Approximation of the convolution function for the special case where a stellar line is blended by a telluric line (µ_s = µ_t). Upper panel: Artificial line profiles representing a stellar line (red line), a telluric line (dashed green line), and the instrumental line profile (dotted blue line, normalized for the sake of clarity). Middle panel: Left and right side of Eq. (A.1). Lower panel: Difference between the left and right side of Eq. (A.1) expressed by a − b. The dashed line indicates the limit set by the product of the line depths, .