Tangential-coma-induced bias between the outer and inner edges of the muon ring for a parabolic telescope with f_# = 1.2, muon Cherenkov angle θ_c = 1°, and inclination angle ν = 1°, shown without plate-scale correction (left) and with plate-scale correction (right). The expected bias for parallel light, typically used for a plate-scale correction, is indicated by the black dotted line. Note the different vertical-axis scales in the two panels.