Cross-section through the centre of, and perpendicular to the orbital plane. The effect of an increase in α on the edge-on view of the spiral model, with α the angle between the two dashed lines. A narrow spiral with an opening angle of α = 10^° (left panel), an intermediate spiral with opening angle α = 35^° (middle panel), and a shell spiral with α = 180^° (right panel) are shown. The red horizontal dotted line represents the location of the edge-on orbital plane. The colour scheme used is qualitative, and serves to demonstrate the Gaussian nature of the S(r,θ,φ) function.