Angular geometry of the spherical triangle centered at pixel i and defined by the zenith direction unit vector, u_i, the observer’s direction unit vector, u_ob, and the star direction unit vector, u_s. The sides of the spherical triangle are: the observer–zenith angle θ_i, the star–zenith angle θ_0i, and the pixel-based phase angle α_i, all centered at pixel i. The angle between sides θ_0i and θ_i is the azimuthal difference angle ϕ_i − ϕ_0i.