be done, or a new error analysis session may be started.

Footnotes

... dimension^2.1

i.e. In this case there are three dimensions to be sampled: major axis, axial ratio and major axis position angle. There are 5 grid points to be sampled per dimension, resulting in  points to be sampled.

... modelfit.^2.2

i.e. The command modelfit 10 will be executed in Difmap.