(a)–(l) Evaluated focal spots listed from left to right edge of the CCD. The number of pixels on the CCD screen is 64×64=4096. The intensity patterns on the screen are obtained by the fast Fourier transform of MATLAB software. The intensities are normalized with respect to the maximum intensity. From (a) to (l), each picture moves by one pixel on the screen of the CCD. For (l), the focal point totally moves by 12 pixels on the screen. For (m)–(p), the focal point moves by 16 pixels, 17 pixels, 18 pixels, and 19 pixels, respectively. The distances between the focal point of (a) and that of (b)–(l) are in the the range of memory effect, while the distances between the focal point of (a) and that of (m)–(p) are outside the range of memory effect.