Relation between the source coordinate system (x′, y′, z′) and the observer coordinate system (x,y,z). Changing from the observer system to the source system involves a rotation by an angle α about the z′-axis to an intermediate coordinate system (x^∗,y^∗,z^∗), followed by a rotation by an angle β about the y^∗-axis (which thus is also the y-axis).