CRLB ratio versus exposure time for a moving source with V_x = 2′′/min, b_s = 2000e⁻/arcsec/min, FWHM = 1.0′′, Δx = 0.3′′, d = 0, and f_s = 800e⁻/min. The star symbols correspond to the maximum of each curve. The vertical dashed line is T_s, the exposure time corresponding to a drifting parameter equal to the FWHM (left: RON equals zero and Δ_T equals 0, 1, 10, and 100 s; right: Δ_T equals zero and RON equals 0, 2, 5, and 25e⁻/pix). The hatched areas correspond to exposure times long enough to allow source detection (here, the source is assumed to be detectable for a peak S/N higher than 3.5).