Measurement of the FeI 630.1 nm line core formation depth δ/sinθ (θ denotes the heliocentric angle). Left column, from top panel to bottom panel: center-to-limb variations of the formation depth of the images at line cord levels 1 to 5 (orange dots), and their linear fit with the a and b coefficients given in Table 1 (black dots). Right column: difference between the measured values of δ/sinθ and their linear fit (orange dots) and analytical fit of this difference with the c and d coefficients given in Table 1 (black dots). Results obtained in the southern hemisphere (denoted by S on the figures) are shown as functions of the variable −1 + μ, whereas results obtained in the northern hemisphere (denoted by N) are shown are as functions of 1 − μ.