3. Data Evaluation

3.1 Evaluating a “good” measurement

The image on the right of figure 2 shows a typical “good” measurement. The image of each pepperpot hole is clearly visible, the signal to noise ration is good.

In order to extract the emittance from this image first start the pepperpot evaluation program QPepperpot and read in an image from a measurement file (File -> Open image). The image file must be a valid pixmap file e.g. “screenimage.bmp”. Then enter the calibration values, scaling factor in pixels/mm (32.6 in Figure 4) and the center of the pepperpot plate in pixels (320,240).. Only the horizontal value will be used. The settings dialog box (Settings -> Configure Pepperpot) is foreseen for this purpose.




Figure 3: QPepperpot's Main Window




Figure 4: The Settings Dialog Box



The next step is to calculate the projection of the image to the horizontal axis (Tools->Projections or the Toolbar button showing the histogram)



Figure 5: Projection: Raw Data (blue), Filtered Data (red), Peaks (black)



The Tools menu in the menu bar allows selection of options for display of the projection. You may

In Figure 5 all curves are shown on a single plot: The blue curve shows the raw data. On the red curve the projection is filtered and the black curve shows the peaks found. The mean noise value and its variance, calculated over the first 50 points is also displayed. You may also read the peak positions by pushing the first mouse button

Now that the positions of the image of all 8 pepperpot holes is known and the calibration has been entered the emittance ellipse can be determined. Tools -> Emittance will display the results.

Figure 6 shows a typical emittance plot for a diverging beam.





Figure 6: Emittance Plot for a Diverging Beam