σ_U to σ_V ratio versus flux (in ADU). The circles are for the stars in the VST images: upper circles are for stars in the image VST_F, while lower circles are for the stars in VST_S (see text). The dashed curve corresponds to the ratio of the two exact expressions of the CRLB, Eqs. (30) and (36), with the parameters corresponding to the image VST_F. The two upper horizontal lines correspond to the approximations of this ratio for the image VST_F in the case of fast-moving sources (expression (41) for faint sources and expression (42) for bright sources). The two lower horizontal lines correspond to the approximations of this ratio for the image VST_S in the case of slow-moving sources (expression (43) for faint sources and expression (44) for bright sources).