Determining the scaled disc model. Top: ƒ_y > 1. Bottom: ƒ_y < 1. The coordinate system is centred at (0, 0). The midpoint of the disc is shifted up to (0, δy). A (blue) circle is drawn with radius R_circle (Eq. (3)). The circle is then scaled in the y direction by ƒ_y and this scaling is compensated with a scaling in the x direction with ƒ_x (grey). The midpoint of the disc is then shifted to (δx, δy), shearing the ellipse to the proper midpoint (orange). From this ellipse, the disc parameters can be determined.