Normalized geometric area, A_∞(θ)/A_∞(0) of an obstructed mirror shell with α₀ = 0.2 deg, δ = 0, and L₁ = L₂, as a function of the off-axis angle, for different values of the obstruction parameter Φ, with Φ ≈ Ψ ≈ Σ and L = L ^∗ . The curves are traced using the analytical formulae reported in Sect. 4.1. The curve for the unobstructed mirror (the solid line) is also valid for any obstructed mirror with Φ > 2α₀.