An EBSD system must be calibrated to work correctly. This typically, involves measuring the sample-to-screen distance and the pattern-centre position on the phosphor screen, (the pattern centre is the point on the screen closest to the generation point of the diffraction pattern on the sample).

There are several methods for calibrating an EBSD system. The preferred method is to collect diffraction patterns from the same point on a sample with the detector placed at different insertion positions (Figure 1) and at different working distance. Using image correlation, it is then possible to identify the pattern centre at these different geometries and be measuring the zoom factors between the images it is possible to calculate the distance to the sample. 

In a modern EBSD system this process is automated so that patterns are collected at these different geometries and a matrix is created, so that as the system is used it is always in calibration, what ever the geometry conditions.

Figure 1 The pattern centre is found by correlating features on the EBSPs at different detector positions.
The pattern centre is the zoom point on those EBSPs - the one point on the EBSP which does not move when the detector moves.