of beam expander in order to reduce the beam divergence angle.
The role of the iris diaphragm is to adjust the focal spot size and
the energy of the focal spot. And the attenuators play the role of
coarse tuning of the energy of the focal spot.
Since the focal spot used in our experiment is larger than the size
of a single CCD pixel, an array of 4 4 pixels, i.e., 25.8 μm 25.8 μm,
was chosen as the new pixel to make sure that it is larger than the
focal spot. Each moving step is set as 0.6 μm, and the range of
movement is 25.8 μm 25.8 μm. The focal spot is divided into 86
bands. When the focal spot size is larger than that of a pixel, the gray
value of which can be directly read out by CCD, in the initial moment