Unfortunately not all measurements are as clean as the one shown in the previous section. If the camera settings as not perfect e.g. the aperture is not opened enough, if the contrast settings are not perfect … we may end up trying to analyze an image who’s projection looks like the one shown in Figure 7. Since even a human has a hard time to see the hole-images, it is even more difficult for a program to automatically evaluate such an image.
Figure 7: Projection of a very noisy image
The pepperpot evaluation programs help through low pass filtering and offset suppression (Tools->Show filtered and Tools -> Subtract Offset).
Figure 8: The low-pass filtered projection
If
you try to automatically find the peaks however, the program will
only find four out of the eight peaks. These peaks can be copied to
the settings dialog
box with the “copy
to manual peaks” button. After having copied the peaks, switch off
automatic peak finding (Tool->Automatically find peaks must be
switched off). You may now define the peaks by hand in the settings
dialog box. The plot
will show you how you placed them. All 8 peaks must be defined.
Clicking the left mouse button on the projection plot brings up a
cursor whose current position is shown. This may help you when
finding the peaks. You may also have a closer look at the background
in order to see if there are any systematic errors by switching on
the zooming. Click and drag to zoom into the picture.
Once you finished defining the peaks by hand you may again try to plot the emittance. The emittance plot will use the manually defined peaks instead of the automatically found ones.