Top row: variations of the square root of the CRB of logN as a function of the column density and the excitation temperature. Second and third row: correlation coefficients between efficient estimators of (T_ex, logN), and (logN, σ_V) in the second and third rows. respectively (defined in Eqs. (23) and (24)). Bottom row: variations of functions of the opacities. Left: only ¹³CO(1− 0) is analyzed. Middle: only ¹³CO(2−1) is analyzed. Right: both ¹³CO(1−0) and ¹³CO(2− 1) are analyzed simultaneously. In (a) and (b) pixels in grey correspond to T_ex and N values whichlead to singular Fisher matrices. For this analysis, σ_b = 100 mK, Δ_V = 1.1 km s⁻¹ and σ_V = 0.61 km s⁻¹.