What is the source of the nearly periodic bright points in the horizontal axis of Fig. 4.4...

What is the source of the nearly periodic bright points in the horizontal axis of Fig. 4.41(b)?

FIGURE 4.41 (a) Test pattern of size 688 × 688 pixels, and (b) its Fourier spectrum.The spectrum is double the image size due to padding but is shown in half size so that it fits in the page. The superimposed circles have radii equal to 10, 30, 60, 160, and 460 with respect to the full-size spectrum image. These radii enclose 87.0, 93.1, 95.7, 97.8, and 99.2% of the padded image power, respectively.