Angular diameter of the shadow as a function of the mass ratio q = M_spike / M_BH (black solid line). The yellow shaded area is the filled contour corresponding to the uncertainty on the shadow due to the error on the total angular Schwarzschild radius, Δθ_S,tot = 6.43  μas. The other contours show the uncertainty due to resolution, for the baselines (Hawaii-ALMA and Bure-SPT), which allow the array to achieve its best resolution, and for two observing frequencies. The 1  μas resolution is an optimistic case.