Square root of the CRLB versus exposure time for sources with a speed of 2′′/min and for which the fluxes per minute f_sfrom top to bottom of the figure are 600, 2000, 10 000, and 60 000e⁻/min (FWHM = 1.0′′, Δx = 0.3′′, b_s = 2000e⁻/arcsec/min, d = 0, and RON = 0), respectively. The star symbols correspond to the optimum exposure time for each source. The vertical dashed line is the lower limit T_O of the optimum exposure time when the flux is completely dominated by the sky background.