Simulated noise in spatially resolved line profile reconstructions from observations during an exoplanet transit. Columns from left to right: reconstructions from resolutions λ/Δλ = 600 000, 300 000, 150 000 and 75 000, sampled with two pixels per spectral resolution element. Two top rows: cooler ‘K2 V’ and a hotter ‘F3 V’ star, differing in line strengths, widths, and limb darkening. Photometric noise levels during transit in the spectral continuum are: σ = 0.01% (thin line) and 0.001% (bold). Noise-free profiles at each resolution are plotted as gray dots. Inside each frame and top down, profiles are for disk center positions μ = 1, 0.79, 0.41, and 0.09. Bottom row: reconstructed profiles at the extremely low noise level of σ = 0.0001%, which is required to also recover well the bisectors (tenfold expanded scale at top).