Example of hyperfine fits with degeneracy between the T_ex and τ parameters. The upper frame shows χ² values over a (T_ex, τ) plane, with the χ² minimum at the centre of the image (the white cross). The white and cyan contours correspond to a 1% and 40% increase over the minimum χ². The bottom frame shows the spectrum (black line), the original model (spectrum without noise; red line), and the spectrum corresponding to the cyan cross in frame a (dashed cyan line).