Empirical χ² distribution functions from the 25 residual noise maps compared with the theoretical cumulative probability density. The black stair line is the empirical distribution function, the blue solid line is the theoretical χ² distribution for 3N_pix − 1 = 36863 degrees of freedom. The red dashed line is the least squares fit of the χ² distribution to the experimental distribution (d.o.f. being the fitting parameter). The horizontal axis is translated to the expected center of the distribution,  ⟨ χ² ⟩  = d.o.f. = 36,863, and scaled by the expected deviation,