Contours of constant χ²-values (confidence levels {68.3%, 95.4%, 99.7%}, solid lines; two degrees of freedom) for fits to the atlas observation. ρ_J was fixed and the contours were calculated for the μ-value of the observation. The dotted ellipse represents a confidence interval of 68.3% for one degree of freedom. It is used to derive the errors of the single parameters by projections onto one dimensional intervals (see Press et al. 2002).