Square root of the CRLB in mas as a function of pixel size Δx for sources observed with a two-dimensional array detector when the background flux per pixel is given by Eq. (24) (b = 6000e⁻/arcsec², D = 0 e⁻, RON = 5 e⁻, and F = 6000e⁻). The solid lines (thin, normal, and bold) are computed with expression (27) for a stationary source with an image quality FWHM of 0.5, 1.0, and 1.5′′, respectively (and all centered on a given pixel). The lines with a plus and with triangles correspond to the same curves, but in the moving case when the drifting parameter is optimum (L = L_o). The lines with a plus and with triangles correspond to the CRLB along the direction normal and parallel to the drifting motion, respectively.